Chapter 5 MySQL Server Administration

Table of Contents

5.1 The MySQL Server
5.1.1 Configuring the Server
5.1.2 Server Configuration Defaults
5.1.3 Server Option, System Variable, and Status Variable Reference
5.1.4 Server System Variable Reference
5.1.5 Server Status Variable Reference
5.1.6 Server Command Options
5.1.7 Server System Variables
5.1.8 Using System Variables
5.1.9 Server Status Variables
5.1.10 Server SQL Modes
5.1.11 IPv6 Support
5.1.12 MySQL Server Time Zone Support
5.1.13 Server-Side Help Support
5.1.14 Server Tracking of Client Session State Changes
5.1.15 The Server Shutdown Process
5.2 The MySQL Data Directory
5.3 The mysql System Database
5.4 MySQL Server Logs
5.4.1 Selecting General Query Log and Slow Query Log Output Destinations
5.4.2 The Error Log
5.4.3 The General Query Log
5.4.4 The Binary Log
5.4.5 The Slow Query Log
5.4.6 The DDL Log
5.4.7 Server Log Maintenance
5.5 MySQL Server Plugins
5.5.1 Installing and Uninstalling Plugins
5.5.2 Obtaining Server Plugin Information
5.5.3 MySQL Enterprise Thread Pool
5.5.4 The Rewriter Query Rewrite Plugin
5.5.5 Version Tokens
5.6 MySQL Server User-Defined Functions
5.6.1 Installing and Uninstalling User-Defined Functions
5.6.2 Obtaining User-Defined Function Information
5.7 Running Multiple MySQL Instances on One Machine
5.7.1 Setting Up Multiple Data Directories
5.7.2 Running Multiple MySQL Instances on Windows
5.7.3 Running Multiple MySQL Instances on Unix
5.7.4 Using Client Programs in a Multiple-Server Environment
5.8 Tracing mysqld Using DTrace
5.8.1 mysqld DTrace Probe Reference

MySQL Server (mysqld) is the main program that does most of the work in a MySQL installation. This chapter provides an overview of MySQL Server and covers general server administration:

For additional information on administrative topics, see also:

5.1 The MySQL Server

mysqld is the MySQL server. The following discussion covers these MySQL server configuration topics:

  • Startup options that the server supports. You can specify these options on the command line, through configuration files, or both.

  • Server system variables. These variables reflect the current state and values of the startup options, some of which can be modified while the server is running.

  • Server status variables. These variables contain counters and statistics about runtime operation.

  • How to set the server SQL mode. This setting modifies certain aspects of SQL syntax and semantics, for example for compatibility with code from other database systems, or to control the error handling for particular situations.

  • Configuring and using IPv6 support.

  • Configuring and using time zone support.

  • Server-side help capabilities.

  • The server shutdown process. There are performance and reliability considerations depending on the type of table (transactional or nontransactional) and whether you use replication.

For listings of MySQL server variables and options that have been added, deprecated, or removed in MySQL 5.7, see Section 1.5, “Server and Status Variables and Options Added, Deprecated, or Removed in MySQL 5.7”.

Note

Not all storage engines are supported by all MySQL server binaries and configurations. To find out how to determine which storage engines your MySQL server installation supports, see Section 13.7.5.16, “SHOW ENGINES Statement”.

5.1.1 Configuring the Server

The MySQL server, mysqld, has many command options and system variables that can be set at startup to configure its operation. To determine the default command option and system variable values used by the server, execute this command:

shell> mysqld --verbose --help

The command produces a list of all mysqld options and configurable system variables. Its output includes the default option and variable values and looks something like this:

abort-slave-event-count           0
allow-suspicious-udfs             FALSE
archive                           ON
auto-increment-increment          1
auto-increment-offset             1
autocommit                        TRUE
automatic-sp-privileges           TRUE
avoid-temporal-upgrade            FALSE
back-log                          80
basedir                           /home/jon/bin/mysql-5.7/
...
tmpdir                            /tmp
transaction-alloc-block-size      8192
transaction-isolation             REPEATABLE-READ
transaction-prealloc-size         4096
transaction-read-only             FALSE
transaction-write-set-extraction  OFF
updatable-views-with-limit        YES
validate-user-plugins             TRUE
verbose                           TRUE
wait-timeout                      28800

To see the current system variable values actually used by the server as it runs, connect to it and execute this statement:

mysql> SHOW VARIABLES;

To see some statistical and status indicators for a running server, execute this statement:

mysql> SHOW STATUS;

System variable and status information also is available using the mysqladmin command:

shell> mysqladmin variables
shell> mysqladmin extended-status

For a full description of all command options, system variables, and status variables, see these sections:

More detailed monitoring information is available from the Performance Schema; see Chapter 25, MySQL Performance Schema. In addition, the MySQL sys schema is a set of objects that provides convenient access to data collected by the Performance Schema; see Chapter 26, MySQL sys Schema.

MySQL uses algorithms that are very scalable, so you can usually run with very little memory. However, normally better performance results from giving MySQL more memory.

When tuning a MySQL server, the two most important variables to configure are key_buffer_size and table_open_cache. You should first feel confident that you have these set appropriately before trying to change any other variables.

The following examples indicate some typical variable values for different runtime configurations.

  • If you have at least 1-2GB of memory and many tables and want maximum performance with a moderate number of clients, use something like this:

    shell> mysqld_safe --key_buffer_size=384M --table_open_cache=4000 \
               --sort_buffer_size=4M --read_buffer_size=1M &
    
  • If you have only 256MB of memory and only a few tables, but you still do a lot of sorting, you can use something like this:

    shell> mysqld_safe --key_buffer_size=64M --sort_buffer_size=1M
    

    If there are very many simultaneous connections, swapping problems may occur unless mysqld has been configured to use very little memory for each connection. mysqld performs better if you have enough memory for all connections.

  • With little memory and lots of connections, use something like this:

    shell> mysqld_safe --key_buffer_size=512K --sort_buffer_size=100K \
               --read_buffer_size=100K &
    

    Or even this:

    shell> mysqld_safe --key_buffer_size=512K --sort_buffer_size=16K \
               --table_open_cache=32 --read_buffer_size=8K \
               --net_buffer_length=1K &
    

If you are performing GROUP BY or ORDER BY operations on tables that are much larger than your available memory, increase the value of read_rnd_buffer_size to speed up the reading of rows following sorting operations.

If you specify an option on the command line for mysqld or mysqld_safe, it remains in effect only for that invocation of the server. To use the option every time the server runs, put it in an option file. See Section 4.2.2.2, “Using Option Files”.

5.1.2 Server Configuration Defaults

The MySQL server has many operating parameters, which you can change at server startup using command-line options or configuration files (option files). It is also possible to change many parameters at runtime. For general instructions on setting parameters at startup or runtime, see Section 5.1.6, “Server Command Options”, and Section 5.1.7, “Server System Variables”.

On Windows, MySQL Installer interacts with the user and creates a file named my.ini in the base installation directory as the default option file. If you install on Windows from a Zip archive, you can copy the my-default.ini template file in the base installation directory to my.ini and use the latter as the default option file.

Note

As of MySQL 5.7.18, my-default.ini is no longer included in or installed by distribution packages.

Note

On Windows, the .ini or .cnf option file extension might not be displayed.

After completing the installation process, you can edit the default option file at any time to modify the parameters used by the server. For example, to use a parameter setting in the file that is commented with a # character at the beginning of the line, remove the #, and modify the parameter value if necessary. To disable a setting, either add a # to the beginning of the line or remove it.

For non-Windows platforms, no default option file is created during either the server installation or the data directory initialization process. Create your option file by following the instructions given in Section 4.2.2.2, “Using Option Files”. Without an option file, the server just starts with its default settings—see Section 5.1.2, “Server Configuration Defaults” on how to check those settings.

For additional information about option file format and syntax, see Section 4.2.2.2, “Using Option Files”.

5.1.3 Server Option, System Variable, and Status Variable Reference

The following table lists all command-line options, system variables, and status variables applicable within mysqld.

The table lists command-line options (Cmd-line), options valid in configuration files (Option file), server system variables (System Var), and status variables (Status var) in one unified list, with an indication of where each option or variable is valid. If a server option set on the command line or in an option file differs from the name of the corresponding system variable, the variable name is noted immediately below the corresponding option. For system and status variables, the scope of the variable (Var Scope) is Global, Session, or both. Please see the corresponding item descriptions for details on setting and using the options and variables. Where appropriate, direct links to further information about the items are provided.

For a version of this table that is specific to NDB Cluster, see Section 21.3.2.5, “NDB Cluster mysqld Option and Variable Reference”.

Table 5.1 Command-Line Option, System Variable, and Status Variable Summary

Name Cmd-Line Option File System Var Status Var Var Scope Dynamic
abort-slave-event-count Yes Yes
Aborted_clients Yes Global No
Aborted_connects Yes Global No
allow-suspicious-udfs Yes Yes
ansi Yes Yes
audit-log Yes Yes
audit_log_buffer_size Yes Yes Yes Global No
audit_log_compression Yes Yes Yes Global No
audit_log_connection_policy Yes Yes Yes Global Yes
audit_log_current_session Yes Both No
Audit_log_current_size Yes Global No
audit_log_encryption Yes Yes Yes Global No
Audit_log_event_max_drop_size Yes Global No
Audit_log_events Yes Global No
Audit_log_events_filtered Yes Global No
Audit_log_events_lost Yes Global No
Audit_log_events_written Yes Global No
audit_log_exclude_accounts Yes Yes Yes Global Yes
audit_log_file Yes Yes Yes Global No
audit_log_filter_id Yes Both No
audit_log_flush Yes Global Yes
audit_log_format Yes Yes Yes Global No
audit_log_include_accounts Yes Yes Yes Global Yes
audit_log_policy Yes Yes Yes Global No
audit_log_read_buffer_size Yes Yes Yes Varies Varies
audit_log_rotate_on_size Yes Yes Yes Global Yes
audit_log_statement_policy Yes Yes Yes Global Yes
audit_log_strategy Yes Yes Yes Global No
Audit_log_total_size Yes Global No
Audit_log_write_waits Yes Global No
authentication_ldap_sasl_auth_method_name Yes Yes Yes Global Yes
authentication_ldap_sasl_bind_base_dn Yes Yes Yes Global Yes
authentication_ldap_sasl_bind_root_dn Yes Yes Yes Global Yes
authentication_ldap_sasl_bind_root_pwd Yes Yes Yes Global Yes
authentication_ldap_sasl_ca_path Yes Yes Yes Global Yes
authentication_ldap_sasl_group_search_attr Yes Yes Yes Global Yes
authentication_ldap_sasl_group_search_filter Yes Yes Yes Global Yes
authentication_ldap_sasl_init_pool_size Yes Yes Yes Global Yes
authentication_ldap_sasl_log_status Yes Yes Yes Global Yes
authentication_ldap_sasl_max_pool_size Yes Yes Yes Global Yes
authentication_ldap_sasl_server_host Yes Yes Yes Global Yes
authentication_ldap_sasl_server_port Yes Yes Yes Global Yes
authentication_ldap_sasl_tls Yes Yes Yes Global Yes
authentication_ldap_sasl_user_search_attr Yes Yes Yes Global Yes
authentication_ldap_simple_auth_method_name Yes Yes Yes Global Yes
authentication_ldap_simple_bind_base_dn Yes Yes Yes Global Yes
authentication_ldap_simple_bind_root_dn Yes Yes Yes Global Yes
authentication_ldap_simple_bind_root_pwd Yes Yes Yes Global Yes
authentication_ldap_simple_ca_path Yes Yes Yes Global Yes
authentication_ldap_simple_group_search_attr Yes Yes Yes Global Yes
authentication_ldap_simple_group_search_filter Yes Yes Yes Global Yes
authentication_ldap_simple_init_pool_size Yes Yes Yes Global Yes
authentication_ldap_simple_log_status Yes Yes Yes Global Yes
authentication_ldap_simple_max_pool_size Yes Yes Yes Global Yes
authentication_ldap_simple_server_host Yes Yes Yes Global Yes
authentication_ldap_simple_server_port Yes Yes Yes Global Yes
authentication_ldap_simple_tls Yes Yes Yes Global Yes
authentication_ldap_simple_user_search_attr Yes Yes Yes Global Yes
authentication_windows_log_level Yes Yes Yes Global No
authentication_windows_use_principal_name Yes Yes Yes Global No
auto_generate_certs Yes Yes Yes Global No
auto_increment_increment Yes Yes Yes Both Yes
auto_increment_offset Yes Yes Yes Both Yes
autocommit Yes Yes Yes Both Yes
automatic_sp_privileges Yes Yes Yes Global Yes
avoid_temporal_upgrade Yes Yes Yes Global Yes
back_log Yes Yes Yes Global No
basedir Yes Yes Yes Global No
big_tables Yes Yes Yes Both Yes
bind_address Yes Yes Yes Global No
Binlog_cache_disk_use Yes Global No
binlog_cache_size Yes Yes Yes Global Yes
Binlog_cache_use Yes Global No
binlog-checksum Yes Yes
binlog_checksum Yes Yes Yes Global Yes
binlog_direct_non_transactional_updates Yes Yes Yes Both Yes
binlog-do-db Yes Yes
binlog_error_action Yes Yes Yes Global Yes
binlog_format Yes Yes Yes Both Yes
binlog_group_commit_sync_delay Yes Yes Yes Global Yes
binlog_group_commit_sync_no_delay_count Yes Yes Yes Global Yes
binlog_gtid_simple_recovery Yes Yes Yes Global No
binlog-ignore-db Yes Yes
binlog_max_flush_queue_time Yes Yes Yes Global Yes
binlog_order_commits Yes Yes Yes Global Yes
binlog-row-event-max-size Yes Yes
binlog_row_image Yes Yes Yes Both Yes
binlog_rows_query_log_events Yes Yes Yes Both Yes
Binlog_stmt_cache_disk_use Yes Global No
binlog_stmt_cache_size Yes Yes Yes Global Yes
Binlog_stmt_cache_use Yes Global No
binlog_transaction_dependency_history_size Yes Yes Yes Global Yes
binlog_transaction_dependency_tracking Yes Yes Yes Global Yes
block_encryption_mode Yes Yes Yes Both Yes
bootstrap Yes Yes
bulk_insert_buffer_size Yes Yes Yes Both Yes
Bytes_received Yes Both No
Bytes_sent Yes Both No
character_set_client Yes Both Yes
character-set-client-handshake Yes Yes
character_set_connection Yes Both Yes
character_set_database (note 1) Yes Both Yes
character_set_filesystem Yes Yes Yes Both Yes
character_set_results Yes Both Yes
character_set_server Yes Yes Yes Both Yes
character_set_system Yes Global No
character_sets_dir Yes Yes Yes Global No
check_proxy_users Yes Yes Yes Global Yes
chroot Yes Yes
collation_connection Yes Both Yes
collation_database (note 1) Yes Both Yes
collation_server Yes Yes Yes Both Yes
Com_admin_commands Yes Both No
Com_alter_db Yes Both No
Com_alter_db_upgrade Yes Both No
Com_alter_event Yes Both No
Com_alter_function Yes Both No
Com_alter_procedure Yes Both No
Com_alter_server Yes Both No
Com_alter_table Yes Both No
Com_alter_tablespace Yes Both No
Com_alter_user Yes Both No
Com_analyze Yes Both No
Com_assign_to_keycache Yes Both No
Com_begin Yes Both No
Com_binlog Yes Both No
Com_call_procedure Yes Both No
Com_change_db Yes Both No
Com_change_master Yes Both No
Com_change_repl_filter Yes Both No
Com_check Yes Both No
Com_checksum Yes Both No
Com_commit Yes Both No
Com_create_db Yes Both No
Com_create_event Yes Both No
Com_create_function Yes Both No
Com_create_index Yes Both No
Com_create_procedure Yes Both No
Com_create_server Yes Both No
Com_create_table Yes Both No
Com_create_trigger Yes Both No
Com_create_udf Yes Both No
Com_create_user Yes Both No
Com_create_view Yes Both No
Com_dealloc_sql Yes Both No
Com_delete Yes Both No
Com_delete_multi Yes Both No
Com_do Yes Both No
Com_drop_db Yes Both No
Com_drop_event Yes Both No
Com_drop_function Yes Both No
Com_drop_index Yes Both No
Com_drop_procedure Yes Both No
Com_drop_server Yes Both No
Com_drop_table Yes Both No
Com_drop_trigger Yes Both No
Com_drop_user Yes Both No
Com_drop_view Yes Both No
Com_empty_query Yes Both No
Com_execute_sql Yes Both No
Com_explain_other Yes Both No
Com_flush Yes Both No
Com_get_diagnostics Yes Both No
Com_grant Yes Both No
Com_group_replication_start Yes Global No
Com_group_replication_stop Yes Global No
Com_ha_close Yes Both No
Com_ha_open Yes Both No
Com_ha_read Yes Both No
Com_help Yes Both No
Com_insert Yes Both No
Com_insert_select Yes Both No
Com_install_plugin Yes Both No
Com_kill Yes Both No
Com_load Yes Both No
Com_lock_tables Yes Both No
Com_optimize Yes Both No
Com_preload_keys Yes Both No
Com_prepare_sql Yes Both No
Com_purge Yes Both No
Com_purge_before_date Yes Both No
Com_release_savepoint Yes Both No
Com_rename_table Yes Both No
Com_rename_user Yes Both No
Com_repair Yes Both No
Com_replace Yes Both No
Com_replace_select Yes Both No
Com_reset Yes Both No
Com_resignal Yes Both No
Com_revoke Yes Both No
Com_revoke_all Yes Both No
Com_rollback Yes Both No
Com_rollback_to_savepoint Yes Both No
Com_savepoint Yes Both No
Com_select Yes Both No
Com_set_option Yes Both No
Com_show_authors Yes Both No
Com_show_binlog_events Yes Both No
Com_show_binlogs Yes Both No
Com_show_charsets Yes Both No
Com_show_collations Yes Both No
Com_show_contributors Yes Both No
Com_show_create_db Yes Both No
Com_show_create_event Yes Both No
Com_show_create_func Yes Both No
Com_show_create_proc Yes Both No
Com_show_create_table Yes Both No
Com_show_create_trigger Yes Both No
Com_show_create_user Yes Both No
Com_show_databases Yes Both No
Com_show_engine_logs Yes Both No
Com_show_engine_mutex Yes Both No
Com_show_engine_status Yes Both No
Com_show_errors Yes Both No
Com_show_events Yes Both No
Com_show_fields Yes Both No
Com_show_function_code Yes Both No
Com_show_function_status Yes Both No
Com_show_grants Yes Both No
Com_show_keys Yes Both No
Com_show_master_status Yes Both No
Com_show_ndb_status Yes Both No
Com_show_open_tables Yes Both No
Com_show_plugins Yes Both No
Com_show_privileges Yes Both No
Com_show_procedure_code Yes Both No
Com_show_procedure_status Yes Both No
Com_show_processlist Yes Both No
Com_show_profile Yes Both No
Com_show_profiles Yes Both No
Com_show_relaylog_events Yes Both No
Com_show_slave_hosts Yes Both No
Com_show_slave_status Yes Both No
Com_show_status Yes Both No
Com_show_storage_engines Yes Both No
Com_show_table_status Yes Both No
Com_show_tables Yes Both No
Com_show_triggers Yes Both No
Com_show_variables Yes Both No
Com_show_warnings Yes Both No
Com_shutdown Yes Both No
Com_signal Yes Both No
Com_slave_start Yes Both No
Com_slave_stop Yes Both No
Com_stmt_close Yes Both No
Com_stmt_execute Yes Both No
Com_stmt_fetch Yes Both No
Com_stmt_prepare Yes Both No
Com_stmt_reprepare Yes Both No
Com_stmt_reset Yes Both No
Com_stmt_send_long_data Yes Both No
Com_truncate Yes Both No
Com_uninstall_plugin Yes Both No
Com_unlock_tables Yes Both No
Com_update Yes Both No
Com_update_multi Yes Both No
Com_xa_commit Yes Both No
Com_xa_end Yes Both No
Com_xa_prepare Yes Both No
Com_xa_recover Yes Both No
Com_xa_rollback Yes Both No
Com_xa_start Yes Both No
completion_type Yes Yes Yes Both Yes
Compression Yes Session No
concurrent_insert Yes Yes Yes Global Yes
connect_timeout Yes Yes Yes Global Yes
Connection_control_delay_generated Yes Global No
connection_control_failed_connections_threshold Yes Yes Yes Global Yes
connection_control_max_connection_delay Yes Yes Yes Global Yes
connection_control_min_connection_delay Yes Yes Yes Global Yes
Connection_errors_accept Yes Global No
Connection_errors_internal Yes Global No
Connection_errors_max_connections Yes Global No
Connection_errors_peer_address Yes Global No
Connection_errors_select Yes Global No
Connection_errors_tcpwrap Yes Global No
Connections Yes Global No
console Yes Yes
core-file Yes Yes
core_file Yes Global No
Created_tmp_disk_tables Yes Both No
Created_tmp_files Yes Global No
Created_tmp_tables Yes Both No
daemon_memcached_enable_binlog Yes Yes Yes Global No
daemon_memcached_engine_lib_name Yes Yes Yes Global No
daemon_memcached_engine_lib_path Yes Yes Yes Global No
daemon_memcached_option Yes Yes Yes Global No
daemon_memcached_r_batch_size Yes Yes Yes Global No
daemon_memcached_w_batch_size Yes Yes Yes Global No
daemonize Yes Yes
datadir Yes Yes Yes Global No
date_format Yes Global No
datetime_format Yes Global No
debug Yes Yes Yes Both Yes
debug_sync Yes Session Yes
debug-sync-timeout Yes Yes
default_authentication_plugin Yes Yes Yes Global No
default_password_lifetime Yes Yes Yes Global Yes
default_storage_engine Yes Yes Yes Both Yes
default-time-zone Yes Yes
default_tmp_storage_engine Yes Yes Yes Both Yes
default_week_format Yes Yes Yes Both Yes
defaults-extra-file Yes
defaults-file Yes
defaults-group-suffix Yes
delay_key_write Yes Yes Yes Global Yes
Delayed_errors Yes Global No
delayed_insert_limit Yes Yes Yes Global Yes
Delayed_insert_threads Yes Global No
delayed_insert_timeout Yes Yes Yes Global Yes
delayed_queue_size Yes Yes Yes Global Yes
Delayed_writes Yes Global No
des-key-file Yes Yes
disable-partition-engine-check Yes Yes
disabled_storage_engines Yes Yes Yes Global No
disconnect_on_expired_password Yes Yes Yes Global No
disconnect-slave-event-count Yes Yes
div_precision_increment Yes Yes Yes Both Yes
early-plugin-load Yes Yes
end_markers_in_json Yes Yes Yes Both Yes
enforce_gtid_consistency Yes Yes Yes Global Varies
eq_range_index_dive_limit Yes Yes Yes Both Yes
error_count Yes Session No
event_scheduler Yes Yes Yes Global Yes
exit-info Yes Yes
expire_logs_days Yes Yes Yes Global Yes
explicit_defaults_for_timestamp Yes Yes Yes Both Yes
external-locking Yes Yes
- Variable: skip_external_locking
external_user Yes Session No
federated Yes Yes
Firewall_access_denied Yes Global No
Firewall_access_granted Yes Global No
Firewall_cached_entries Yes Global No
flush Yes Yes Yes Global Yes
Flush_commands Yes Global No
flush_time Yes Yes Yes Global Yes
foreign_key_checks Yes Both Yes
ft_boolean_syntax Yes Yes Yes Global Yes
ft_max_word_len Yes Yes Yes Global No
ft_min_word_len Yes Yes Yes Global No
ft_query_expansion_limit Yes Yes Yes Global No
ft_stopword_file Yes Yes Yes Global No
gdb Yes Yes
general_log Yes Yes Yes Global Yes
general_log_file Yes Yes Yes Global Yes
group_concat_max_len Yes Yes Yes Both Yes
group_replication_allow_local_disjoint_gtids_join Yes Yes Yes Global Yes
group_replication_allow_local_lower_version_join Yes Yes Yes Global Yes
group_replication_auto_increment_increment Yes Yes Yes Global Yes
group_replication_bootstrap_group Yes Yes Yes Global Yes
group_replication_components_stop_timeout Yes Yes Yes Global Yes
group_replication_compression_threshold Yes Yes Yes Global Yes
group_replication_enforce_update_everywhere_checks Yes Yes Yes Global Yes
group_replication_exit_state_action Yes Yes Yes Global Yes
group_replication_flow_control_applier_threshold Yes Yes Yes Global Yes
group_replication_flow_control_certifier_threshold Yes Yes Yes Global Yes
group_replication_flow_control_mode Yes Yes Yes Global Yes
group_replication_force_members Yes Yes Yes Global Yes
group_replication_group_name Yes Yes Yes Global Yes
group_replication_group_seeds Yes Yes Yes Global Yes
group_replication_gtid_assignment_block_size Yes Yes Yes Global Yes
group_replication_ip_whitelist Yes Yes Yes Global Yes
group_replication_local_address Yes Yes Yes Global Yes
group_replication_member_weight Yes Yes Yes Global Yes
group_replication_poll_spin_loops Yes Yes Yes Global Yes
group_replication_primary_member Yes Global No
group_replication_recovery_complete_at Yes Yes Yes Global Yes
group_replication_recovery_reconnect_interval Yes Yes Yes Global Yes
group_replication_recovery_retry_count Yes Yes Yes Global Yes
group_replication_recovery_ssl_ca Yes Yes Yes Global Yes
group_replication_recovery_ssl_capath Yes Yes Yes Global Yes
group_replication_recovery_ssl_cert Yes Yes Yes Global Yes
group_replication_recovery_ssl_cipher Yes Yes Yes Global Yes
group_replication_recovery_ssl_crl Yes Yes Yes Global Yes
group_replication_recovery_ssl_crlpath Yes Yes Yes Global Yes
group_replication_recovery_ssl_key Yes Yes Yes Global Yes
group_replication_recovery_ssl_verify_server_cert Yes Yes Yes Global Yes
group_replication_recovery_use_ssl Yes Yes Yes Global Yes
group_replication_single_primary_mode Yes Yes Yes Global Yes
group_replication_ssl_mode Yes Yes Yes Global Yes
group_replication_start_on_boot Yes Yes Yes Global Yes
group_replication_transaction_size_limit Yes Yes Yes Global Yes
group_replication_unreachable_majority_timeout Yes Yes Yes Global Yes
gtid_executed Yes Varies No
gtid_executed_compression_period Yes Yes Yes Global Yes
gtid_mode Yes Yes Yes Global Varies
gtid_next Yes Session Yes
gtid_owned Yes Both No
gtid_purged Yes Global Yes
Handler_commit Yes Both No
Handler_delete Yes Both No
Handler_discover Yes Both No
Handler_external_lock Yes Both No
Handler_mrr_init Yes Both No
Handler_prepare Yes Both No
Handler_read_first Yes Both No
Handler_read_key Yes Both No
Handler_read_last Yes Both No
Handler_read_next Yes Both No
Handler_read_prev Yes Both No
Handler_read_rnd Yes Both No
Handler_read_rnd_next Yes Both No
Handler_rollback Yes Both No
Handler_savepoint Yes Both No
Handler_savepoint_rollback Yes Both No
Handler_update Yes Both No
Handler_write Yes Both No
have_compress Yes Global No
have_crypt Yes Global No
have_dynamic_loading Yes Global No
have_geometry Yes Global No
have_openssl Yes Global No
have_profiling Yes Global No
have_query_cache Yes Global No
have_rtree_keys Yes Global No
have_ssl Yes Global No
have_statement_timeout Yes Global No
have_symlink Yes Global No
help Yes Yes
host_cache_size Yes Yes Yes Global Yes
hostname Yes Global No
identity Yes Session Yes
ignore_builtin_innodb Yes Yes Yes Global No
ignore-db-dir Yes Yes
ignore_db_dirs Yes Global No
init_connect Yes Yes Yes Global Yes
init_file Yes Yes Yes Global No
init_slave Yes Yes Yes Global Yes
initialize Yes Yes
initialize-insecure Yes Yes
innodb Yes Yes
innodb_adaptive_flushing Yes Yes Yes Global Yes
innodb_adaptive_flushing_lwm Yes Yes Yes Global Yes
innodb_adaptive_hash_index Yes Yes Yes Global Yes
innodb_adaptive_hash_index_parts Yes Yes Yes Global No
innodb_adaptive_max_sleep_delay Yes Yes Yes Global Yes
innodb_api_bk_commit_interval Yes Yes Yes Global Yes
innodb_api_disable_rowlock Yes Yes Yes Global No
innodb_api_enable_binlog Yes Yes Yes Global No
innodb_api_enable_mdl Yes Yes Yes Global No
innodb_api_trx_level Yes Yes Yes Global Yes
innodb_autoextend_increment Yes Yes Yes Global Yes
innodb_autoinc_lock_mode Yes Yes Yes Global No
Innodb_available_undo_logs Yes Global No
innodb_background_drop_list_empty Yes Yes Yes Global Yes
Innodb_buffer_pool_bytes_data Yes Global No
Innodb_buffer_pool_bytes_dirty Yes Global No
innodb_buffer_pool_chunk_size Yes Yes Yes Global No
innodb_buffer_pool_dump_at_shutdown Yes Yes Yes Global Yes
innodb_buffer_pool_dump_now Yes Yes Yes Global Yes
innodb_buffer_pool_dump_pct Yes Yes Yes Global Yes
Innodb_buffer_pool_dump_status Yes Global No
innodb_buffer_pool_filename Yes Yes Yes Global Yes
innodb_buffer_pool_instances Yes Yes Yes Global No
innodb_buffer_pool_load_abort Yes Yes Yes Global Yes
innodb_buffer_pool_load_at_startup Yes Yes Yes Global No
innodb_buffer_pool_load_now Yes Yes Yes Global Yes
Innodb_buffer_pool_load_status Yes Global No
Innodb_buffer_pool_pages_data Yes Global No
Innodb_buffer_pool_pages_dirty Yes Global No
Innodb_buffer_pool_pages_flushed Yes Global No
Innodb_buffer_pool_pages_free Yes Global No
Innodb_buffer_pool_pages_latched Yes Global No
Innodb_buffer_pool_pages_misc Yes Global No
Innodb_buffer_pool_pages_total Yes Global No
Innodb_buffer_pool_read_ahead Yes Global No
Innodb_buffer_pool_read_ahead_evicted Yes Global No
Innodb_buffer_pool_read_ahead_rnd Yes Global No
Innodb_buffer_pool_read_requests Yes Global No
Innodb_buffer_pool_reads Yes Global No
Innodb_buffer_pool_resize_status Yes Global No
innodb_buffer_pool_size Yes Yes Yes Global Varies
Innodb_buffer_pool_wait_free Yes Global No
Innodb_buffer_pool_write_requests Yes Global No
innodb_change_buffer_max_size Yes Yes Yes Global Yes
innodb_change_buffering Yes Yes Yes Global Yes
innodb_change_buffering_debug Yes Yes Yes Global Yes
innodb_checksum_algorithm Yes Yes Yes Global Yes
innodb_checksums Yes Yes Yes Global No
innodb_cmp_per_index_enabled Yes Yes Yes Global Yes
innodb_commit_concurrency Yes Yes Yes Global Yes
innodb_compress_debug Yes Yes Yes Global Yes
innodb_compression_failure_threshold_pct Yes Yes Yes Global Yes
innodb_compression_level Yes Yes Yes Global Yes
innodb_compression_pad_pct_max Yes Yes Yes Global Yes
innodb_concurrency_tickets Yes Yes Yes Global Yes
innodb_data_file_path Yes Yes Yes Global No
Innodb_data_fsyncs Yes Global No
innodb_data_home_dir Yes Yes Yes Global No
Innodb_data_pending_fsyncs Yes Global No
Innodb_data_pending_reads Yes Global No
Innodb_data_pending_writes Yes Global No
Innodb_data_read Yes Global No
Innodb_data_reads Yes Global No
Innodb_data_writes Yes Global No
Innodb_data_written Yes Global No
Innodb_dblwr_pages_written Yes Global No
Innodb_dblwr_writes Yes Global No
innodb_deadlock_detect Yes Yes Yes Global Yes
innodb_default_row_format Yes Yes Yes Global Yes
innodb_disable_resize_buffer_pool_debug Yes Yes Yes Global Yes
innodb_disable_sort_file_cache Yes Yes Yes Global Yes
innodb_doublewrite Yes Yes Yes Global No
innodb_fast_shutdown Yes Yes Yes Global Yes
innodb_fil_make_page_dirty_debug Yes Yes Yes Global Yes
innodb_file_format Yes Yes Yes Global Yes
innodb_file_format_check Yes Yes Yes Global No
innodb_file_format_max Yes Yes Yes Global Yes
innodb_file_per_table Yes Yes Yes Global Yes
innodb_fill_factor Yes Yes Yes Global Yes
innodb_flush_log_at_timeout Yes Yes Yes Global Yes
innodb_flush_log_at_trx_commit Yes Yes Yes Global Yes
innodb_flush_method Yes Yes Yes Global No
innodb_flush_neighbors Yes Yes Yes Global Yes
innodb_flush_sync Yes Yes Yes Global Yes
innodb_flushing_avg_loops Yes Yes Yes Global Yes
innodb_force_load_corrupted Yes Yes Yes Global No
innodb_force_recovery Yes Yes Yes Global No
innodb_ft_aux_table Yes Global Yes
innodb_ft_cache_size Yes Yes Yes Global No
innodb_ft_enable_diag_print Yes Yes Yes Global Yes
innodb_ft_enable_stopword Yes Yes Yes Both Yes
innodb_ft_max_token_size Yes Yes Yes Global No
innodb_ft_min_token_size Yes Yes Yes Global No
innodb_ft_num_word_optimize Yes Yes Yes Global Yes
innodb_ft_result_cache_limit Yes Yes Yes Global Yes
innodb_ft_server_stopword_table Yes Yes Yes Global Yes
innodb_ft_sort_pll_degree Yes Yes Yes Global No
innodb_ft_total_cache_size Yes Yes Yes Global No
innodb_ft_user_stopword_table Yes Yes Yes Both Yes
Innodb_have_atomic_builtins Yes Global No
innodb_io_capacity Yes Yes Yes Global Yes
innodb_io_capacity_max Yes Yes Yes Global Yes
innodb_large_prefix Yes Yes Yes Global Yes
innodb_limit_optimistic_insert_debug Yes Yes Yes Global Yes
innodb_lock_wait_timeout Yes Yes Yes Both Yes
innodb_locks_unsafe_for_binlog Yes Yes Yes Global No
innodb_log_buffer_size Yes Yes Yes Global No
innodb_log_checkpoint_now Yes Yes Yes Global Yes
innodb_log_checksums Yes Yes Yes Global Yes
innodb_log_compressed_pages Yes Yes Yes Global Yes
innodb_log_file_size Yes Yes Yes Global No
innodb_log_files_in_group Yes Yes Yes Global No
innodb_log_group_home_dir Yes Yes Yes Global No
Innodb_log_waits Yes Global No
innodb_log_write_ahead_size Yes Yes Yes Global Yes
Innodb_log_write_requests Yes Global No
Innodb_log_writes Yes Global No
innodb_lru_scan_depth Yes Yes Yes Global Yes
innodb_max_dirty_pages_pct Yes Yes Yes Global Yes
innodb_max_dirty_pages_pct_lwm Yes Yes Yes Global Yes
innodb_max_purge_lag Yes Yes Yes Global Yes
innodb_max_purge_lag_delay Yes Yes Yes Global Yes
innodb_max_undo_log_size Yes Yes Yes Global Yes
innodb_merge_threshold_set_all_debug Yes Yes Yes Global Yes
innodb_monitor_disable Yes Yes Yes Global Yes
innodb_monitor_enable Yes Yes Yes Global Yes
innodb_monitor_reset Yes Yes Yes Global Yes
innodb_monitor_reset_all Yes Yes Yes Global Yes
Innodb_num_open_files Yes Global No
innodb_numa_interleave Yes Yes Yes Global No
innodb_old_blocks_pct Yes Yes Yes Global Yes
innodb_old_blocks_time Yes Yes Yes Global Yes
innodb_online_alter_log_max_size Yes Yes Yes Global Yes
innodb_open_files Yes Yes Yes Global No
innodb_optimize_fulltext_only Yes Yes Yes Global Yes
Innodb_os_log_fsyncs Yes Global No
Innodb_os_log_pending_fsyncs Yes Global No
Innodb_os_log_pending_writes Yes Global No
Innodb_os_log_written Yes Global No
innodb_page_cleaners Yes Yes Yes Global No
Innodb_page_size Yes Global No
innodb_page_size Yes Yes Yes Global No
Innodb_pages_created Yes Global No
Innodb_pages_read Yes Global No
Innodb_pages_written Yes Global No
innodb_print_all_deadlocks Yes Yes Yes Global Yes
innodb_purge_batch_size Yes Yes Yes Global Yes
innodb_purge_rseg_truncate_frequency Yes Yes Yes Global Yes
innodb_purge_threads Yes Yes Yes Global No
innodb_random_read_ahead Yes Yes Yes Global Yes
innodb_read_ahead_threshold Yes Yes Yes Global Yes
innodb_read_io_threads Yes Yes Yes Global No
innodb_read_only Yes Yes Yes Global No
innodb_replication_delay Yes Yes Yes Global Yes
innodb_rollback_on_timeout Yes Yes Yes Global No
innodb_rollback_segments Yes Yes Yes Global Yes
Innodb_row_lock_current_waits Yes Global No
Innodb_row_lock_time Yes Global No
Innodb_row_lock_time_avg Yes Global No
Innodb_row_lock_time_max Yes Global No
Innodb_row_lock_waits Yes Global No
Innodb_rows_deleted Yes Global No
Innodb_rows_inserted Yes Global No
Innodb_rows_read Yes Global No
Innodb_rows_updated Yes Global No
innodb_saved_page_number_debug Yes Yes Yes Global Yes
innodb_sort_buffer_size Yes Yes Yes Global No
innodb_spin_wait_delay Yes Yes Yes Global Yes
innodb_stats_auto_recalc Yes Yes Yes Global Yes
innodb_stats_include_delete_marked Yes Yes Yes Global Yes
innodb_stats_method Yes Yes Yes Global Yes
innodb_stats_on_metadata Yes Yes Yes Global Yes
innodb_stats_persistent Yes Yes Yes Global Yes
innodb_stats_persistent_sample_pages Yes Yes Yes Global Yes
innodb_stats_sample_pages Yes Yes Yes Global Yes
innodb_stats_transient_sample_pages Yes Yes Yes Global Yes
innodb-status-file Yes Yes
innodb_status_output Yes Yes Yes Global Yes
innodb_status_output_locks Yes Yes Yes Global Yes
innodb_strict_mode Yes Yes Yes Both Yes
innodb_support_xa Yes Yes Yes Both Yes
innodb_sync_array_size Yes Yes Yes Global No
innodb_sync_debug Yes Yes Yes Global No
innodb_sync_spin_loops Yes Yes Yes Global Yes
innodb_table_locks Yes Yes Yes Both Yes
innodb_temp_data_file_path Yes Yes Yes Global No
innodb_thread_concurrency Yes Yes Yes Global Yes
innodb_thread_sleep_delay Yes Yes Yes Global Yes
innodb_tmpdir Yes Yes Yes Both Yes
Innodb_truncated_status_writes Yes Global No
innodb_trx_purge_view_update_only_debug Yes Yes Yes Global Yes
innodb_trx_rseg_n_slots_debug Yes Yes Yes Global Yes
innodb_undo_directory Yes Yes Yes Global No
innodb_undo_log_truncate Yes Yes Yes Global Yes
innodb_undo_logs Yes Yes Yes Global Yes
innodb_undo_tablespaces Yes Yes Yes Global No
innodb_use_native_aio Yes Yes Yes Global No
innodb_version Yes Global No
innodb_write_io_threads Yes Yes Yes Global No
insert_id Yes Session Yes
install Yes
install-manual Yes
interactive_timeout Yes Yes Yes Both Yes
internal_tmp_disk_storage_engine Yes Yes Yes Global Yes
join_buffer_size Yes Yes Yes Both Yes
keep_files_on_create Yes Yes Yes Both Yes
Key_blocks_not_flushed Yes Global No
Key_blocks_unused Yes Global No
Key_blocks_used Yes Global No
key_buffer_size Yes Yes Yes Global Yes
key_cache_age_threshold Yes Yes Yes Global Yes
key_cache_block_size Yes Yes Yes Global Yes
key_cache_division_limit Yes Yes Yes Global Yes
Key_read_requests Yes Global No
Key_reads Yes Global No
Key_write_requests Yes Global No
Key_writes Yes Global No
keyring_aws_cmk_id Yes Yes Yes Global Yes
keyring_aws_conf_file Yes Yes Yes Global No
keyring_aws_data_file Yes Yes Yes Global No
keyring_aws_region Yes Yes Yes Global Yes
keyring_encrypted_file_data Yes Yes Yes Global Yes
keyring_encrypted_file_password Yes Yes Yes Global Yes
keyring_file_data Yes Yes Yes Global Yes
keyring-migration-destination Yes Yes
keyring-migration-host Yes Yes
keyring-migration-password Yes Yes
keyring-migration-port Yes Yes
keyring-migration-socket Yes Yes
keyring-migration-source Yes Yes
keyring-migration-user Yes Yes
keyring_okv_conf_dir Yes Yes Yes Global Yes
keyring_operations Yes Global Yes
language Yes Yes Yes Global No
large_files_support Yes Global No
large_page_size Yes Global No
large_pages Yes Yes Yes Global No
last_insert_id Yes Session Yes
Last_query_cost Yes Session No
Last_query_partial_plans Yes Session No
lc_messages Yes Yes Yes Both Yes
lc_messages_dir Yes Yes Yes Global No
lc_time_names Yes Yes Yes Both Yes
license Yes Global No
local_infile Yes Yes Yes Global Yes
local-service Yes
lock_wait_timeout Yes Yes Yes Both Yes
Locked_connects Yes Global No
locked_in_memory Yes Global No
log-bin Yes Yes
log_bin Yes Global No
log_bin_basename Yes Global No
log_bin_index Yes Yes Yes Global No
log_bin_trust_function_creators Yes Yes Yes Global Yes
log_bin_use_v1_row_events Yes Yes Yes Global No
log_builtin_as_identified_by_password Yes Yes Yes Global Yes
log_error Yes Yes Yes Global No
log_error_verbosity Yes Yes Yes Global Yes
log-isam Yes Yes
log_output Yes Yes Yes Global Yes
log_queries_not_using_indexes Yes Yes Yes Global Yes
log-raw Yes Yes
log-short-format Yes Yes
log_slave_updates Yes Yes Yes Global No
log_slow_admin_statements Yes Yes Yes Global Yes
log_slow_slave_statements Yes Yes Yes Global Yes
log_statements_unsafe_for_binlog Yes Yes Yes Global Yes
log_syslog Yes Yes Yes Global Yes
log_syslog_facility Yes Yes Yes Global Yes
log_syslog_include_pid Yes Yes Yes Global Yes
log_syslog_tag Yes Yes Yes Global Yes
log-tc Yes Yes
log-tc-size Yes Yes
log_throttle_queries_not_using_indexes Yes Yes Yes Global Yes
log_timestamps Yes Yes Yes Global Yes
log_warnings Yes Yes Yes Global Yes
long_query_time Yes Yes Yes Both Yes
low_priority_updates Yes Yes Yes Both Yes
lower_case_file_system Yes Global No
lower_case_table_names Yes Yes Yes Global No
master-info-file Yes Yes
master_info_repository Yes Yes Yes Global Yes
master-retry-count Yes Yes
master_verify_checksum Yes Yes Yes Global Yes
max_allowed_packet Yes Yes Yes Both Yes
max_binlog_cache_size Yes Yes Yes Global Yes
max-binlog-dump-events Yes Yes
max_binlog_size Yes Yes Yes Global Yes
max_binlog_stmt_cache_size Yes Yes Yes Global Yes
max_connect_errors Yes Yes Yes Global Yes
max_connections Yes Yes Yes Global Yes
max_delayed_threads Yes Yes Yes Both Yes
max_digest_length Yes Yes Yes Global No
max_error_count Yes Yes Yes Both Yes
max_execution_time Yes Yes Yes Both Yes
Max_execution_time_exceeded Yes Both No
Max_execution_time_set Yes Both No
Max_execution_time_set_failed Yes Both No
max_heap_table_size Yes Yes Yes Both Yes
max_insert_delayed_threads Yes Both Yes
max_join_size Yes Yes Yes Both Yes
max_length_for_sort_data Yes Yes Yes Both Yes
max_points_in_geometry Yes Yes Yes Both Yes
max_prepared_stmt_count Yes Yes Yes Global Yes
max_relay_log_size Yes Yes Yes Global Yes
max_seeks_for_key Yes Yes Yes Both Yes
max_sort_length Yes Yes Yes Both Yes
max_sp_recursion_depth Yes Yes Yes Both Yes
max_tmp_tables Yes Both Yes
Max_used_connections Yes Global No
Max_used_connections_time Yes Global No
max_user_connections Yes Yes Yes Both Yes
max_write_lock_count Yes Yes Yes Global Yes
mecab_charset Yes Global No
mecab_rc_file Yes Yes Yes Global No
memlock Yes Yes
- Variable: locked_in_memory
metadata_locks_cache_size Yes Yes Yes Global No
metadata_locks_hash_instances Yes Yes Yes Global No
min_examined_row_limit Yes Yes Yes Both Yes
multi_range_count Yes Yes Yes Both Yes
myisam-block-size Yes Yes
myisam_data_pointer_size Yes Yes Yes Global Yes
myisam_max_sort_file_size Yes Yes Yes Global Yes
myisam_mmap_size Yes Yes Yes Global No
myisam_recover_options Yes Yes Yes Global No
myisam_repair_threads Yes Yes Yes Both Yes
myisam_sort_buffer_size Yes Yes Yes Both Yes
myisam_stats_method Yes Yes Yes Both Yes
myisam_use_mmap Yes Yes Yes Global Yes
mysql_firewall_mode Yes Yes Yes Global Yes
mysql_firewall_trace Yes Yes Yes Global Yes
mysql_native_password_proxy_users Yes Yes Yes Global Yes
mysqlx Yes Yes
Mysqlx_address Yes Global No
mysqlx_bind_address Yes Yes Yes Global No
Mysqlx_bytes_received Yes Both No
Mysqlx_bytes_sent Yes Both No
mysqlx_connect_timeout Yes Yes Yes Global Yes
Mysqlx_connection_accept_errors Yes Both No
Mysqlx_connection_errors Yes Both No
Mysqlx_connections_accepted Yes Global No
Mysqlx_connections_closed Yes Global No
Mysqlx_connections_rejected Yes Global No
Mysqlx_crud_create_view Yes Both No
Mysqlx_crud_delete Yes Both No
Mysqlx_crud_drop_view Yes Both No
Mysqlx_crud_find Yes Both No
Mysqlx_crud_insert Yes Both No
Mysqlx_crud_modify_view Yes Both No
Mysqlx_crud_update Yes Both No
Mysqlx_errors_sent Yes Both No
Mysqlx_errors_unknown_message_type Yes Both No
Mysqlx_expect_close Yes Both No
Mysqlx_expect_open Yes Both No
mysqlx_idle_worker_thread_timeout Yes Yes Yes Global Yes
Mysqlx_init_error Yes Both No
mysqlx_max_allowed_packet Yes Yes Yes Global Yes
mysqlx_max_connections Yes Yes Yes Global Yes
mysqlx_min_worker_threads Yes Yes Yes Global Yes
Mysqlx_notice_other_sent Yes Both No
Mysqlx_notice_warning_sent Yes Both No
Mysqlx_port Yes Global No
mysqlx_port Yes Yes Yes Global No
mysqlx_port_open_timeout Yes Yes Yes Global No
Mysqlx_rows_sent Yes Both No
Mysqlx_sessions Yes Global No
Mysqlx_sessions_accepted Yes Global No
Mysqlx_sessions_closed Yes Global No
Mysqlx_sessions_fatal_error Yes Global No
Mysqlx_sessions_killed Yes Global No
Mysqlx_sessions_rejected Yes Global No
Mysqlx_socket Yes Global No
mysqlx_socket Yes Yes Yes Global No
Mysqlx_ssl_accept_renegotiates Yes Global No
Mysqlx_ssl_accepts Yes Global No
Mysqlx_ssl_active Yes Both No
mysqlx_ssl_ca Yes Yes Yes Global No
mysqlx_ssl_capath Yes Yes Yes Global No
mysqlx_ssl_cert Yes Yes Yes Global No
Mysqlx_ssl_cipher Yes Both No
mysqlx_ssl_cipher Yes Yes Yes Global No
Mysqlx_ssl_cipher_list Yes Both No
mysqlx_ssl_crl Yes Yes Yes Global No
mysqlx_ssl_crlpath Yes Yes Yes Global No
Mysqlx_ssl_ctx_verify_depth Yes Both No
Mysqlx_ssl_ctx_verify_mode Yes Both No
Mysqlx_ssl_finished_accepts Yes Global No
mysqlx_ssl_key Yes Yes Yes Global No
Mysqlx_ssl_server_not_after Yes Global No
Mysqlx_ssl_server_not_before Yes Global No
Mysqlx_ssl_verify_depth Yes Global No
Mysqlx_ssl_verify_mode Yes Global No
Mysqlx_ssl_version Yes Both No
Mysqlx_stmt_create_collection Yes Both No
Mysqlx_stmt_create_collection_index Yes Both No
Mysqlx_stmt_disable_notices Yes Both No
Mysqlx_stmt_drop_collection Yes Both No
Mysqlx_stmt_drop_collection_index Yes Both No
Mysqlx_stmt_enable_notices Yes Both No
Mysqlx_stmt_ensure_collection Yes Both No
Mysqlx_stmt_execute_mysqlx Yes Both No
Mysqlx_stmt_execute_sql Yes Both No
Mysqlx_stmt_execute_xplugin Yes Both No
Mysqlx_stmt_kill_client Yes Both No
Mysqlx_stmt_list_clients Yes Both No
Mysqlx_stmt_list_notices Yes Both No
Mysqlx_stmt_list_objects Yes Both No
Mysqlx_stmt_ping Yes Both No
Mysqlx_worker_threads Yes Global No
Mysqlx_worker_threads_active Yes Global No
named_pipe Yes Yes Yes Global No
named_pipe_full_access_group Yes Yes Yes Global No
ndb_allow_copying_alter_table Yes Yes Yes Both Yes
Ndb_api_bytes_received_count Yes Global No
Ndb_api_bytes_received_count_session Yes Session No
Ndb_api_bytes_received_count_slave Yes Global No
Ndb_api_bytes_sent_count Yes Global No
Ndb_api_bytes_sent_count_session Yes Session No
Ndb_api_bytes_sent_count_slave Yes Global No
Ndb_api_event_bytes_count Yes Global No
Ndb_api_event_bytes_count_injector Yes Global No
Ndb_api_event_data_count Yes Global No
Ndb_api_event_data_count_injector Yes Global No
Ndb_api_event_nondata_count Yes Global No
Ndb_api_event_nondata_count_injector Yes Global No
Ndb_api_pk_op_count Yes Global No
Ndb_api_pk_op_count_session Yes Session No
Ndb_api_pk_op_count_slave Yes Global No
Ndb_api_pruned_scan_count Yes Global No
Ndb_api_pruned_scan_count_session Yes Session No
Ndb_api_pruned_scan_count_slave Yes Global No
Ndb_api_range_scan_count Yes Global No
Ndb_api_range_scan_count_session Yes Session No
Ndb_api_range_scan_count_slave Yes Global No
Ndb_api_read_row_count Yes Global No
Ndb_api_read_row_count_session Yes Session No
Ndb_api_read_row_count_slave Yes Global No
Ndb_api_scan_batch_count Yes Global No
Ndb_api_scan_batch_count_session Yes Session No
Ndb_api_scan_batch_count_slave Yes Global No
Ndb_api_table_scan_count Yes Global No
Ndb_api_table_scan_count_session Yes Session No
Ndb_api_table_scan_count_slave Yes Global No
Ndb_api_trans_abort_count Yes Global No
Ndb_api_trans_abort_count_session Yes Session No
Ndb_api_trans_abort_count_slave Yes Global No
Ndb_api_trans_close_count Yes Global No
Ndb_api_trans_close_count_session Yes Session No
Ndb_api_trans_close_count_slave Yes Global No
Ndb_api_trans_commit_count Yes Global No
Ndb_api_trans_commit_count_session Yes Session No
Ndb_api_trans_commit_count_slave Yes Global No
Ndb_api_trans_local_read_row_count Yes Global No
Ndb_api_trans_local_read_row_count_session Yes Session No
Ndb_api_trans_local_read_row_count_slave Yes Global No
Ndb_api_trans_start_count Yes Global No
Ndb_api_trans_start_count_session Yes Session No
Ndb_api_trans_start_count_slave Yes Global No
Ndb_api_uk_op_count Yes Global No
Ndb_api_uk_op_count_session Yes Session No
Ndb_api_uk_op_count_slave Yes Global No
Ndb_api_wait_exec_complete_count Yes Global No
Ndb_api_wait_exec_complete_count_session Yes Session No
Ndb_api_wait_exec_complete_count_slave Yes Global No
Ndb_api_wait_meta_request_count Yes Global No
Ndb_api_wait_meta_request_count_session Yes Session No
Ndb_api_wait_meta_request_count_slave Yes Global No
Ndb_api_wait_nanos_count Yes Global No
Ndb_api_wait_nanos_count_session Yes Session No
Ndb_api_wait_nanos_count_slave Yes Global No
Ndb_api_wait_scan_result_count Yes Global No
Ndb_api_wait_scan_result_count_session Yes Session No
Ndb_api_wait_scan_result_count_slave Yes Global No
ndb_autoincrement_prefetch_sz Yes Yes Yes Both Yes
ndb_batch_size Yes Yes Yes Global No
ndb_blob_read_batch_bytes Yes Yes Yes Both Yes
ndb_blob_write_batch_bytes Yes Yes Yes Both Yes
ndb_cache_check_time Yes Yes Yes Global Yes
ndb_clear_apply_status Yes Yes Global Yes
ndb_cluster_connection_pool Yes Yes Yes Global No
ndb_cluster_connection_pool_nodeids Yes Yes Yes Global No
Ndb_cluster_node_id Yes Global No
Ndb_config_from_host Yes Both No
Ndb_config_from_port Yes Both No
Ndb_conflict_fn_epoch Yes Global No
Ndb_conflict_fn_epoch_trans Yes Global No
Ndb_conflict_fn_epoch2 Yes Global No
Ndb_conflict_fn_epoch2_trans Yes Global No
Ndb_conflict_fn_max Yes Global No
Ndb_conflict_fn_old Yes Global No
Ndb_conflict_last_conflict_epoch Yes Global No
Ndb_conflict_last_stable_epoch Yes Global No
Ndb_conflict_reflected_op_discard_count Yes Global No
Ndb_conflict_reflected_op_prepare_count Yes Global No
Ndb_conflict_refresh_op_count Yes Global No
Ndb_conflict_trans_conflict_commit_count Yes Global No
Ndb_conflict_trans_detect_iter_count Yes Global No
Ndb_conflict_trans_reject_count Yes Global No
Ndb_conflict_trans_row_conflict_count Yes Global No
Ndb_conflict_trans_row_reject_count Yes Global No
ndb-connectstring Yes Yes
ndb_data_node_neighbour Yes Yes Yes Global Yes
ndb_default_column_format Yes Yes Yes Global Yes
ndb_default_column_format Yes Yes Yes Global Yes
ndb_deferred_constraints Yes Yes Yes Both Yes
ndb_deferred_constraints Yes Yes Yes Both Yes
ndb_distribution Yes Yes Yes Global Yes
ndb_distribution Yes Yes Yes Global Yes
Ndb_epoch_delete_delete_count Yes Global No
ndb_eventbuffer_free_percent Yes Yes Yes Global Yes
ndb_eventbuffer_max_alloc Yes Yes Yes Global Yes
Ndb_execute_count Yes Global No
ndb_extra_logging Yes Yes Yes Global Yes
ndb_force_send Yes Yes Yes Both Yes
ndb_fully_replicated Yes Yes Yes Both Yes
ndb_index_stat_enable Yes Yes Yes Both Yes
ndb_index_stat_option Yes Yes Yes Both Yes
ndb_join_pushdown Yes Both Yes
Ndb_last_commit_epoch_server Yes Global No
Ndb_last_commit_epoch_session Yes Session No
ndb_log_apply_status Yes Yes Yes Global No
ndb_log_apply_status Yes Yes Yes Global No
ndb_log_bin Yes Yes Both Yes
ndb_log_binlog_index Yes Yes Global Yes
ndb_log_empty_epochs Yes Yes Yes Global Yes
ndb_log_empty_epochs Yes Yes Yes Global Yes
ndb_log_empty_update Yes Yes Yes Global Yes
ndb_log_empty_update Yes Yes Yes Global Yes
ndb_log_exclusive_reads Yes Yes Yes Both Yes
ndb_log_exclusive_reads Yes Yes Yes Both Yes
ndb_log_orig Yes Yes Yes Global No
ndb_log_orig Yes Yes Yes Global No
ndb_log_transaction_id Yes Yes Yes Global No
ndb_log_transaction_id Yes Global No
ndb_log_update_as_write Yes Yes Yes Global Yes
ndb_log_update_minimal Yes Yes Yes Global Yes
ndb_log_updated_only Yes Yes Yes Global Yes
ndb-mgmd-host Yes Yes
ndb_nodeid Yes Yes Yes Global No
Ndb_number_of_data_nodes Yes Global No
ndb_optimization_delay Yes Yes Yes Global Yes
ndb_optimized_node_selection Yes Yes Yes Global No
Ndb_pruned_scan_count Yes Global No
Ndb_pushed_queries_defined Yes Global No
Ndb_pushed_queries_dropped Yes Global No
Ndb_pushed_queries_executed Yes Global No
Ndb_pushed_reads Yes Global No
ndb_read_backup Yes Yes Yes Global Yes
ndb_recv_thread_activation_threshold Yes Yes Yes Global Yes
ndb_recv_thread_cpu_mask Yes Yes Yes Global Yes
ndb_report_thresh_binlog_epoch_slip Yes Yes Yes Global Yes
ndb_report_thresh_binlog_mem_usage Yes Yes Yes Global Yes
ndb_row_checksum Yes Both Yes
Ndb_scan_count Yes Global No
ndb_show_foreign_key_mock_tables Yes Yes Yes Global Yes
ndb_slave_conflict_role Yes Yes Yes Global Yes
Ndb_slave_max_replicated_epoch Yes Global No
Ndb_system_name Yes Global No
ndb_table_no_logging Yes Session Yes
ndb_table_temporary Yes Session Yes
ndb-transid-mysql-connection-map Yes
ndb_use_copying_alter_table Yes Both No
ndb_use_exact_count Yes Both Yes
ndb_use_transactions Yes Yes Yes Both Yes
ndb_version Yes Global No
ndb_version_string Yes Global No
ndb_wait_connected Yes Yes Yes Global No
ndb_wait_setup Yes Yes Yes Global No
ndbcluster Yes Yes
ndbinfo_database Yes Global No
ndbinfo_max_bytes Yes Yes Both Yes
ndbinfo_max_rows Yes Yes Both Yes
ndbinfo_offline Yes Global Yes
ndbinfo_show_hidden Yes Yes Both Yes
ndbinfo_table_prefix Yes Yes Both Yes
ndbinfo_version Yes Global No
net_buffer_length Yes Yes Yes Both Yes
net_read_timeout Yes Yes Yes Both Yes
net_retry_count Yes Yes Yes Both Yes
net_write_timeout Yes Yes Yes Both Yes
new Yes Yes Yes Both Yes
ngram_token_size Yes Yes Yes Global No
no-defaults Yes
Not_flushed_delayed_rows Yes Global No
offline_mode Yes Yes Yes Global Yes
old Yes Yes Yes Global No
old_alter_table Yes Yes Yes Both Yes
old_passwords Yes Yes Yes Both Yes
old-style-user-limits Yes Yes
Ongoing_anonymous_gtid_violating_transaction_count Yes Global No
Ongoing_anonymous_transaction_count Yes Global No
Ongoing_automatic_gtid_violating_transaction_count Yes Global No
Open_files Yes Global No
open_files_limit Yes Yes Yes Global No
Open_streams Yes Global No
Open_table_definitions Yes Global No
Open_tables Yes Both No
Opened_files Yes Global No
Opened_table_definitions Yes Both No
Opened_tables Yes Both No
optimizer_prune_level Yes Yes Yes Both Yes
optimizer_search_depth Yes Yes Yes Both Yes
optimizer_switch Yes Yes Yes Both Yes
optimizer_trace Yes Yes Yes Both Yes
optimizer_trace_features Yes Yes Yes Both Yes
optimizer_trace_limit Yes Yes Yes Both Yes
optimizer_trace_max_mem_size Yes Yes Yes Both Yes
optimizer_trace_offset Yes Yes Yes Both Yes
parser_max_mem_size Yes Yes Yes Both Yes
partition Yes Yes
performance_schema Yes Yes Yes Global No
Performance_schema_accounts_lost Yes Global No
performance_schema_accounts_size Yes Yes Yes Global No
Performance_schema_cond_classes_lost Yes Global No
Performance_schema_cond_instances_lost Yes Global No
performance-schema-consumer-events-stages-current Yes Yes
performance-schema-consumer-events-stages-history Yes Yes
performance-schema-consumer-events-stages-history-long Yes Yes
performance-schema-consumer-events-statements-current Yes Yes
performance-schema-consumer-events-statements-history Yes Yes
performance-schema-consumer-events-statements-history-long Yes Yes
performance-schema-consumer-events-transactions-current Yes Yes
performance-schema-consumer-events-transactions-history Yes Yes
performance-schema-consumer-events-transactions-history-long Yes Yes
performance-schema-consumer-events-waits-current Yes Yes
performance-schema-consumer-events-waits-history Yes Yes
performance-schema-consumer-events-waits-history-long Yes Yes
performance-schema-consumer-global-instrumentation Yes Yes
performance-schema-consumer-statements-digest Yes Yes
performance-schema-consumer-thread-instrumentation Yes Yes
Performance_schema_digest_lost Yes Global No
performance_schema_digests_size Yes Yes Yes Global No
performance_schema_events_stages_history_long_size Yes Yes Yes Global No
performance_schema_events_stages_history_size Yes Yes Yes Global No
performance_schema_events_statements_history_long_size Yes Yes Yes Global No
performance_schema_events_statements_history_size Yes Yes Yes Global No
performance_schema_events_transactions_history_long_size Yes Yes Yes Global No
performance_schema_events_transactions_history_size Yes Yes Yes Global No
performance_schema_events_waits_history_long_size Yes Yes Yes Global No
performance_schema_events_waits_history_size Yes Yes Yes Global No
Performance_schema_file_classes_lost Yes Global No
Performance_schema_file_handles_lost Yes Global No
Performance_schema_file_instances_lost Yes Global No
Performance_schema_hosts_lost Yes Global No
performance_schema_hosts_size Yes Yes Yes Global No
Performance_schema_index_stat_lost Yes Global No
performance-schema-instrument Yes Yes
Performance_schema_locker_lost Yes Global No
performance_schema_max_cond_classes Yes Yes Yes Global No
performance_schema_max_cond_instances Yes Yes Yes Global No
performance_schema_max_digest_length Yes Yes Yes Global No
performance_schema_max_file_classes Yes Yes Yes Global No
performance_schema_max_file_handles Yes Yes Yes Global No
performance_schema_max_file_instances Yes Yes Yes Global No
performance_schema_max_index_stat Yes Yes Yes Global No
performance_schema_max_memory_classes Yes Yes Yes Global No
performance_schema_max_metadata_locks Yes Yes Yes Global No
performance_schema_max_mutex_classes Yes Yes Yes Global No
performance_schema_max_mutex_instances Yes Yes Yes Global No
performance_schema_max_prepared_statements_instances Yes Yes Yes Global No
performance_schema_max_program_instances Yes Yes Yes Global No
performance_schema_max_rwlock_classes Yes Yes Yes Global No
performance_schema_max_rwlock_instances Yes Yes Yes Global No
performance_schema_max_socket_classes Yes Yes Yes Global No
performance_schema_max_socket_instances Yes Yes Yes Global No
performance_schema_max_sql_text_length Yes Yes Yes Global No
performance_schema_max_stage_classes Yes Yes Yes Global No
performance_schema_max_statement_classes Yes Yes Yes Global No
performance_schema_max_statement_stack Yes Yes Yes Global No
performance_schema_max_table_handles Yes Yes Yes Global No
performance_schema_max_table_instances Yes Yes Yes Global No
performance_schema_max_table_lock_stat Yes Yes Yes Global No
performance_schema_max_thread_classes Yes Yes Yes Global No
performance_schema_max_thread_instances Yes Yes Yes Global No
Performance_schema_memory_classes_lost Yes Global No
Performance_schema_metadata_lock_lost Yes Global No
Performance_schema_mutex_classes_lost Yes Global No
Performance_schema_mutex_instances_lost Yes Global No
Performance_schema_nested_statement_lost Yes Global No
Performance_schema_prepared_statements_lost Yes Global No
Performance_schema_program_lost Yes Global No
Performance_schema_rwlock_classes_lost Yes Global No
Performance_schema_rwlock_instances_lost Yes Global No
Performance_schema_session_connect_attrs_lost Yes Global No
performance_schema_session_connect_attrs_size Yes Yes Yes Global No
performance_schema_setup_actors_size Yes Yes Yes Global No
performance_schema_setup_objects_size Yes Yes Yes Global No
Performance_schema_socket_classes_lost Yes Global No
Performance_schema_socket_instances_lost Yes Global No
Performance_schema_stage_classes_lost Yes Global No
Performance_schema_statement_classes_lost Yes Global No
Performance_schema_table_handles_lost Yes Global No
Performance_schema_table_instances_lost Yes Global No
Performance_schema_table_lock_stat_lost Yes Global No
Performance_schema_thread_classes_lost Yes Global No
Performance_schema_thread_instances_lost Yes Global No
Performance_schema_users_lost Yes Global No
performance_schema_users_size Yes Yes Yes Global No
pid_file Yes Yes Yes Global No
plugin_dir Yes Yes Yes Global No
plugin_load Yes Yes Yes Global No
plugin_load_add Yes Yes Yes Global No
plugin-xxx Yes Yes
port Yes Yes Yes Global No
port-open-timeout Yes Yes
preload_buffer_size Yes Yes Yes Both Yes
Prepared_stmt_count Yes Global No
print-defaults Yes
profiling Yes Both Yes
profiling_history_size Yes Yes Yes Both Yes
protocol_version Yes Global No
proxy_user Yes Session No
pseudo_slave_mode Yes Session Yes
pseudo_thread_id Yes Session Yes
Qcache_free_blocks Yes Global No
Qcache_free_memory Yes Global No
Qcache_hits Yes Global No
Qcache_inserts Yes Global No
Qcache_lowmem_prunes Yes Global No
Qcache_not_cached Yes Global No
Qcache_queries_in_cache Yes Global No
Qcache_total_blocks Yes Global No
Queries Yes Both No
query_alloc_block_size Yes Yes Yes Both Yes
query_cache_limit Yes Yes Yes Global Yes
query_cache_min_res_unit Yes Yes Yes Global Yes
query_cache_size Yes Yes Yes Global Yes
query_cache_type Yes Yes Yes Both Yes
query_cache_wlock_invalidate Yes Yes Yes Both Yes
query_prealloc_size Yes Yes Yes Both Yes
Questions Yes Both No
rand_seed1 Yes Session Yes
rand_seed2 Yes Session Yes
range_alloc_block_size Yes Yes Yes Both Yes
range_optimizer_max_mem_size Yes Yes Yes Both Yes
rbr_exec_mode Yes Both Yes
read_buffer_size Yes Yes Yes Both Yes
read_only Yes Yes Yes Global Yes
read_rnd_buffer_size Yes Yes Yes Both Yes
relay_log Yes Yes Yes Global No
relay_log_basename Yes Global No
relay_log_index Yes Yes Yes Global No
relay_log_info_file Yes Yes Yes Global No
relay_log_info_repository Yes Yes Yes Global Yes
relay_log_purge Yes Yes Yes Global Yes
relay_log_recovery Yes Yes Yes Global No
relay_log_space_limit Yes Yes Yes Global No
remove Yes
replicate-do-db Yes Yes
replicate-do-table Yes Yes
replicate-ignore-db Yes Yes
replicate-ignore-table Yes Yes
replicate-rewrite-db Yes Yes
replicate-same-server-id Yes Yes
replicate-wild-do-table Yes Yes
replicate-wild-ignore-table Yes Yes
report_host Yes Yes Yes Global No
report_password Yes Yes Yes Global No
report_port Yes Yes Yes Global No
report_user Yes Yes Yes Global No
require_secure_transport Yes Yes Yes Global Yes
rewriter_enabled Yes Global Yes
Rewriter_number_loaded_rules Yes Global No
Rewriter_number_reloads Yes Global No
Rewriter_number_rewritten_queries Yes Global No
Rewriter_reload_error Yes Global No
rewriter_verbose Yes Global Yes
Rpl_semi_sync_master_clients Yes Global No
rpl_semi_sync_master_enabled Yes Yes Yes Global Yes
Rpl_semi_sync_master_net_avg_wait_time Yes Global No
Rpl_semi_sync_master_net_wait_time Yes Global No
Rpl_semi_sync_master_net_waits Yes Global No
Rpl_semi_sync_master_no_times Yes Global No
Rpl_semi_sync_master_no_tx Yes Global No
Rpl_semi_sync_master_status Yes Global No
Rpl_semi_sync_master_timefunc_failures Yes Global No
rpl_semi_sync_master_timeout Yes Yes Yes Global Yes
rpl_semi_sync_master_trace_level Yes Yes Yes Global Yes
Rpl_semi_sync_master_tx_avg_wait_time Yes Global No
Rpl_semi_sync_master_tx_wait_time Yes Global No
Rpl_semi_sync_master_tx_waits Yes Global No
rpl_semi_sync_master_wait_for_slave_count Yes Yes Yes Global Yes
rpl_semi_sync_master_wait_no_slave Yes Yes Yes Global Yes
rpl_semi_sync_master_wait_point Yes Yes Yes Global Yes
Rpl_semi_sync_master_wait_pos_backtraverse Yes Global No
Rpl_semi_sync_master_wait_sessions Yes Global No
Rpl_semi_sync_master_yes_tx Yes Global No
rpl_semi_sync_slave_enabled Yes Yes Yes Global Yes
Rpl_semi_sync_slave_status Yes Global No
rpl_semi_sync_slave_trace_level Yes Yes Yes Global Yes
rpl_stop_slave_timeout Yes Yes Yes Global Yes
Rsa_public_key Yes Global No
safe-user-create Yes Yes
secure_auth Yes Yes Yes Global Yes
secure_file_priv Yes Yes Yes Global No
Select_full_join Yes Both No
Select_full_range_join Yes Both No
Select_range Yes Both No
Select_range_check Yes Both No
Select_scan Yes Both No
server_id Yes Yes Yes Global Yes
server_id_bits Yes Yes Yes Global No
server_uuid Yes Global No
session_track_gtids Yes Yes Yes Both Yes
session_track_schema Yes Yes Yes Both Yes
session_track_state_change Yes Yes Yes Both Yes
session_track_system_variables Yes Yes Yes Both Yes
session_track_transaction_info Yes Yes Yes Both Yes
sha256_password_auto_generate_rsa_keys Yes Yes Yes Global No
sha256_password_private_key_path Yes Yes Yes Global No
sha256_password_proxy_users Yes Yes Yes Global Yes
sha256_password_public_key_path Yes Yes Yes Global No
shared_memory Yes Yes Yes Global No
shared_memory_base_name Yes Yes Yes Global No
show_compatibility_56 Yes Yes Yes Global Yes
show_create_table_verbosity Yes Yes Yes Both Yes
show_old_temporals Yes Yes Yes Both Yes
show-slave-auth-info Yes Yes
skip-character-set-client-handshake Yes Yes
skip_external_locking Yes Yes Yes Global No
skip-grant-tables Yes Yes
skip-host-cache Yes Yes
skip_name_resolve Yes Yes Yes Global No
skip-ndbcluster Yes Yes
skip_networking Yes Yes Yes Global No
skip-new Yes Yes
skip-partition Yes Yes
skip_show_database Yes Yes Yes Global No
skip-slave-start Yes Yes
skip-ssl Yes Yes
skip-stack-trace Yes Yes
slave_allow_batching Yes Yes Yes Global Yes
slave_checkpoint_group Yes Yes Yes Global Yes
slave_checkpoint_period Yes Yes Yes Global Yes
slave_compressed_protocol Yes Yes Yes Global Yes
slave_exec_mode Yes Yes Yes Global Yes
Slave_heartbeat_period Yes Global No
Slave_last_heartbeat Yes Global No
slave_load_tmpdir Yes Yes Yes Global No
slave_max_allowed_packet Yes Yes Yes Global Yes
slave_net_timeout Yes Yes Yes Global Yes
Slave_open_temp_tables Yes Global No
slave_parallel_type Yes Yes Yes Global Yes
slave_parallel_workers Yes Yes Yes Global Yes
slave_pending_jobs_size_max Yes Yes Yes Global Yes
slave_preserve_commit_order Yes Yes Yes Global Yes
Slave_received_heartbeats Yes Global No
Slave_retried_transactions Yes Global No
Slave_rows_last_search_algorithm_used Yes Global No
slave_rows_search_algorithms Yes Yes Yes Global Yes
Slave_running Yes Global No
slave_skip_errors Yes Yes Yes Global No
slave-sql-verify-checksum Yes Yes
slave_sql_verify_checksum Yes Yes Yes Global Yes
slave_transaction_retries Yes Yes Yes Global Yes
slave_type_conversions Yes Yes Yes Global No
Slow_launch_threads Yes Both No
slow_launch_time Yes Yes Yes Global Yes
Slow_queries Yes Both No
slow_query_log Yes Yes Yes Global Yes
slow_query_log_file Yes Yes Yes Global Yes
slow-start-timeout Yes Yes
socket Yes Yes Yes Global No
sort_buffer_size Yes Yes Yes Both Yes
Sort_merge_passes Yes Both No
Sort_range Yes Both No
Sort_rows Yes Both No
Sort_scan Yes Both No
sporadic-binlog-dump-fail Yes Yes
sql_auto_is_null Yes Both Yes
sql_big_selects Yes Both Yes
sql_buffer_result Yes Both Yes
sql_log_bin Yes Session Yes
sql_log_off Yes Both Yes
sql_mode Yes Yes Yes Both Yes
sql_notes Yes Both Yes
sql_quote_show_create Yes Both Yes
sql_safe_updates Yes Both Yes
sql_select_limit Yes Both Yes
sql_slave_skip_counter Yes Global Yes
sql_warnings Yes Both Yes
ssl Yes Yes
Ssl_accept_renegotiates Yes Global No
Ssl_accepts Yes Global No
ssl_ca Yes Yes Yes Global No
Ssl_callback_cache_hits Yes Global No
ssl_capath Yes Yes Yes Global No
ssl_cert Yes Yes Yes Global No
Ssl_cipher Yes Both No
ssl_cipher Yes Yes Yes Global No
Ssl_cipher_list Yes Both No
Ssl_client_connects Yes Global No
Ssl_connect_renegotiates Yes Global No
ssl_crl Yes Yes Yes Global No
ssl_crlpath Yes Yes Yes Global No
Ssl_ctx_verify_depth Yes Global No
Ssl_ctx_verify_mode Yes Global No
Ssl_default_timeout Yes Both No
Ssl_finished_accepts Yes Global No
Ssl_finished_connects Yes Global No
ssl_key Yes Yes Yes Global No
Ssl_server_not_after Yes Both No
Ssl_server_not_before Yes Both No
Ssl_session_cache_hits Yes Global No
Ssl_session_cache_misses Yes Global No
Ssl_session_cache_mode Yes Global No
Ssl_session_cache_overflows Yes Global No
Ssl_session_cache_size Yes Global No
Ssl_session_cache_timeouts Yes Global No
Ssl_sessions_reused Yes Both No
Ssl_used_session_cache_entries Yes Global No
Ssl_verify_depth Yes Both No
Ssl_verify_mode Yes Both No
Ssl_version Yes Both No
standalone Yes Yes
stored_program_cache Yes Yes Yes Global Yes
super-large-pages Yes Yes
super_read_only Yes Yes Yes Global Yes
symbolic-links Yes Yes
sync_binlog Yes Yes Yes Global Yes
sync_frm Yes Yes Yes Global Yes
sync_master_info Yes Yes Yes Global Yes
sync_relay_log Yes Yes Yes Global Yes
sync_relay_log_info Yes Yes Yes Global Yes
sysdate-is-now Yes Yes
system_time_zone Yes Global No
table_definition_cache Yes Yes Yes Global Yes
Table_locks_immediate Yes Global No
Table_locks_waited Yes Global No
table_open_cache Yes Yes Yes Global Yes
Table_open_cache_hits Yes Both No
table_open_cache_instances Yes Yes Yes Global No
Table_open_cache_misses Yes Both No
Table_open_cache_overflows Yes Both No
tc-heuristic-recover Yes Yes
Tc_log_max_pages_used Yes Global No
Tc_log_page_size Yes Global No
Tc_log_page_waits Yes Global No
temp-pool Yes Yes
thread_cache_size Yes Yes Yes Global Yes
thread_handling Yes Yes Yes Global No
thread_pool_algorithm Yes Yes Yes Global No
thread_pool_high_priority_connection Yes Yes Yes Both Yes
thread_pool_max_unused_threads Yes Yes Yes Global Yes
thread_pool_prio_kickup_timer Yes Yes Yes Both Yes
thread_pool_size Yes Yes Yes Global No
thread_pool_stall_limit Yes Yes Yes Global Yes
thread_stack Yes Yes Yes Global No
Threads_cached Yes Global No
Threads_connected Yes Global No
Threads_created Yes Global No
Threads_running Yes Global No
time_format Yes Global No
time_zone Yes Both Yes
timestamp Yes Session Yes
tls_version Yes Yes Yes Global No
tmp_table_size Yes Yes Yes Both Yes
tmpdir Yes Yes Yes Global No
transaction_alloc_block_size Yes Yes Yes Both Yes
transaction_allow_batching Yes Session Yes
transaction_isolation Yes Yes Both Yes
- Variable: tx_isolation Yes Both Yes
transaction_prealloc_size Yes Yes Yes Both Yes
transaction_read_only Yes Yes Both Yes
- Variable: tx_read_only Yes Both Yes
transaction_write_set_extraction Yes Yes Yes Both Yes
tx_isolation Yes Both Yes
tx_read_only Yes Both Yes
unique_checks Yes Both Yes
updatable_views_with_limit Yes Yes Yes Both Yes
Uptime Yes Global No
Uptime_since_flush_status Yes Global No
user Yes Yes
validate-password Yes Yes
validate_password_check_user_name Yes Yes Yes Global Yes
validate_password_dictionary_file Yes Yes Yes Global Varies
validate_password_dictionary_file_last_parsed Yes Global No
validate_password_dictionary_file_words_count Yes Global No
validate_password_length Yes Yes Yes Global Yes
validate_password_mixed_case_count Yes Yes Yes Global Yes
validate_password_number_count Yes Yes Yes Global Yes
validate_password_policy Yes Yes Yes Global Yes
validate_password_special_char_count Yes Yes Yes Global Yes
validate_user_plugins Yes Yes Yes Global No
verbose Yes Yes
version Yes Global No
version_comment Yes Global No
version_compile_machine Yes Global No
version_compile_os Yes Global No
version_tokens_session Yes Yes Yes Both Yes
version_tokens_session_number Yes Yes Yes Both No
wait_timeout Yes Yes Yes Both Yes
warning_count Yes Session No

Notes:

1. This option is dynamic, but only the server should set this information. You should not set the value of this variable manually.

5.1.4 Server System Variable Reference

The following table lists all system variables applicable within mysqld.

The table lists command-line options (Cmd-line), options valid in configuration files (Option file), server system variables (System Var), and status variables (Status var) in one unified list, with an indication of where each option or variable is valid. If a server option set on the command line or in an option file differs from the name of the corresponding system variable, the variable name is noted immediately below the corresponding option. The scope of the variable (Var Scope) is Global, Session, or both. Please see the corresponding item descriptions for details on setting and using the variables. Where appropriate, direct links to further information about the items are provided.

Table 5.2 System Variable Summary

Name Cmd-Line Option File System Var Var Scope Dynamic
audit_log_buffer_size Yes Yes Yes Global No
audit_log_compression Yes Yes Yes Global No
audit_log_connection_policy Yes Yes Yes Global Yes
audit_log_current_session Yes Both No
audit_log_encryption Yes Yes Yes Global No
audit_log_exclude_accounts Yes Yes Yes Global Yes
audit_log_file Yes Yes Yes Global No
audit_log_filter_id Yes Both No
audit_log_flush Yes Global Yes
audit_log_format Yes Yes Yes Global No
audit_log_include_accounts Yes Yes Yes Global Yes
audit_log_policy Yes Yes Yes Global No
audit_log_read_buffer_size Yes Yes Yes Varies Varies
audit_log_rotate_on_size Yes Yes Yes Global Yes
audit_log_statement_policy Yes Yes Yes Global Yes
audit_log_strategy Yes Yes Yes Global No
authentication_ldap_sasl_auth_method_name Yes Yes Yes Global Yes
authentication_ldap_sasl_bind_base_dn Yes Yes Yes Global Yes
authentication_ldap_sasl_bind_root_dn Yes Yes Yes Global Yes
authentication_ldap_sasl_bind_root_pwd Yes Yes Yes Global Yes
authentication_ldap_sasl_ca_path Yes Yes Yes Global Yes
authentication_ldap_sasl_group_search_attr Yes Yes Yes Global Yes
authentication_ldap_sasl_group_search_filter Yes Yes Yes Global Yes
authentication_ldap_sasl_init_pool_size Yes Yes Yes Global Yes
authentication_ldap_sasl_log_status Yes Yes Yes Global Yes
authentication_ldap_sasl_max_pool_size Yes Yes Yes Global Yes
authentication_ldap_sasl_server_host Yes Yes Yes Global Yes
authentication_ldap_sasl_server_port Yes Yes Yes Global Yes
authentication_ldap_sasl_tls Yes Yes Yes Global Yes
authentication_ldap_sasl_user_search_attr Yes Yes Yes Global Yes
authentication_ldap_simple_auth_method_name Yes Yes Yes Global Yes
authentication_ldap_simple_bind_base_dn Yes Yes Yes Global Yes
authentication_ldap_simple_bind_root_dn Yes Yes Yes Global Yes
authentication_ldap_simple_bind_root_pwd Yes Yes Yes Global Yes
authentication_ldap_simple_ca_path Yes Yes Yes Global Yes
authentication_ldap_simple_group_search_attr Yes Yes Yes Global Yes
authentication_ldap_simple_group_search_filter Yes Yes Yes Global Yes
authentication_ldap_simple_init_pool_size Yes Yes Yes Global Yes
authentication_ldap_simple_log_status Yes Yes Yes Global Yes
authentication_ldap_simple_max_pool_size Yes Yes Yes Global Yes
authentication_ldap_simple_server_host Yes Yes Yes Global Yes
authentication_ldap_simple_server_port Yes Yes Yes Global Yes
authentication_ldap_simple_tls Yes Yes Yes Global Yes
authentication_ldap_simple_user_search_attr Yes Yes Yes Global Yes
authentication_windows_log_level Yes Yes Yes Global No
authentication_windows_use_principal_name Yes Yes Yes Global No
auto_generate_certs Yes Yes Yes Global No
auto_increment_increment Yes Yes Yes Both Yes
auto_increment_offset Yes Yes Yes Both Yes
autocommit Yes Yes Yes Both Yes
automatic_sp_privileges Yes Yes Yes Global Yes
avoid_temporal_upgrade Yes Yes Yes Global Yes
back_log Yes Yes Yes Global No
basedir Yes Yes Yes Global No
big_tables Yes Yes Yes Both Yes
bind_address Yes Yes Yes Global No
binlog_cache_size Yes Yes Yes Global Yes
binlog_checksum Yes Yes Yes Global Yes
binlog_direct_non_transactional_updates Yes Yes Yes Both Yes
binlog_error_action Yes Yes Yes Global Yes
binlog_format Yes Yes Yes Both Yes
binlog_group_commit_sync_delay Yes Yes Yes Global Yes
binlog_group_commit_sync_no_delay_count Yes Yes Yes Global Yes
binlog_gtid_simple_recovery Yes Yes Yes Global No
binlog_max_flush_queue_time Yes Yes Yes Global Yes
binlog_order_commits Yes Yes Yes Global Yes
binlog_row_image Yes Yes Yes Both Yes
binlog_rows_query_log_events Yes Yes Yes Both Yes
binlog_stmt_cache_size Yes Yes Yes Global Yes
binlog_transaction_dependency_history_size Yes Yes Yes Global Yes
binlog_transaction_dependency_tracking Yes Yes Yes Global Yes
block_encryption_mode Yes Yes Yes Both Yes
bulk_insert_buffer_size Yes Yes Yes Both Yes
character_set_client Yes Both Yes
character_set_connection Yes Both Yes
character_set_database (note 1) Yes Both Yes
character_set_filesystem Yes Yes Yes Both Yes
character_set_results Yes Both Yes
character_set_server Yes Yes Yes Both Yes
character_set_system Yes Global No
character_sets_dir Yes Yes Yes Global No
check_proxy_users Yes Yes Yes Global Yes
collation_connection Yes Both Yes
collation_database (note 1) Yes Both Yes
collation_server Yes Yes Yes Both Yes
completion_type Yes Yes Yes Both Yes
concurrent_insert Yes Yes Yes Global Yes
connect_timeout Yes Yes Yes Global Yes
connection_control_failed_connections_threshold Yes Yes Yes Global Yes
connection_control_max_connection_delay Yes Yes Yes Global Yes
connection_control_min_connection_delay Yes Yes Yes Global Yes
core_file Yes Global No
daemon_memcached_enable_binlog Yes Yes Yes Global No
daemon_memcached_engine_lib_name Yes Yes Yes Global No
daemon_memcached_engine_lib_path Yes Yes Yes Global No
daemon_memcached_option Yes Yes Yes Global No
daemon_memcached_r_batch_size Yes Yes Yes Global No
daemon_memcached_w_batch_size Yes Yes Yes Global No
datadir Yes Yes Yes Global No
date_format Yes Global No
datetime_format Yes Global No
debug Yes Yes Yes Both Yes
debug_sync Yes Session Yes
default_authentication_plugin Yes Yes Yes Global No
default_password_lifetime Yes Yes Yes Global Yes
default_storage_engine Yes Yes Yes Both Yes
default_tmp_storage_engine Yes Yes Yes Both Yes
default_week_format Yes Yes Yes Both Yes
delay_key_write Yes Yes Yes Global Yes
delayed_insert_limit Yes Yes Yes Global Yes
delayed_insert_timeout Yes Yes Yes Global Yes
delayed_queue_size Yes Yes Yes Global Yes
disabled_storage_engines Yes Yes Yes Global No
disconnect_on_expired_password Yes Yes Yes Global No
div_precision_increment Yes Yes Yes Both Yes
end_markers_in_json Yes Yes Yes Both Yes
enforce_gtid_consistency Yes Yes Yes Global Varies
eq_range_index_dive_limit Yes Yes Yes Both Yes
error_count Yes Session No
event_scheduler Yes Yes Yes Global Yes
expire_logs_days Yes Yes Yes Global Yes
explicit_defaults_for_timestamp Yes Yes Yes Both Yes
external_user Yes Session No
flush Yes Yes Yes Global Yes
flush_time Yes Yes Yes Global Yes
foreign_key_checks Yes Both Yes
ft_boolean_syntax Yes Yes Yes Global Yes
ft_max_word_len Yes Yes Yes Global No
ft_min_word_len Yes Yes Yes Global No
ft_query_expansion_limit Yes Yes Yes Global No
ft_stopword_file Yes Yes Yes Global No
general_log Yes Yes Yes Global Yes
general_log_file Yes Yes Yes Global Yes
group_concat_max_len Yes Yes Yes Both Yes
group_replication_allow_local_disjoint_gtids_join Yes Yes Yes Global Yes
group_replication_allow_local_lower_version_join Yes Yes Yes Global Yes
group_replication_auto_increment_increment Yes Yes Yes Global Yes
group_replication_bootstrap_group Yes Yes Yes Global Yes
group_replication_components_stop_timeout Yes Yes Yes Global Yes
group_replication_compression_threshold Yes Yes Yes Global Yes
group_replication_enforce_update_everywhere_checks Yes Yes Yes Global Yes
group_replication_exit_state_action Yes Yes Yes Global Yes
group_replication_flow_control_applier_threshold Yes Yes Yes Global Yes
group_replication_flow_control_certifier_threshold Yes Yes Yes Global Yes
group_replication_flow_control_mode Yes Yes Yes Global Yes
group_replication_force_members Yes Yes Yes Global Yes
group_replication_group_name Yes Yes Yes Global Yes
group_replication_group_seeds Yes Yes Yes Global Yes
group_replication_gtid_assignment_block_size Yes Yes Yes Global Yes
group_replication_ip_whitelist Yes Yes Yes Global Yes
group_replication_local_address Yes Yes Yes Global Yes
group_replication_member_weight Yes Yes Yes Global Yes
group_replication_poll_spin_loops Yes Yes Yes Global Yes
group_replication_recovery_complete_at Yes Yes Yes Global Yes
group_replication_recovery_reconnect_interval Yes Yes Yes Global Yes
group_replication_recovery_retry_count Yes Yes Yes Global Yes
group_replication_recovery_ssl_ca Yes Yes Yes Global Yes
group_replication_recovery_ssl_capath Yes Yes Yes Global Yes
group_replication_recovery_ssl_cert Yes Yes Yes Global Yes
group_replication_recovery_ssl_cipher Yes Yes Yes Global Yes
group_replication_recovery_ssl_crl Yes Yes Yes Global Yes
group_replication_recovery_ssl_crlpath Yes Yes Yes Global Yes
group_replication_recovery_ssl_key Yes Yes Yes Global Yes
group_replication_recovery_ssl_verify_server_cert Yes Yes Yes Global Yes
group_replication_recovery_use_ssl Yes Yes Yes Global Yes
group_replication_single_primary_mode Yes Yes Yes Global Yes
group_replication_ssl_mode Yes Yes Yes Global Yes
group_replication_start_on_boot Yes Yes Yes Global Yes
group_replication_transaction_size_limit Yes Yes Yes Global Yes
group_replication_unreachable_majority_timeout Yes Yes Yes Global Yes
gtid_executed Yes Varies No
gtid_executed_compression_period Yes Yes Yes Global Yes
gtid_mode Yes Yes Yes Global Varies
gtid_next Yes Session Yes
gtid_owned Yes Both No
gtid_purged Yes Global Yes
have_compress Yes Global No
have_crypt Yes Global No
have_dynamic_loading Yes Global No
have_geometry Yes Global No
have_openssl Yes Global No
have_profiling Yes Global No
have_query_cache Yes Global No
have_rtree_keys Yes Global No
have_ssl Yes Global No
have_statement_timeout Yes Global No
have_symlink Yes Global No
host_cache_size Yes Yes Yes Global Yes
hostname Yes Global No
identity Yes Session Yes
ignore_builtin_innodb Yes Yes Yes Global No
ignore_db_dirs Yes Global No
init_connect Yes Yes Yes Global Yes
init_file Yes Yes Yes Global No
init_slave Yes Yes Yes Global Yes
innodb_adaptive_flushing Yes Yes Yes Global Yes
innodb_adaptive_flushing_lwm Yes Yes Yes Global Yes
innodb_adaptive_hash_index Yes Yes Yes Global Yes
innodb_adaptive_hash_index_parts Yes Yes Yes Global No
innodb_adaptive_max_sleep_delay Yes Yes Yes Global Yes
innodb_api_bk_commit_interval Yes Yes Yes Global Yes
innodb_api_disable_rowlock Yes Yes Yes Global No
innodb_api_enable_binlog Yes Yes Yes Global No
innodb_api_enable_mdl Yes Yes Yes Global No
innodb_api_trx_level Yes Yes Yes Global Yes
innodb_autoextend_increment Yes Yes Yes Global Yes
innodb_autoinc_lock_mode Yes Yes Yes Global No
innodb_background_drop_list_empty Yes Yes Yes Global Yes
innodb_buffer_pool_chunk_size Yes Yes Yes Global No
innodb_buffer_pool_dump_at_shutdown Yes Yes Yes Global Yes
innodb_buffer_pool_dump_now Yes Yes Yes Global Yes
innodb_buffer_pool_dump_pct Yes Yes Yes Global Yes
innodb_buffer_pool_filename Yes Yes Yes Global Yes
innodb_buffer_pool_instances Yes Yes Yes Global No
innodb_buffer_pool_load_abort Yes Yes Yes Global Yes
innodb_buffer_pool_load_at_startup Yes Yes Yes Global No
innodb_buffer_pool_load_now Yes Yes Yes Global Yes
innodb_buffer_pool_size Yes Yes Yes Global Varies
innodb_change_buffer_max_size Yes Yes Yes Global Yes
innodb_change_buffering Yes Yes Yes Global Yes
innodb_change_buffering_debug Yes Yes Yes Global Yes
innodb_checksum_algorithm Yes Yes Yes Global Yes
innodb_checksums Yes Yes Yes Global No
innodb_cmp_per_index_enabled Yes Yes Yes Global Yes
innodb_commit_concurrency Yes Yes Yes Global Yes
innodb_compress_debug Yes Yes Yes Global Yes
innodb_compression_failure_threshold_pct Yes Yes Yes Global Yes
innodb_compression_level Yes Yes Yes Global Yes
innodb_compression_pad_pct_max Yes Yes Yes Global Yes
innodb_concurrency_tickets Yes Yes Yes Global Yes
innodb_data_file_path Yes Yes Yes Global No
innodb_data_home_dir Yes Yes Yes Global No
innodb_deadlock_detect Yes Yes Yes Global Yes
innodb_default_row_format Yes Yes Yes Global Yes
innodb_disable_resize_buffer_pool_debug Yes Yes Yes Global Yes
innodb_disable_sort_file_cache Yes Yes Yes Global Yes
innodb_doublewrite Yes Yes Yes Global No
innodb_fast_shutdown Yes Yes Yes Global Yes
innodb_fil_make_page_dirty_debug Yes Yes Yes Global Yes
innodb_file_format Yes Yes Yes Global Yes
innodb_file_format_check Yes Yes Yes Global No
innodb_file_format_max Yes Yes Yes Global Yes
innodb_file_per_table Yes Yes Yes Global Yes
innodb_fill_factor Yes Yes Yes Global Yes
innodb_flush_log_at_timeout Yes Yes Yes Global Yes
innodb_flush_log_at_trx_commit Yes Yes Yes Global Yes
innodb_flush_method Yes Yes Yes Global No
innodb_flush_neighbors Yes Yes Yes Global Yes
innodb_flush_sync Yes Yes Yes Global Yes
innodb_flushing_avg_loops Yes Yes Yes Global Yes
innodb_force_load_corrupted Yes Yes Yes Global No
innodb_force_recovery Yes Yes Yes Global No
innodb_ft_aux_table Yes Global Yes
innodb_ft_cache_size Yes Yes Yes Global No
innodb_ft_enable_diag_print Yes Yes Yes Global Yes
innodb_ft_enable_stopword Yes Yes Yes Both Yes
innodb_ft_max_token_size Yes Yes Yes Global No
innodb_ft_min_token_size Yes Yes Yes Global No
innodb_ft_num_word_optimize Yes Yes Yes Global Yes
innodb_ft_result_cache_limit Yes Yes Yes Global Yes
innodb_ft_server_stopword_table Yes Yes Yes Global Yes
innodb_ft_sort_pll_degree Yes Yes Yes Global No
innodb_ft_total_cache_size Yes Yes Yes Global No
innodb_ft_user_stopword_table Yes Yes Yes Both Yes
innodb_io_capacity Yes Yes Yes Global Yes
innodb_io_capacity_max Yes Yes Yes Global Yes
innodb_large_prefix Yes Yes Yes Global Yes
innodb_limit_optimistic_insert_debug Yes Yes Yes Global Yes
innodb_lock_wait_timeout Yes Yes Yes Both Yes
innodb_locks_unsafe_for_binlog Yes Yes Yes Global No
innodb_log_buffer_size Yes Yes Yes Global No
innodb_log_checkpoint_now Yes Yes Yes Global Yes
innodb_log_checksums Yes Yes Yes Global Yes
innodb_log_compressed_pages Yes Yes Yes Global Yes
innodb_log_file_size Yes Yes Yes Global No
innodb_log_files_in_group Yes Yes Yes Global No
innodb_log_group_home_dir Yes Yes Yes Global No
innodb_log_write_ahead_size Yes Yes Yes Global Yes
innodb_lru_scan_depth Yes Yes Yes Global Yes
innodb_max_dirty_pages_pct Yes Yes Yes Global Yes
innodb_max_dirty_pages_pct_lwm Yes Yes Yes Global Yes
innodb_max_purge_lag Yes Yes Yes Global Yes
innodb_max_purge_lag_delay Yes Yes Yes Global Yes
innodb_max_undo_log_size Yes Yes Yes Global Yes
innodb_merge_threshold_set_all_debug Yes Yes Yes Global Yes
innodb_monitor_disable Yes Yes Yes Global Yes
innodb_monitor_enable Yes Yes Yes Global Yes
innodb_monitor_reset Yes Yes Yes Global Yes
innodb_monitor_reset_all Yes Yes Yes Global Yes
innodb_numa_interleave Yes Yes Yes Global No
innodb_old_blocks_pct Yes Yes Yes Global Yes
innodb_old_blocks_time Yes Yes Yes Global Yes
innodb_online_alter_log_max_size Yes Yes Yes Global Yes
innodb_open_files Yes Yes Yes Global No
innodb_optimize_fulltext_only Yes Yes Yes Global Yes
innodb_page_cleaners Yes Yes Yes Global No
innodb_page_size Yes Yes Yes Global No
innodb_print_all_deadlocks Yes Yes Yes Global Yes
innodb_purge_batch_size Yes Yes Yes Global Yes
innodb_purge_rseg_truncate_frequency Yes Yes Yes Global Yes
innodb_purge_threads Yes Yes Yes Global No
innodb_random_read_ahead Yes Yes Yes Global Yes
innodb_read_ahead_threshold Yes Yes Yes Global Yes
innodb_read_io_threads Yes Yes Yes Global No
innodb_read_only Yes Yes Yes Global No
innodb_replication_delay Yes Yes Yes Global Yes
innodb_rollback_on_timeout Yes Yes Yes Global No
innodb_rollback_segments Yes Yes Yes Global Yes
innodb_saved_page_number_debug Yes Yes Yes Global Yes
innodb_sort_buffer_size Yes Yes Yes Global No
innodb_spin_wait_delay Yes Yes Yes Global Yes
innodb_stats_auto_recalc Yes Yes Yes Global Yes
innodb_stats_include_delete_marked Yes Yes Yes Global Yes
innodb_stats_method Yes Yes Yes Global Yes
innodb_stats_on_metadata Yes Yes Yes Global Yes
innodb_stats_persistent Yes Yes Yes Global Yes
innodb_stats_persistent_sample_pages Yes Yes Yes Global Yes
innodb_stats_sample_pages Yes Yes Yes Global Yes
innodb_stats_transient_sample_pages Yes Yes Yes Global Yes
innodb_status_output Yes Yes Yes Global Yes
innodb_status_output_locks Yes Yes Yes Global Yes
innodb_strict_mode Yes Yes Yes Both Yes
innodb_support_xa Yes Yes Yes Both Yes
innodb_sync_array_size Yes Yes Yes Global No
innodb_sync_debug Yes Yes Yes Global No
innodb_sync_spin_loops Yes Yes Yes Global Yes
innodb_table_locks Yes Yes Yes Both Yes
innodb_temp_data_file_path Yes Yes Yes Global No
innodb_thread_concurrency Yes Yes Yes Global Yes
innodb_thread_sleep_delay Yes Yes Yes Global Yes
innodb_tmpdir Yes Yes Yes Both Yes
innodb_trx_purge_view_update_only_debug Yes Yes Yes Global Yes
innodb_trx_rseg_n_slots_debug Yes Yes Yes Global Yes
innodb_undo_directory Yes Yes Yes Global No
innodb_undo_log_truncate Yes Yes Yes Global Yes
innodb_undo_logs Yes Yes Yes Global Yes
innodb_undo_tablespaces Yes Yes Yes Global No
innodb_use_native_aio Yes Yes Yes Global No
innodb_version Yes Global No
innodb_write_io_threads Yes Yes Yes Global No
insert_id Yes Session Yes
interactive_timeout Yes Yes Yes Both Yes
internal_tmp_disk_storage_engine Yes Yes Yes Global Yes
join_buffer_size Yes Yes Yes Both Yes
keep_files_on_create Yes Yes Yes Both Yes
key_buffer_size Yes Yes Yes Global Yes
key_cache_age_threshold Yes Yes Yes Global Yes
key_cache_block_size Yes Yes Yes Global Yes
key_cache_division_limit Yes Yes Yes Global Yes
keyring_aws_cmk_id Yes Yes Yes Global Yes
keyring_aws_conf_file Yes Yes Yes Global No
keyring_aws_data_file Yes Yes Yes Global No
keyring_aws_region Yes Yes Yes Global Yes
keyring_encrypted_file_data Yes Yes Yes Global Yes
keyring_encrypted_file_password Yes Yes Yes Global Yes
keyring_file_data Yes Yes Yes Global Yes
keyring_okv_conf_dir Yes Yes Yes Global Yes
keyring_operations Yes Global Yes
language Yes Yes Yes Global No
large_files_support Yes Global No
large_page_size Yes Global No
large_pages Yes Yes Yes Global No
last_insert_id Yes Session Yes
lc_messages Yes Yes Yes Both Yes
lc_messages_dir Yes Yes Yes Global No
lc_time_names Yes Yes Yes Both Yes
license Yes Global No
local_infile Yes Yes Yes Global Yes
lock_wait_timeout Yes Yes Yes Both Yes
locked_in_memory Yes Global No
log_bin Yes Global No
log_bin_basename Yes Global No
log_bin_index Yes Yes Yes Global No
log_bin_trust_function_creators Yes Yes Yes Global Yes
log_bin_use_v1_row_events Yes Yes Yes Global No
log_builtin_as_identified_by_password Yes Yes Yes Global Yes
log_error Yes Yes Yes Global No
log_error_verbosity Yes Yes Yes Global Yes
log_output Yes Yes Yes Global Yes
log_queries_not_using_indexes Yes Yes Yes Global Yes
log_slave_updates Yes Yes Yes Global No
log_slow_admin_statements Yes Yes Yes Global Yes
log_slow_slave_statements Yes Yes Yes Global Yes
log_statements_unsafe_for_binlog Yes Yes Yes Global Yes
log_syslog Yes Yes Yes Global Yes
log_syslog_facility Yes Yes Yes Global Yes
log_syslog_include_pid Yes Yes Yes Global Yes
log_syslog_tag Yes Yes Yes Global Yes
log_throttle_queries_not_using_indexes Yes Yes Yes Global Yes
log_timestamps Yes Yes Yes Global Yes
log_warnings Yes Yes Yes Global Yes
long_query_time Yes Yes Yes Both Yes
low_priority_updates Yes Yes Yes Both Yes
lower_case_file_system Yes Global No
lower_case_table_names Yes Yes Yes Global No
master_info_repository Yes Yes Yes Global Yes
master_verify_checksum Yes Yes Yes Global Yes
max_allowed_packet Yes Yes Yes Both Yes
max_binlog_cache_size Yes Yes Yes Global Yes
max_binlog_size Yes Yes Yes Global Yes
max_binlog_stmt_cache_size Yes Yes Yes Global Yes
max_connect_errors Yes Yes Yes Global Yes
max_connections Yes Yes Yes Global Yes
max_delayed_threads Yes Yes Yes Both Yes
max_digest_length Yes Yes Yes Global No
max_error_count Yes Yes Yes Both Yes
max_execution_time Yes Yes Yes Both Yes
max_heap_table_size Yes Yes Yes Both Yes
max_insert_delayed_threads Yes Both Yes
max_join_size Yes Yes Yes Both Yes
max_length_for_sort_data Yes Yes Yes Both Yes
max_points_in_geometry Yes Yes Yes Both Yes
max_prepared_stmt_count Yes Yes Yes Global Yes
max_relay_log_size Yes Yes Yes Global Yes
max_seeks_for_key Yes Yes Yes Both Yes
max_sort_length Yes Yes Yes Both Yes
max_sp_recursion_depth Yes Yes Yes Both Yes
max_tmp_tables Yes Both Yes
max_user_connections Yes Yes Yes Both Yes
max_write_lock_count Yes Yes Yes Global Yes
mecab_rc_file Yes Yes Yes Global No
metadata_locks_cache_size Yes Yes Yes Global No
metadata_locks_hash_instances Yes Yes Yes Global No
min_examined_row_limit Yes Yes Yes Both Yes
multi_range_count Yes Yes Yes Both Yes
myisam_data_pointer_size Yes Yes Yes Global Yes
myisam_max_sort_file_size Yes Yes Yes Global Yes
myisam_mmap_size Yes Yes Yes Global No
myisam_recover_options Yes Yes Yes Global No
myisam_repair_threads Yes Yes Yes Both Yes
myisam_sort_buffer_size Yes Yes Yes Both Yes
myisam_stats_method Yes Yes Yes Both Yes
myisam_use_mmap Yes Yes Yes Global Yes
mysql_firewall_mode Yes Yes Yes Global Yes
mysql_firewall_trace Yes Yes Yes Global Yes
mysql_native_password_proxy_users Yes Yes Yes Global Yes
mysqlx_bind_address Yes Yes Yes Global No
mysqlx_connect_timeout Yes Yes Yes Global Yes
mysqlx_idle_worker_thread_timeout Yes Yes Yes Global Yes
mysqlx_max_allowed_packet Yes Yes Yes Global Yes
mysqlx_max_connections Yes Yes Yes Global Yes
mysqlx_min_worker_threads Yes Yes Yes Global Yes
mysqlx_port Yes Yes Yes Global No
mysqlx_port_open_timeout Yes Yes Yes Global No
mysqlx_socket Yes Yes Yes Global No
mysqlx_ssl_ca Yes Yes Yes Global No
mysqlx_ssl_capath Yes Yes Yes Global No
mysqlx_ssl_cert Yes Yes Yes Global No
mysqlx_ssl_cipher Yes Yes Yes Global No
mysqlx_ssl_crl Yes Yes Yes Global No
mysqlx_ssl_crlpath Yes Yes Yes Global No
mysqlx_ssl_key Yes Yes Yes Global No
named_pipe Yes Yes Yes Global No
named_pipe_full_access_group Yes Yes Yes Global No
ndb_allow_copying_alter_table Yes Yes Yes Both Yes
ndb_autoincrement_prefetch_sz Yes Yes Yes Both Yes
ndb_batch_size Yes Yes Yes Global No
ndb_blob_read_batch_bytes Yes Yes Yes Both Yes
ndb_blob_write_batch_bytes Yes Yes Yes Both Yes
ndb_cache_check_time Yes Yes Yes Global Yes
ndb_clear_apply_status Yes Yes Global Yes
ndb_cluster_connection_pool Yes Yes Yes Global No
ndb_cluster_connection_pool_nodeids Yes Yes Yes Global No
Ndb_conflict_last_conflict_epoch Yes Global No
ndb_data_node_neighbour Yes Yes Yes Global Yes
ndb_default_column_format Yes Yes Yes Global Yes
ndb_default_column_format Yes Yes Yes Global Yes
ndb_deferred_constraints Yes Yes Yes Both Yes
ndb_deferred_constraints Yes Yes Yes Both Yes
ndb_distribution Yes Yes Yes Global Yes
ndb_distribution Yes Yes Yes Global Yes
ndb_eventbuffer_free_percent Yes Yes Yes Global Yes
ndb_eventbuffer_max_alloc Yes Yes Yes Global Yes
ndb_extra_logging Yes Yes Yes Global Yes
ndb_force_send Yes Yes Yes Both Yes
ndb_fully_replicated Yes Yes Yes Both Yes
ndb_index_stat_enable Yes Yes Yes Both Yes
ndb_index_stat_option Yes Yes Yes Both Yes
ndb_join_pushdown Yes Both Yes
ndb_log_apply_status Yes Yes Yes Global No
ndb_log_apply_status Yes Yes Yes Global No
ndb_log_bin Yes Yes Both Yes
ndb_log_binlog_index Yes Yes Global Yes
ndb_log_empty_epochs Yes Yes Yes Global Yes
ndb_log_empty_epochs Yes Yes Yes Global Yes
ndb_log_empty_update Yes Yes Yes Global Yes
ndb_log_empty_update Yes Yes Yes Global Yes
ndb_log_exclusive_reads Yes Yes Yes Both Yes
ndb_log_exclusive_reads Yes Yes Yes Both Yes
ndb_log_orig Yes Yes Yes Global No
ndb_log_orig Yes Yes Yes Global No
ndb_log_transaction_id Yes Yes Yes Global No
ndb_log_transaction_id Yes Global No
ndb_log_update_as_write Yes Yes Yes Global Yes
ndb_log_update_minimal Yes Yes Yes Global Yes
ndb_log_updated_only Yes Yes Yes Global Yes
ndb_optimization_delay Yes Yes Yes Global Yes
ndb_optimized_node_selection Yes Yes Yes Global No
ndb_read_backup Yes Yes Yes Global Yes
ndb_recv_thread_activation_threshold Yes Yes Yes Global Yes
ndb_recv_thread_cpu_mask Yes Yes Yes Global Yes
ndb_report_thresh_binlog_epoch_slip Yes Yes Yes Global Yes
ndb_report_thresh_binlog_mem_usage Yes Yes Yes Global Yes
ndb_row_checksum Yes Both Yes
ndb_show_foreign_key_mock_tables Yes Yes Yes Global Yes
ndb_slave_conflict_role Yes Yes Yes Global Yes
Ndb_slave_max_replicated_epoch Yes Global No
Ndb_system_name Yes Global No
ndb_table_no_logging Yes Session Yes
ndb_table_temporary Yes Session Yes
ndb_use_copying_alter_table Yes Both No
ndb_use_exact_count Yes Both Yes
ndb_use_transactions Yes Yes Yes Both Yes
ndb_version Yes Global No
ndb_version_string Yes Global No
ndb_wait_connected Yes Yes Yes Global No
ndb_wait_setup Yes Yes Yes Global No
ndbinfo_database Yes Global No
ndbinfo_max_bytes Yes Yes Both Yes
ndbinfo_max_rows Yes Yes Both Yes
ndbinfo_offline Yes Global Yes
ndbinfo_show_hidden Yes Yes Both Yes
ndbinfo_table_prefix Yes Yes Both Yes
ndbinfo_version Yes Global No
net_buffer_length Yes Yes Yes Both Yes
net_read_timeout Yes Yes Yes Both Yes
net_retry_count Yes Yes Yes Both Yes
net_write_timeout Yes Yes Yes Both Yes
new Yes Yes Yes Both Yes
ngram_token_size Yes Yes Yes Global No
offline_mode Yes Yes Yes Global Yes
old Yes Yes Yes Global No
old_alter_table Yes Yes Yes Both Yes
old_passwords Yes Yes Yes Both Yes
open_files_limit Yes Yes Yes Global No
optimizer_prune_level Yes Yes Yes Both Yes
optimizer_search_depth Yes Yes Yes Both Yes
optimizer_switch Yes Yes Yes Both Yes
optimizer_trace Yes Yes Yes Both Yes
optimizer_trace_features Yes Yes Yes Both Yes
optimizer_trace_limit Yes Yes Yes Both Yes
optimizer_trace_max_mem_size Yes Yes Yes Both Yes
optimizer_trace_offset Yes Yes Yes Both Yes
parser_max_mem_size Yes Yes Yes Both Yes
performance_schema Yes Yes Yes Global No
performance_schema_accounts_size Yes Yes Yes Global No
performance_schema_digests_size Yes Yes Yes Global No
performance_schema_events_stages_history_long_size Yes Yes Yes Global No
performance_schema_events_stages_history_size Yes Yes Yes Global No
performance_schema_events_statements_history_long_size Yes Yes Yes Global No
performance_schema_events_statements_history_size Yes Yes Yes Global No
performance_schema_events_transactions_history_long_size Yes Yes Yes Global No
performance_schema_events_transactions_history_size Yes Yes Yes Global No
performance_schema_events_waits_history_long_size Yes Yes Yes Global No
performance_schema_events_waits_history_size Yes Yes Yes Global No
performance_schema_hosts_size Yes Yes Yes Global No
performance_schema_max_cond_classes Yes Yes Yes Global No
performance_schema_max_cond_instances Yes Yes Yes Global No
performance_schema_max_digest_length Yes Yes Yes Global No
performance_schema_max_file_classes Yes Yes Yes Global No
performance_schema_max_file_handles Yes Yes Yes Global No
performance_schema_max_file_instances Yes Yes Yes Global No
performance_schema_max_index_stat Yes Yes Yes Global No
performance_schema_max_memory_classes Yes Yes Yes Global No
performance_schema_max_metadata_locks Yes Yes Yes Global No
performance_schema_max_mutex_classes Yes Yes Yes Global No
performance_schema_max_mutex_instances Yes Yes Yes Global No
performance_schema_max_prepared_statements_instances Yes Yes Yes Global No
performance_schema_max_program_instances Yes Yes Yes Global No
performance_schema_max_rwlock_classes Yes Yes Yes Global No
performance_schema_max_rwlock_instances Yes Yes Yes Global No
performance_schema_max_socket_classes Yes Yes Yes Global No
performance_schema_max_socket_instances Yes Yes Yes Global No
performance_schema_max_sql_text_length Yes Yes Yes Global No
performance_schema_max_stage_classes Yes Yes Yes Global No
performance_schema_max_statement_classes Yes Yes Yes Global No
performance_schema_max_statement_stack Yes Yes Yes Global No
performance_schema_max_table_handles Yes Yes Yes Global No
performance_schema_max_table_instances Yes Yes Yes Global No
performance_schema_max_table_lock_stat Yes Yes Yes Global No
performance_schema_max_thread_classes Yes Yes Yes Global No
performance_schema_max_thread_instances Yes Yes Yes Global No
performance_schema_session_connect_attrs_size Yes Yes Yes Global No
performance_schema_setup_actors_size Yes Yes Yes Global No
performance_schema_setup_objects_size Yes Yes Yes Global No
performance_schema_users_size Yes Yes Yes Global No
pid_file Yes Yes Yes Global No
plugin_dir Yes Yes Yes Global No
plugin_load Yes Yes Yes Global No
plugin_load_add Yes Yes Yes Global No
port Yes Yes Yes Global No
preload_buffer_size Yes Yes Yes Both Yes
profiling Yes Both Yes
profiling_history_size Yes Yes Yes Both Yes
protocol_version Yes Global No
proxy_user Yes Session No
pseudo_slave_mode Yes Session Yes
pseudo_thread_id Yes Session Yes
query_alloc_block_size Yes Yes Yes Both Yes
query_cache_limit Yes Yes Yes Global Yes
query_cache_min_res_unit Yes Yes Yes Global Yes
query_cache_size Yes Yes Yes Global Yes
query_cache_type Yes Yes Yes Both Yes
query_cache_wlock_invalidate Yes Yes Yes Both Yes
query_prealloc_size Yes Yes Yes Both Yes
rand_seed1 Yes Session Yes
rand_seed2 Yes Session Yes
range_alloc_block_size Yes Yes Yes Both Yes
range_optimizer_max_mem_size Yes Yes Yes Both Yes
rbr_exec_mode Yes Both Yes
read_buffer_size Yes Yes Yes Both Yes
read_only Yes Yes Yes Global Yes
read_rnd_buffer_size Yes Yes Yes Both Yes
relay_log Yes Yes Yes Global No
relay_log_basename Yes Global No
relay_log_index Yes Yes Yes Global No
relay_log_info_file Yes Yes Yes Global No
relay_log_info_repository Yes Yes Yes Global Yes
relay_log_purge Yes Yes Yes Global Yes
relay_log_recovery Yes Yes Yes Global No
relay_log_space_limit Yes Yes Yes Global No
report_host Yes Yes Yes Global No
report_password Yes Yes Yes Global No
report_port Yes Yes Yes Global No
report_user Yes Yes Yes Global No
require_secure_transport Yes Yes Yes Global Yes
rewriter_enabled Yes Global Yes
rewriter_verbose Yes Global Yes
rpl_semi_sync_master_enabled Yes Yes Yes Global Yes
rpl_semi_sync_master_timeout Yes Yes Yes Global Yes
rpl_semi_sync_master_trace_level Yes Yes Yes Global Yes
rpl_semi_sync_master_wait_for_slave_count Yes Yes Yes Global Yes
rpl_semi_sync_master_wait_no_slave Yes Yes Yes Global Yes
rpl_semi_sync_master_wait_point Yes Yes Yes Global Yes
rpl_semi_sync_slave_enabled Yes Yes Yes Global Yes
rpl_semi_sync_slave_trace_level Yes Yes Yes Global Yes
rpl_stop_slave_timeout Yes Yes Yes Global Yes
secure_auth Yes Yes Yes Global Yes
secure_file_priv Yes Yes Yes Global No
server_id Yes Yes Yes Global Yes
server_id_bits Yes Yes Yes Global No
server_uuid Yes Global No
session_track_gtids Yes Yes Yes Both Yes
session_track_schema Yes Yes Yes Both Yes
session_track_state_change Yes Yes Yes Both Yes
session_track_system_variables Yes Yes Yes Both Yes
session_track_transaction_info Yes Yes Yes Both Yes
sha256_password_auto_generate_rsa_keys Yes Yes Yes Global No
sha256_password_private_key_path Yes Yes Yes Global No
sha256_password_proxy_users Yes Yes Yes Global Yes
sha256_password_public_key_path Yes Yes Yes Global No
shared_memory Yes Yes Yes Global No
shared_memory_base_name Yes Yes Yes Global No
show_compatibility_56 Yes Yes Yes Global Yes
show_create_table_verbosity Yes Yes Yes Both Yes
show_old_temporals Yes Yes Yes Both Yes
skip_external_locking Yes Yes Yes Global No
skip_name_resolve Yes Yes Yes Global No
skip_networking Yes Yes Yes Global No
skip_show_database Yes Yes Yes Global No
slave_allow_batching Yes Yes Yes Global Yes
slave_checkpoint_group Yes Yes Yes Global Yes
slave_checkpoint_period Yes Yes Yes Global Yes
slave_compressed_protocol Yes Yes Yes Global Yes
slave_exec_mode Yes Yes Yes Global Yes
slave_load_tmpdir Yes Yes Yes Global No
slave_max_allowed_packet Yes Yes Yes Global Yes
slave_net_timeout Yes Yes Yes Global Yes
slave_parallel_type Yes Yes Yes Global Yes
slave_parallel_workers Yes Yes Yes Global Yes
slave_pending_jobs_size_max Yes Yes Yes Global Yes
slave_preserve_commit_order Yes Yes Yes Global Yes
slave_rows_search_algorithms Yes Yes Yes Global Yes
slave_skip_errors Yes Yes Yes Global No
slave_sql_verify_checksum Yes Yes Yes Global Yes
slave_transaction_retries Yes Yes Yes Global Yes
slave_type_conversions Yes Yes Yes Global No
slow_launch_time Yes Yes Yes Global Yes
slow_query_log Yes Yes Yes Global Yes
slow_query_log_file Yes Yes Yes Global Yes
socket Yes Yes Yes Global No
sort_buffer_size Yes Yes Yes Both Yes
sql_auto_is_null Yes Both Yes
sql_big_selects Yes Both Yes
sql_buffer_result Yes Both Yes
sql_log_bin Yes Session Yes
sql_log_off Yes Both Yes
sql_mode Yes Yes Yes Both Yes
sql_notes Yes Both Yes
sql_quote_show_create Yes Both Yes
sql_safe_updates Yes Both Yes
sql_select_limit Yes Both Yes
sql_slave_skip_counter Yes Global Yes
sql_warnings Yes Both Yes
ssl_ca Yes Yes Yes Global No
ssl_capath Yes Yes Yes Global No
ssl_cert Yes Yes Yes Global No
ssl_cipher Yes Yes Yes Global No
ssl_crl Yes Yes Yes Global No
ssl_crlpath Yes Yes Yes Global No
ssl_key Yes Yes Yes Global No
stored_program_cache Yes Yes Yes Global Yes
super_read_only Yes Yes Yes Global Yes
sync_binlog Yes Yes Yes Global Yes
sync_frm Yes Yes Yes Global Yes
sync_master_info Yes Yes Yes Global Yes
sync_relay_log Yes Yes Yes Global Yes
sync_relay_log_info Yes Yes Yes Global Yes
system_time_zone Yes Global No
table_definition_cache Yes Yes Yes Global Yes
table_open_cache Yes Yes Yes Global Yes
table_open_cache_instances Yes Yes Yes Global No
thread_cache_size Yes Yes Yes Global Yes
thread_handling Yes Yes Yes Global No
thread_pool_algorithm Yes Yes Yes Global No
thread_pool_high_priority_connection Yes Yes Yes Both Yes
thread_pool_max_unused_threads Yes Yes Yes Global Yes
thread_pool_prio_kickup_timer Yes Yes Yes Both Yes
thread_pool_size Yes Yes Yes Global No
thread_pool_stall_limit Yes Yes Yes Global Yes
thread_stack Yes Yes Yes Global No
time_format Yes Global No
time_zone Yes Both Yes
timestamp Yes Session Yes
tls_version Yes Yes Yes Global No
tmp_table_size Yes Yes Yes Both Yes
tmpdir Yes Yes Yes Global No
transaction_alloc_block_size Yes Yes Yes Both Yes
transaction_allow_batching Yes Session Yes
transaction_isolation Yes Yes Yes
- Variable: tx_isolation Yes Both Yes
transaction_prealloc_size Yes Yes Yes Both Yes
transaction_read_only Yes Yes Yes
- Variable: tx_read_only Yes Both Yes
transaction_write_set_extraction Yes Yes Yes Both Yes
tx_isolation Yes Both Yes
tx_read_only Yes Both Yes
unique_checks Yes Both Yes
updatable_views_with_limit Yes Yes Yes Both Yes
validate_password_check_user_name Yes Yes Yes Global Yes
validate_password_dictionary_file Yes Yes Yes Global Varies
validate_password_length Yes Yes Yes Global Yes
validate_password_mixed_case_count Yes Yes Yes Global Yes
validate_password_number_count Yes Yes Yes Global Yes
validate_password_policy Yes Yes Yes Global Yes
validate_password_special_char_count Yes Yes Yes Global Yes
validate_user_plugins Yes Yes Yes Global No
version Yes Global No
version_comment Yes Global No
version_compile_machine Yes Global No
version_compile_os Yes Global No
version_tokens_session Yes Yes Yes Both Yes
version_tokens_session_number Yes Yes Yes Both No
wait_timeout Yes Yes Yes Both Yes
warning_count Yes Session No

Notes:

1. This option is dynamic, but only the server should set this information. You should not set the value of this variable manually.

5.1.5 Server Status Variable Reference

The following table lists all status variables applicable within mysqld.

The table lists each variable's data type and scope. The last column indicates whether the scope for each variable is Global, Session, or both. Please see the corresponding item descriptions for details on setting and using the variables. Where appropriate, direct links to further information about the items are provided.

Table 5.3 Status Variable Summary

Variable Name Variable Type Variable Scope
Aborted_clients Integer Global
Aborted_connects Integer Global
Audit_log_current_size Integer Global
Audit_log_event_max_drop_size Integer Global
Audit_log_events Integer Global
Audit_log_events_filtered Integer Global
Audit_log_events_lost Integer Global
Audit_log_events_written Integer Global
Audit_log_total_size Integer Global
Audit_log_write_waits Integer Global
Binlog_cache_disk_use Integer Global
Binlog_cache_use Integer Global
Binlog_stmt_cache_disk_use Integer Global
Binlog_stmt_cache_use Integer Global
Bytes_received Integer Both
Bytes_sent Integer Both
Com_admin_commands Integer Both
Com_alter_db Integer Both
Com_alter_db_upgrade Integer Both
Com_alter_event Integer Both
Com_alter_function Integer Both
Com_alter_procedure Integer Both
Com_alter_server Integer Both
Com_alter_table Integer Both
Com_alter_tablespace Integer Both
Com_alter_user Integer Both
Com_analyze Integer Both
Com_assign_to_keycache Integer Both
Com_begin Integer Both
Com_binlog Integer Both
Com_call_procedure Integer Both
Com_change_db Integer Both
Com_change_master Integer Both
Com_change_repl_filter Integer Both
Com_check Integer Both
Com_checksum Integer Both
Com_commit Integer Both
Com_create_db Integer Both
Com_create_event Integer Both
Com_create_function Integer Both
Com_create_index Integer Both
Com_create_procedure Integer Both
Com_create_server Integer Both
Com_create_table Integer Both
Com_create_trigger Integer Both
Com_create_udf Integer Both
Com_create_user Integer Both
Com_create_view Integer Both
Com_dealloc_sql Integer Both
Com_delete Integer Both
Com_delete_multi Integer Both
Com_do Integer Both
Com_drop_db Integer Both
Com_drop_event Integer Both
Com_drop_function Integer Both
Com_drop_index Integer Both
Com_drop_procedure Integer Both
Com_drop_server Integer Both
Com_drop_table Integer Both
Com_drop_trigger Integer Both
Com_drop_user Integer Both
Com_drop_view Integer Both
Com_empty_query Integer Both
Com_execute_sql Integer Both
Com_explain_other Integer Both
Com_flush Integer Both
Com_get_diagnostics Integer Both
Com_grant Integer Both
Com_group_replication_start Integer Global
Com_group_replication_stop Integer Global
Com_ha_close Integer Both
Com_ha_open Integer Both
Com_ha_read Integer Both
Com_help Integer Both
Com_insert Integer Both
Com_insert_select Integer Both
Com_install_plugin Integer Both
Com_kill Integer Both
Com_load Integer Both
Com_lock_tables Integer Both
Com_optimize Integer Both
Com_preload_keys Integer Both
Com_prepare_sql Integer Both
Com_purge Integer Both
Com_purge_before_date Integer Both
Com_release_savepoint Integer Both
Com_rename_table Integer Both
Com_rename_user Integer Both
Com_repair Integer Both
Com_replace Integer Both
Com_replace_select Integer Both
Com_reset Integer Both
Com_resignal Integer Both
Com_revoke Integer Both
Com_revoke_all Integer Both
Com_rollback Integer Both
Com_rollback_to_savepoint Integer Both
Com_savepoint Integer Both
Com_select Integer Both
Com_set_option Integer Both
Com_show_authors Integer Both
Com_show_binlog_events Integer Both
Com_show_binlogs Integer Both
Com_show_charsets Integer Both
Com_show_collations Integer Both
Com_show_contributors Integer Both
Com_show_create_db Integer Both
Com_show_create_event Integer Both
Com_show_create_func Integer Both
Com_show_create_proc Integer Both
Com_show_create_table Integer Both
Com_show_create_trigger Integer Both
Com_show_create_user Integer Both
Com_show_databases Integer Both
Com_show_engine_logs Integer Both
Com_show_engine_mutex Integer Both
Com_show_engine_status Integer Both
Com_show_errors Integer Both
Com_show_events Integer Both
Com_show_fields Integer Both
Com_show_function_code Integer Both
Com_show_function_status Integer Both
Com_show_grants Integer Both
Com_show_keys Integer Both
Com_show_master_status Integer Both
Com_show_ndb_status Integer Both
Com_show_open_tables Integer Both
Com_show_plugins Integer Both
Com_show_privileges Integer Both
Com_show_procedure_code Integer Both
Com_show_procedure_status Integer Both
Com_show_processlist Integer Both
Com_show_profile Integer Both
Com_show_profiles Integer Both
Com_show_relaylog_events Integer Both
Com_show_slave_hosts Integer Both
Com_show_slave_status Integer Both
Com_show_status Integer Both
Com_show_storage_engines Integer Both
Com_show_table_status Integer Both
Com_show_tables Integer Both
Com_show_triggers Integer Both
Com_show_variables Integer Both
Com_show_warnings Integer Both
Com_shutdown Integer Both
Com_signal Integer Both
Com_slave_start Integer Both
Com_slave_stop Integer Both
Com_stmt_close Integer Both
Com_stmt_execute Integer Both
Com_stmt_fetch Integer Both
Com_stmt_prepare Integer Both
Com_stmt_reprepare Integer Both
Com_stmt_reset Integer Both
Com_stmt_send_long_data Integer Both
Com_truncate Integer Both
Com_uninstall_plugin Integer Both
Com_unlock_tables Integer Both
Com_update Integer Both
Com_update_multi Integer Both
Com_xa_commit Integer Both
Com_xa_end Integer Both
Com_xa_prepare Integer Both
Com_xa_recover Integer Both
Com_xa_rollback Integer Both
Com_xa_start Integer Both
Compression Integer Session
Connection_control_delay_generated Integer Global
Connection_errors_accept Integer Global
Connection_errors_internal Integer Global
Connection_errors_max_connections Integer Global
Connection_errors_peer_address Integer Global
Connection_errors_select Integer Global
Connection_errors_tcpwrap Integer Global
Connections Integer Global
Created_tmp_disk_tables Integer Both
Created_tmp_files Integer Global
Created_tmp_tables Integer Both
Delayed_errors Integer Global
Delayed_insert_threads Integer Global
Delayed_writes Integer Global
Firewall_access_denied Integer Global
Firewall_access_granted Integer Global
Firewall_cached_entries Integer Global
Flush_commands Integer Global
group_replication_primary_member String Global
Handler_commit Integer Both
Handler_delete Integer Both
Handler_discover Integer Both
Handler_external_lock Integer Both
Handler_mrr_init Integer Both
Handler_prepare Integer Both
Handler_read_first Integer Both
Handler_read_key Integer Both
Handler_read_last Integer Both
Handler_read_next Integer Both
Handler_read_prev Integer Both
Handler_read_rnd Integer Both
Handler_read_rnd_next Integer Both
Handler_rollback Integer Both
Handler_savepoint Integer Both
Handler_savepoint_rollback Integer Both
Handler_update Integer Both
Handler_write Integer Both
Innodb_available_undo_logs Integer Global
Innodb_buffer_pool_bytes_data Integer Global
Innodb_buffer_pool_bytes_dirty Integer Global
Innodb_buffer_pool_dump_status String Global
Innodb_buffer_pool_load_status String Global
Innodb_buffer_pool_pages_data Integer Global
Innodb_buffer_pool_pages_dirty Integer Global
Innodb_buffer_pool_pages_flushed Integer Global
Innodb_buffer_pool_pages_free Integer Global
Innodb_buffer_pool_pages_latched Integer Global
Innodb_buffer_pool_pages_misc Integer Global
Innodb_buffer_pool_pages_total Integer Global
Innodb_buffer_pool_read_ahead Integer Global
Innodb_buffer_pool_read_ahead_evicted Integer Global
Innodb_buffer_pool_read_ahead_rnd Integer Global
Innodb_buffer_pool_read_requests Integer Global
Innodb_buffer_pool_reads Integer Global
Innodb_buffer_pool_resize_status String Global
Innodb_buffer_pool_wait_free Integer Global
Innodb_buffer_pool_write_requests Integer Global
Innodb_data_fsyncs Integer Global
Innodb_data_pending_fsyncs Integer Global
Innodb_data_pending_reads Integer Global
Innodb_data_pending_writes Integer Global
Innodb_data_read Integer Global
Innodb_data_reads Integer Global
Innodb_data_writes Integer Global
Innodb_data_written Integer Global
Innodb_dblwr_pages_written Integer Global
Innodb_dblwr_writes Integer Global
Innodb_have_atomic_builtins Integer Global
Innodb_log_waits Integer Global
Innodb_log_write_requests Integer Global
Innodb_log_writes Integer Global
Innodb_num_open_files Integer Global
Innodb_os_log_fsyncs Integer Global
Innodb_os_log_pending_fsyncs Integer Global
Innodb_os_log_pending_writes Integer Global
Innodb_os_log_written Integer Global
Innodb_page_size Integer Global
Innodb_pages_created Integer Global
Innodb_pages_read Integer Global
Innodb_pages_written Integer Global
Innodb_row_lock_current_waits Integer Global
Innodb_row_lock_time Integer Global
Innodb_row_lock_time_avg Integer Global
Innodb_row_lock_time_max Integer Global
Innodb_row_lock_waits Integer Global
Innodb_rows_deleted Integer Global
Innodb_rows_inserted Integer Global
Innodb_rows_read Integer Global
Innodb_rows_updated Integer Global
Innodb_truncated_status_writes Integer Global
Key_blocks_not_flushed Integer Global
Key_blocks_unused Integer Global
Key_blocks_used Integer Global
Key_read_requests Integer Global
Key_reads Integer Global
Key_write_requests Integer Global
Key_writes Integer Global
Last_query_cost Numeric Session
Last_query_partial_plans Integer Session
Locked_connects Integer Global
Max_execution_time_exceeded Integer Both
Max_execution_time_set Integer Both
Max_execution_time_set_failed Integer Both
Max_used_connections Integer Global
Max_used_connections_time Datetime Global
mecab_charset String Global
Mysqlx_address String Global
Mysqlx_bytes_received Integer Both
Mysqlx_bytes_sent Integer Both
Mysqlx_connection_accept_errors Integer Both
Mysqlx_connection_errors Integer Both
Mysqlx_connections_accepted Integer Global
Mysqlx_connections_closed Integer Global
Mysqlx_connections_rejected Integer Global
Mysqlx_crud_create_view Integer Both
Mysqlx_crud_delete Integer Both
Mysqlx_crud_drop_view Integer Both
Mysqlx_crud_find Integer Both
Mysqlx_crud_insert Integer Both
Mysqlx_crud_modify_view Integer Both
Mysqlx_crud_update Integer Both
Mysqlx_errors_sent Integer Both
Mysqlx_errors_unknown_message_type Integer Both
Mysqlx_expect_close Integer Both
Mysqlx_expect_open Integer Both
Mysqlx_init_error Integer Both
Mysqlx_notice_other_sent Integer Both
Mysqlx_notice_warning_sent Integer Both
Mysqlx_port String Global
Mysqlx_rows_sent Integer Both
Mysqlx_sessions Integer Global
Mysqlx_sessions_accepted Integer Global
Mysqlx_sessions_closed Integer Global
Mysqlx_sessions_fatal_error Integer Global
Mysqlx_sessions_killed Integer Global
Mysqlx_sessions_rejected Integer Global
Mysqlx_socket String Global
Mysqlx_ssl_accept_renegotiates Integer Global
Mysqlx_ssl_accepts Integer Global
Mysqlx_ssl_active Integer Both
Mysqlx_ssl_cipher Integer Both
Mysqlx_ssl_cipher_list Integer Both
Mysqlx_ssl_ctx_verify_depth Integer Both
Mysqlx_ssl_ctx_verify_mode Integer Both
Mysqlx_ssl_finished_accepts Integer Global
Mysqlx_ssl_server_not_after Integer Global
Mysqlx_ssl_server_not_before Integer Global
Mysqlx_ssl_verify_depth Integer Global
Mysqlx_ssl_verify_mode Integer Global
Mysqlx_ssl_version Integer Both
Mysqlx_stmt_create_collection Integer Both
Mysqlx_stmt_create_collection_index Integer Both
Mysqlx_stmt_disable_notices Integer Both
Mysqlx_stmt_drop_collection Integer Both
Mysqlx_stmt_drop_collection_index Integer Both
Mysqlx_stmt_enable_notices Integer Both
Mysqlx_stmt_ensure_collection String Both
Mysqlx_stmt_execute_mysqlx Integer Both
Mysqlx_stmt_execute_sql Integer Both
Mysqlx_stmt_execute_xplugin Integer Both
Mysqlx_stmt_kill_client Integer Both
Mysqlx_stmt_list_clients Integer Both
Mysqlx_stmt_list_notices Integer Both
Mysqlx_stmt_list_objects Integer Both
Mysqlx_stmt_ping Integer Both
Mysqlx_worker_threads Integer Global
Mysqlx_worker_threads_active Integer Global
Ndb_api_bytes_received_count Integer Global
Ndb_api_bytes_received_count_session Integer Session
Ndb_api_bytes_received_count_slave Integer Global
Ndb_api_bytes_sent_count Integer Global
Ndb_api_bytes_sent_count_session Integer Session
Ndb_api_bytes_sent_count_slave Integer Global
Ndb_api_event_bytes_count Integer Global
Ndb_api_event_bytes_count_injector Integer Global
Ndb_api_event_data_count Integer Global
Ndb_api_event_data_count_injector Integer Global
Ndb_api_event_nondata_count Integer Global
Ndb_api_event_nondata_count_injector Integer Global
Ndb_api_pk_op_count Integer Global
Ndb_api_pk_op_count_session Integer Session
Ndb_api_pk_op_count_slave Integer Global
Ndb_api_pruned_scan_count Integer Global
Ndb_api_pruned_scan_count_session Integer Session
Ndb_api_pruned_scan_count_slave Integer Global
Ndb_api_range_scan_count Integer Global
Ndb_api_range_scan_count_session Integer Session
Ndb_api_range_scan_count_slave Integer Global
Ndb_api_read_row_count Integer Global
Ndb_api_read_row_count_session Integer Session
Ndb_api_read_row_count_slave Integer Global
Ndb_api_scan_batch_count Integer Global
Ndb_api_scan_batch_count_session Integer Session
Ndb_api_scan_batch_count_slave Integer Global
Ndb_api_table_scan_count Integer Global
Ndb_api_table_scan_count_session Integer Session
Ndb_api_table_scan_count_slave Integer Global
Ndb_api_trans_abort_count Integer Global
Ndb_api_trans_abort_count_session Integer Session
Ndb_api_trans_abort_count_slave Integer Global
Ndb_api_trans_close_count Integer Global
Ndb_api_trans_close_count_session Integer Session
Ndb_api_trans_close_count_slave Integer Global
Ndb_api_trans_commit_count Integer Global
Ndb_api_trans_commit_count_session Integer Session
Ndb_api_trans_commit_count_slave Integer Global
Ndb_api_trans_local_read_row_count Integer Global
Ndb_api_trans_local_read_row_count_session Integer Session
Ndb_api_trans_local_read_row_count_slave Integer Global
Ndb_api_trans_start_count Integer Global
Ndb_api_trans_start_count_session Integer Session
Ndb_api_trans_start_count_slave Integer Global
Ndb_api_uk_op_count Integer Global
Ndb_api_uk_op_count_session Integer Session
Ndb_api_uk_op_count_slave Integer Global
Ndb_api_wait_exec_complete_count Integer Global
Ndb_api_wait_exec_complete_count_session Integer Session
Ndb_api_wait_exec_complete_count_slave Integer Global
Ndb_api_wait_meta_request_count Integer Global
Ndb_api_wait_meta_request_count_session Integer Session
Ndb_api_wait_meta_request_count_slave Integer Global
Ndb_api_wait_nanos_count Integer Global
Ndb_api_wait_nanos_count_session Integer Session
Ndb_api_wait_nanos_count_slave Integer Global
Ndb_api_wait_scan_result_count Integer Global
Ndb_api_wait_scan_result_count_session Integer Session
Ndb_api_wait_scan_result_count_slave Integer Global
Ndb_cluster_node_id Integer Global
Ndb_config_from_host Integer Both
Ndb_config_from_port Integer Both
Ndb_conflict_fn_epoch Integer Global
Ndb_conflict_fn_epoch_trans Integer Global
Ndb_conflict_fn_epoch2 Integer Global
Ndb_conflict_fn_epoch2_trans Integer Global
Ndb_conflict_fn_max Integer Global
Ndb_conflict_fn_old Integer Global
Ndb_conflict_last_stable_epoch Integer Global
Ndb_conflict_reflected_op_discard_count Integer Global
Ndb_conflict_reflected_op_prepare_count Integer Global
Ndb_conflict_refresh_op_count Integer Global
Ndb_conflict_trans_conflict_commit_count Integer Global
Ndb_conflict_trans_detect_iter_count Integer Global
Ndb_conflict_trans_reject_count Integer Global
Ndb_conflict_trans_row_conflict_count Integer Global
Ndb_conflict_trans_row_reject_count Integer Global
Ndb_epoch_delete_delete_count Integer Global
Ndb_execute_count Integer Global
Ndb_last_commit_epoch_server Integer Global
Ndb_last_commit_epoch_session Integer Session
Ndb_cluster_node_id Integer Global
Ndb_number_of_data_nodes Integer Global
Ndb_pruned_scan_count Integer Global
Ndb_pushed_queries_defined Integer Global
Ndb_pushed_queries_dropped Integer Global
Ndb_pushed_queries_executed Integer Global
Ndb_pushed_reads Integer Global
Ndb_scan_count Integer Global
Not_flushed_delayed_rows Integer Global
Ongoing_anonymous_gtid_violating_transaction_count Integer Global
Ongoing_anonymous_transaction_count Integer Global
Ongoing_automatic_gtid_violating_transaction_count Integer Global
Open_files Integer Global
Open_streams Integer Global
Open_table_definitions Integer Global
Open_tables Integer Both
Opened_files Integer Global
Opened_table_definitions Integer Both
Opened_tables Integer Both
Performance_schema_accounts_lost Integer Global
Performance_schema_cond_classes_lost Integer Global
Performance_schema_cond_instances_lost Integer Global
Performance_schema_digest_lost Integer Global
Performance_schema_file_classes_lost Integer Global
Performance_schema_file_handles_lost Integer Global
Performance_schema_file_instances_lost Integer Global
Performance_schema_hosts_lost Integer Global
Performance_schema_index_stat_lost Integer Global
Performance_schema_locker_lost Integer Global
Performance_schema_memory_classes_lost Integer Global
Performance_schema_metadata_lock_lost Integer Global
Performance_schema_mutex_classes_lost Integer Global
Performance_schema_mutex_instances_lost Integer Global
Performance_schema_nested_statement_lost Integer Global
Performance_schema_prepared_statements_lost Integer Global
Performance_schema_program_lost Integer Global
Performance_schema_rwlock_classes_lost Integer Global
Performance_schema_rwlock_instances_lost Integer Global
Performance_schema_session_connect_attrs_lost Integer Global
Performance_schema_socket_classes_lost Integer Global
Performance_schema_socket_instances_lost Integer Global
Performance_schema_stage_classes_lost Integer Global
Performance_schema_statement_classes_lost Integer Global
Performance_schema_table_handles_lost Integer Global
Performance_schema_table_instances_lost Integer Global
Performance_schema_table_lock_stat_lost Integer Global
Performance_schema_thread_classes_lost Integer Global
Performance_schema_thread_instances_lost Integer Global
Performance_schema_users_lost Integer Global
Prepared_stmt_count Integer Global
Qcache_free_blocks Integer Global
Qcache_free_memory Integer Global
Qcache_hits Integer Global
Qcache_inserts Integer Global
Qcache_lowmem_prunes Integer Global
Qcache_not_cached Integer Global
Qcache_queries_in_cache Integer Global
Qcache_total_blocks Integer Global
Queries Integer Both
Questions Integer Both
Rewriter_number_loaded_rules Integer Global
Rewriter_number_reloads Integer Global
Rewriter_number_rewritten_queries Integer Global
Rewriter_reload_error Boolean Global
Rpl_semi_sync_master_clients Integer Global
Rpl_semi_sync_master_net_avg_wait_time Integer Global
Rpl_semi_sync_master_net_wait_time Integer Global
Rpl_semi_sync_master_net_waits Integer Global
Rpl_semi_sync_master_no_times Integer Global
Rpl_semi_sync_master_no_tx Integer Global
Rpl_semi_sync_master_status Boolean Global
Rpl_semi_sync_master_timefunc_failures Integer Global
Rpl_semi_sync_master_tx_avg_wait_time Integer Global
Rpl_semi_sync_master_tx_wait_time Integer Global
Rpl_semi_sync_master_tx_waits Integer Global
Rpl_semi_sync_master_wait_pos_backtraverse Integer Global
Rpl_semi_sync_master_wait_sessions Integer Global
Rpl_semi_sync_master_yes_tx Integer Global
Rpl_semi_sync_slave_status Boolean Global
Rsa_public_key String Global
Select_full_join Integer Both
Select_full_range_join Integer Both
Select_range Integer Both
Select_range_check Integer Both
Select_scan Integer Both
Slave_heartbeat_period Numeric Global
Slave_last_heartbeat Datetime Global
Slave_open_temp_tables Integer Global
Slave_received_heartbeats Integer Global
Slave_retried_transactions Integer Global
Slave_rows_last_search_algorithm_used String Global
Slave_running String Global
Slow_launch_threads Integer Both
Slow_queries Integer Both
Sort_merge_passes Integer Both
Sort_range Integer Both
Sort_rows Integer Both
Sort_scan Integer Both
Ssl_accept_renegotiates Integer Global
Ssl_accepts Integer Global
Ssl_callback_cache_hits Integer Global
Ssl_cipher String Both
Ssl_cipher_list String Both
Ssl_client_connects Integer Global
Ssl_connect_renegotiates Integer Global
Ssl_ctx_verify_depth Integer Global
Ssl_ctx_verify_mode Integer Global
Ssl_default_timeout Integer Both
Ssl_finished_accepts Integer Global
Ssl_finished_connects Integer Global
Ssl_server_not_after Integer Both
Ssl_server_not_before Integer Both
Ssl_session_cache_hits Integer Global
Ssl_session_cache_misses Integer Global
Ssl_session_cache_mode String Global
Ssl_session_cache_overflows Integer Global
Ssl_session_cache_size Integer Global
Ssl_session_cache_timeouts Integer Global
Ssl_sessions_reused Integer Both
Ssl_used_session_cache_entries Integer Global
Ssl_verify_depth Integer Both
Ssl_verify_mode Integer Both
Ssl_version String Both
Table_locks_immediate Integer Global
Table_locks_waited Integer Global
Table_open_cache_hits Integer Both
Table_open_cache_misses Integer Both
Table_open_cache_overflows Integer Both
Tc_log_max_pages_used Integer Global
Tc_log_page_size Integer Global
Tc_log_page_waits Integer Global
Threads_cached Integer Global
Threads_connected Integer Global
Threads_created Integer Global
Threads_running Integer Global
Uptime Integer Global
Uptime_since_flush_status Integer Global
validate_password_dictionary_file_last_parsed Datetime Global
validate_password_dictionary_file_words_count Integer Global

5.1.6 Server Command Options

When you start the mysqld server, you can specify program options using any of the methods described in Section 4.2.2, “Specifying Program Options”. The most common methods are to provide options in an option file or on the command line. However, in most cases it is desirable to make sure that the server uses the same options each time it runs. The best way to ensure this is to list them in an option file. See Section 4.2.2.2, “Using Option Files”. That section also describes option file format and syntax.

mysqld reads options from the [mysqld] and [server] groups. mysqld_safe reads options from the [mysqld], [server], [mysqld_safe], and [safe_mysqld] groups. mysql.server reads options from the [mysqld] and [mysql.server] groups.

An embedded MySQL server usually reads options from the [server], [embedded], and [xxxxx_SERVER] groups, where xxxxx is the name of the application into which the server is embedded.

mysqld accepts many command options. For a brief summary, execute this command:

mysqld --help

To see the full list, use this command:

mysqld --verbose --help

Some of the items in the list are actually system variables that can be set at server startup. These can be displayed at runtime using the SHOW VARIABLES statement. Some items displayed by the preceding mysqld command do not appear in SHOW VARIABLES output; this is because they are options only and not system variables.

The following list shows some of the most common server options. Additional options are described in other sections:

Some options control the size of buffers or caches. For a given buffer, the server might need to allocate internal data structures. These structures typically are allocated from the total memory allocated to the buffer, and the amount of space required might be platform dependent. This means that when you assign a value to an option that controls a buffer size, the amount of space actually available might differ from the value assigned. In some cases, the amount might be less than the value assigned. It is also possible that the server will adjust a value upward. For example, if you assign a value of 0 to an option for which the minimal value is 1024, the server will set the value to 1024.

Values for buffer sizes, lengths, and stack sizes are given in bytes unless otherwise specified.

Some options take file name values. Unless otherwise specified, the default file location is the data directory if the value is a relative path name. To specify the location explicitly, use an absolute path name. Suppose that the data directory is /var/mysql/data. If a file-valued option is given as a relative path name, it will be located under /var/mysql/data. If the value is an absolute path name, its location is as given by the path name.

You can also set the values of server system variables at server startup by using variable names as options. To assign a value to a server system variable, use an option of the form --var_name=value. For example, --sort_buffer_size=384M sets the sort_buffer_size variable to a value of 384MB.

When you assign a value to a variable, MySQL might automatically correct the value to stay within a given range, or adjust the value to the closest permissible value if only certain values are permitted.

To restrict the maximum value to which a system variable can be set at runtime with the SET statement, specify this maximum by using an option of the form --maximum-var_name=value at server startup.

You can change the values of most system variables at runtime with the SET statement. See Section 13.7.4.1, “SET Syntax for Variable Assignment”.

Section 5.1.7, “Server System Variables”, provides a full description for all variables, and additional information for setting them at server startup and runtime. For information on changing system variables, see Section 5.1.1, “Configuring the Server”.

  • --help, -?

    Property Value
    Command-Line Format --help

    Display a short help message and exit. Use both the --verbose and --help options to see the full message.

  • --allow-suspicious-udfs

    Property Value
    Command-Line Format --allow-suspicious-udfs[={OFF|ON}]
    Type Boolean
    Default Value OFF

    This option controls whether user-defined functions that have only an xxx symbol for the main function can be loaded. By default, the option is off and only UDFs that have at least one auxiliary symbol can be loaded; this prevents attempts at loading functions from shared object files other than those containing legitimate UDFs. See UDF Security Precautions.

  • --ansi

    Property Value
    Command-Line Format --ansi

    Use standard (ANSI) SQL syntax instead of MySQL syntax. For more precise control over the server SQL mode, use the --sql-mode option instead. See Section 1.8, “MySQL Standards Compliance”, and Section 5.1.10, “Server SQL Modes”.

  • --basedir=dir_name, -b dir_name

    Property Value
    Command-Line Format --basedir=dir_name
    System Variable basedir
    Scope Global
    Dynamic No
    Type Directory name
    Default Value configuration-dependent default

    The path to the MySQL installation directory. This option sets the basedir system variable.

  • --bootstrap

    Property Value
    Command-Line Format --bootstrap
    Deprecated Yes

    This option is used by the mysql_install_db program to create the MySQL privilege tables without having to start a full MySQL server.

    Note

    mysql_install_db is deprecated because its functionality has been integrated into mysqld, the MySQL server. Consequently, the --bootstrap server option that mysql_install_db passes to mysqld is also deprecated. To initialize a MySQL installation, invoke mysqld with the --initialize or --initialize-insecure option. For more information, see Section 2.10.1, “Initializing the Data Directory”. mysql_install_db and the --bootstrap server option will be removed in a future MySQL release.

    --bootstrap is mutually exclusive with --daemonize, --initialize, and --initialize-insecure.

    Global transaction identifiers (GTIDs) are not disabled when --bootstrap is used. --bootstrap was used (Bug #20980271). See Section 16.1.3, “Replication with Global Transaction Identifiers”.

    When the server operates in bootstap mode, some functionality is unavailable that limits the statements permitted in any file named by the init_file system variable. For more information, see the description of that variable. In addition, the disabled_storage_engines system variable has no effect.

  • --character-set-client-handshake

    Property Value
    Command-Line Format --character-set-client-handshake[={OFF|ON}]
    Type Boolean
    Default Value ON

    Do not ignore character set information sent by the client. To ignore client information and use the default server character set, use --skip-character-set-client-handshake; this makes MySQL behave like MySQL 4.0.

  • --chroot=dir_name, -r dir_name

    Property Value
    Command-Line Format --chroot=dir_name
    Type Directory name

    Put the mysqld server in a closed environment during startup by using the chroot() system call. This is a recommended security measure. Use of this option somewhat limits LOAD DATA and SELECT ... INTO OUTFILE.

  • --console

    Property Value
    Command-Line Format --console
    Platform Specific Windows

    (Windows only.) Write the error log to stderr and stdout (the console). mysqld does not close the console window if this option is used.

    --console takes precedence over --log-error if both are given. (In MySQL 5.5 and 5.6, this is reversed: --log-error takes precedence over --console if both are given.)

  • --core-file

    Property Value
    Command-Line Format --core-file[={OFF|ON}]
    Type Boolean
    Default Value OFF

    Write a core file if mysqld dies. The name and location of the core file is system dependent. On Linux, a core file named core.pid is written to the current working directory of the process, which for mysqld is the data directory. pid represents the process ID of the server process. On macOS, a core file named core.pid is written to the /cores directory. On Solaris, use the coreadm command to specify where to write the core file and how to name it.

    For some systems, to get a core file you must also specify the --core-file-size option to mysqld_safe. See Section 4.3.2, “mysqld_safe — MySQL Server Startup Script”. On some systems, such as Solaris, you do not get a core file if you are also using the --user option. There might be additional restrictions or limitations. For example, it might be necessary to execute ulimit -c unlimited before starting the server. Consult your system documentation.

  • --daemonize

    Property Value
    Command-Line Format --daemonize[={OFF|ON}]
    Type Boolean
    Default Value OFF

    This option causes the server to run as a traditional, forking daemon, permitting it to work with operating systems that use systemd for process control. For more information, see Section 2.5.10, “Managing MySQL Server with systemd”.

    --daemonize is mutually exclusive with --bootstrap, --initialize, and --initialize-insecure.

  • --datadir=dir_name, -h dir_name

    Property Value
    Command-Line Format --datadir=dir_name
    System Variable datadir
    Scope Global
    Dynamic No
    Type Directory name

    The path to the MySQL server data directory. This option sets the datadir system variable. See the description of that variable.

  • --debug[=debug_options], -# [debug_options]

    Property Value
    Command-Line Format --debug[=debug_options]
    System Variable debug
    Scope Global, Session
    Dynamic Yes
    Type String
    Default Value (Windows) d:t:i:O,\mysqld.trace
    Default Value (Unix) d:t:i:o,/tmp/mysqld.trace

    If MySQL is configured with the -DWITH_DEBUG=1 CMake option, you can use this option to get a trace file of what mysqld is doing. A typical debug_options string is d:t:o,file_name. The default is d:t:i:o,/tmp/mysqld.trace on Unix and d:t:i:O,\mysqld.trace on Windows.

    Using -DWITH_DEBUG=1 to configure MySQL with debugging support enables you to use the --debug="d,parser_debug" option when you start the server. This causes the Bison parser that is used to process SQL statements to dump a parser trace to the server's standard error output. Typically, this output is written to the error log.

    This option may be given multiple times. Values that begin with + or - are added to or subtracted from the previous value. For example, --debug=T --debug=+P sets the value to P:T.

    For more information, see Section 28.5.3, “The DBUG Package”.

  • --debug-sync-timeout[=N]

    Property Value
    Command-Line Format --debug-sync-timeout[=#]
    Type Integer

    Controls whether the Debug Sync facility for testing and debugging is enabled. Use of Debug Sync requires that MySQL be configured with the -DENABLE_DEBUG_SYNC=1 CMake option (see Section 2.9.7, “MySQL Source-Configuration Options”). If Debug Sync is not compiled in, this option is not available. The option value is a timeout in seconds. The default value is 0, which disables Debug Sync. To enable it, specify a value greater than 0; this value also becomes the default timeout for individual synchronization points. If the option is given without a value, the timeout is set to 300 seconds.

    For a description of the Debug Sync facility and how to use synchronization points, see MySQL Internals: Test Synchronization.

  • --default-time-zone=timezone

    Property Value
    Command-Line Format --default-time-zone=name
    Type String

    Set the default server time zone. This option sets the global time_zone system variable. If this option is not given, the default time zone is the same as the system time zone (given by the value of the system_time_zone system variable.

  • --defaults-extra-file=file_name

    Read this option file after the global option file but (on Unix) before the user option file. If the file does not exist or is otherwise inaccessible, an error occurs. file_name is interpreted relative to the current directory if given as a relative path name rather than a full path name. This must be the first option on the command line if it is used.

    For additional information about this and other option-file options, see Section 4.2.2.3, “Command-Line Options that Affect Option-File Handling”.

  • --defaults-file=file_name

    Read only the given option file. If the file does not exist or is otherwise inaccessible, an error occurs. file_name is interpreted relative to the current directory if given as a relative path name rather than a full path name.

    Note

    This must be the first option on the command line if it is used, except that if the server is started with the --defaults-file and --install (or --install-manual) options, --install (or --install-manual) must be first.

    For additional information about this and other option-file options, see Section 4.2.2.3, “Command-Line Options that Affect Option-File Handling”.

  • --defaults-group-suffix=str

    Read not only the usual option groups, but also groups with the usual names and a suffix of str. For example, mysqld normally reads the [mysqld] group. If the --defaults-group-suffix=_other option is given, mysqld also reads the [mysqld_other] group.

    For additional information about this and other option-file options, see Section 4.2.2.3, “Command-Line Options that Affect Option-File Handling”.

  • --des-key-file=file_name

    Property Value
    Command-Line Format --des-key-file=file_name
    Deprecated Yes

    Read the default DES keys from this file. These keys are used by the DES_ENCRYPT() and DES_DECRYPT() functions.

    Note

    The DES_ENCRYPT() and DES_DECRYPT() functions are deprecated in MySQL 5.7, will be removed in a future MySQL release, and should no longer be used. Consequently, --des-key-file also is deprecated and will be removed.

  • --disable-partition-engine-check

    Property Value
    Command-Line Format --disable-partition-engine-check[={OFF|ON}]
    Introduced 5.7.17
    Deprecated 5.7.17
    Type Boolean
    Default Value (>= 5.7.21) ON
    Default Value (>= 5.7.17, <= 5.7.20) OFF

    Whether to disable the startup check for tables with nonnative partitioning.

    As of MySQL 5.7.17, the generic partitioning handler in the MySQL server is deprecated, and is removed in MySQL 8.0, when the storage engine used for a given table is expected to provide its own (native) partitioning handler. Currently, only the InnoDB and NDB storage engines do this.

    Use of tables with nonnative partitioning results in an ER_WARN_DEPRECATED_SYNTAX warning. In MySQL 5.7.17 through 5.7.20, the server automatically performs a check at startup to identify tables that use nonnative partitioning; for any that are found, the server writes a message to its error log. To disable this check, use the --disable-partition-engine-check option. In MySQL 5.7.21 and later, this check is not performed; in these versions, you must start the server with --disable-partition-engine-check=false, if you wish for the server to check for tables using the generic partitioning handler (Bug #85830, Bug #25846957).

    Use of tables with nonnative partitioning results in an ER_WARN_DEPRECATED_SYNTAX warning. Also, the server performs a check at startup to identify tables that use nonnative partitioning; for any found, the server writes a message to its error log. To disable this check, use the --disable-partition-engine-check option.

    To prepare for migration to MySQL 8.0, any table with nonnative partitioning should be changed to use an engine that provides native partitioning, or be made nonpartitioned. For example, to change a table to InnoDB, execute this statement:

    ALTER TABLE table_name ENGINE = INNODB;
    
  • --early-plugin-load=plugin_list

    Property Value
    Command-Line Format --early-plugin-load=plugin_list
    Introduced 5.7.11
    Type String
    Default Value (>= 5.7.12) empty string
    Default Value (5.7.11) keyring_file plugin library file name

    This option tells the server which plugins to load before loading mandatory built-in plugins and before storage engine initialization. If multiple --early-plugin-load options are given, only the last one is used.

    The option value is a semicolon-separated list of name=plugin_library and plugin_library values. Each name is the name of a plugin to load, and plugin_library is the name of the library file that contains the plugin code. If a plugin library is named without any preceding plugin name, the server loads all plugins in the library. The server looks for plugin library files in the directory named by the plugin_dir system variable.

    For example, if plugins named myplug1 and myplug2 have library files myplug1.so and myplug2.so, use this option to perform an early plugin load:

    shell> mysqld --early-plugin-load="myplug1=myplug1.so;myplug2=myplug2.so"
    

    Quotes are used around the argument value because otherwise a semicolon (;) is interpreted as a special character by some command interpreters. (Unix shells treat it as a command terminator, for example.)

    Each named plugin is loaded early for a single invocation of mysqld only. After a restart, the plugin is not loaded early unless --early-plugin-load is used again.

    If the server is started using --initialize or --initialize-insecure, plugins specified by --early-plugin-load are not loaded.

    If the server is run with --help, plugins specified by --early-plugin-load are loaded but not initialized. This behavior ensures that plugin options are displayed in the help message.

    As of MySQL 5.7.12, the default --early-plugin-load value is empty. To load your chosen keyring plugin, you must use an explicit --early-plugin-load option with a nonempty value.

    Important

    In MySQL 5.7.11, the default --early-plugin-load value was the name of the keyring_file plugin library file, so that plugin was loaded by default. InnoDB tablespace encryption requires the keyring_file plugin to be loaded prior to InnoDB initialization, so this change of default --early-plugin-load value introduces an incompatibility for upgrades from 5.7.11 to 5.7.12 or higher. Administrators who have encrypted InnoDB tablespaces must take explicit action to ensure continued loading of the keyring_file plugin: Start the server with an --early-plugin-load option that names the plugin library file. For additional information, see Section 6.4.4.1, “Keyring Plugin Installation”.

    The InnoDB tablespace encryption feature relies on the keyring_file plugin for encryption key management, and the keyring_file plugin must be loaded prior to storage engine initialization to facilitate InnoDB recovery for encrypted tables. In MySQL 5.7.11, if you do not want to load the keyring_file plugin at server startup, specify an empty string (--early-plugin-load="").

    For information about InnoDB tablespace encryption, see Section 14.14, “InnoDB Data-at-Rest Encryption”. For general information about plugin loading, see Section 5.5.1, “Installing and Uninstalling Plugins”.

  • --exit-info[=flags], -T [flags]

    Property Value
    Command-Line Format --exit-info[=flags]
    Type Integer

    This is a bitmask of different flags that you can use for debugging the mysqld server. Do not use this option unless you know exactly what it does!

  • --external-locking

    Property Value
    Command-Line Format --external-locking[={OFF|ON}]
    Type Boolean
    Default Value OFF

    Enable external locking (system locking), which is disabled by default. If you use this option on a system on which lockd does not fully work (such as Linux), it is easy for mysqld to deadlock.

    To disable external locking explicitly, use --skip-external-locking.

    External locking affects only MyISAM table access. For more information, including conditions under which it can and cannot be used, see Section 8.11.5, “External Locking”.

  • --flush

    Property Value
    Command-Line Format --flush[={OFF|ON}]
    System Variable flush
    Scope Global
    Dynamic Yes
    Type Boolean
    Default Value OFF

    Flush (synchronize) all changes to disk after each SQL statement. Normally, MySQL does a write of all changes to disk only after each SQL statement and lets the operating system handle the synchronizing to disk. See Section B.4.3.3, “What to Do If MySQL Keeps Crashing”.

    Note

    If --flush is specified, the value of flush_time does not matter and changes to flush_time have no effect on flush behavior.

  • --gdb

    Property Value
    Command-Line Format --gdb[={OFF|ON}]
    Type Boolean
    Default Value OFF

    Install an interrupt handler for SIGINT (needed to stop mysqld with ^C to set breakpoints) and disable stack tracing and core file handling. See Section 28.5.1.4, “Debugging mysqld under gdb”.

  • --ignore-db-dir=dir_name

    Property Value
    Command-Line Format --ignore-db-dir=dir_name
    Deprecated 5.7.16
    Type Directory name

    This option tells the server to ignore the given directory name for purposes of the SHOW DATABASES statement or INFORMATION_SCHEMA tables. For example, if a MySQL configuration locates the data directory at the root of a file system on Unix, the system might create a lost+found directory there that the server should ignore. Starting the server with --ignore-db-dir=lost+found causes that name not to be listed as a database.

    To specify more than one name, use this option multiple times, once for each name. Specifying the option with an empty value (that is, as --ignore-db-dir=) resets the directory list to the empty list.

    Instances of this option given at server startup are used to set the ignore_db_dirs system variable.

    This option is deprecated in MySQL 5.7. With the introduction of the data dictionary in MySQL 8.0, it became superfluous and was removed in that version.

  • --initialize

    Property Value
    Command-Line Format --initialize[={OFF|ON}]
    Type Boolean
    Default Value OFF

    This option is used to initialize a MySQL installation by creating the data directory and populating the tables in the mysql system database. For more information, see Section 2.10.1, “Initializing the Data Directory”.

    When the server is started with --initialize, some functionality is unavailable that limits the statements permitted in any file named by the init_file system variable. For more information, see the description of that variable. In addition, the disabled_storage_engines system variable has no effect.

    In MySQL NDB Cluster 7.5.4 and later, the --ndbcluster option is ignored when used together with --initialize. (Bug #81689, Bug #23518923)

    --initialize is mutually exclusive with --bootstrap and --daemonize.

  • --initialize-insecure

    Property Value
    Command-Line Format --initialize-insecure[={OFF|ON}]
    Type Boolean
    Default Value OFF

    This option is used to initialize a MySQL installation by creating the data directory and populating the tables in the mysql system database. This option implies --initialize. For more information, see the description of that option, and Section 2.10.1, “Initializing the Data Directory”.

    --initialize-insecure is mutually exclusive with --bootstrap and --daemonize.

  • --innodb-xxx

    Set an option for the InnoDB storage engine. The InnoDB options are listed in Section 14.15, “InnoDB Startup Options and System Variables”.

  • --install [service_name]

    Property Value
    Command-Line Format --install [service_name]
    Platform Specific Windows

    (Windows only) Install the server as a Windows service that starts automatically during Windows startup. The default service name is MySQL if no service_name value is given. For more information, see Section 2.3.4.8, “Starting MySQL as a Windows Service”.

    Note

    If the server is started with the --defaults-file and --install options, --install must be first.

  • --install-manual [service_name]

    Property Value
    Command-Line Format --install-manual [service_name]
    Platform Specific Windows

    (Windows only) Install the server as a Windows service that must be started manually. It does not start automatically during Windows startup. The default service name is MySQL if no service_name value is given. For more information, see Section 2.3.4.8, “Starting MySQL as a Windows Service”.

    Note

    If the server is started with the --defaults-file and --install-manual options, --install-manual must be first.

  • --language=lang_name, -L lang_name

    Property Value
    Command-Line Format --language=name
    Deprecated Yes; use lc-messages-dir instead
    System Variable language
    Scope Global
    Dynamic No
    Type Directory name
    Default Value /usr/local/mysql/share/mysql/english/

    The language to use for error messages. lang_name can be given as the language name or as the full path name to the directory where the language files are installed. See Section 10.12, “Setting the Error Message Language”.

    --lc-messages-dir and --lc-messages should be used rather than --language, which is deprecated (and handled as an alias for --lc-messages-dir). The --language option will be removed in a future MySQL release.

  • --large-pages

    Property Value
    Command-Line Format --large-pages[={OFF|ON}]
    System Variable large_pages
    Scope Global
    Dynamic No
    Platform Specific Linux
    Type Boolean
    Default Value OFF

    Some hardware/operating system architectures support memory pages greater than the default (usually 4KB). The actual implementation of this support depends on the underlying hardware and operating system. Applications that perform a lot of memory accesses may obtain performance improvements by using large pages due to reduced Translation Lookaside Buffer (TLB) misses.

    MySQL supports the Linux implementation of large page support (which is called HugeTLB in Linux). See Section 8.12.4.2, “Enabling Large Page Support”. For Solaris support of large pages, see the description of the --super-large-pages option.

    --large-pages is disabled by default.

  • --lc-messages=locale_name

    Property Value
    Command-Line Format --lc-messages=name
    System Variable lc_messages
    Scope Global, Session
    Dynamic Yes
    Type String
    Default Value en_US

    The locale to use for error messages. The default is en_US. The server converts the argument to a language name and combines it with the value of --lc-messages-dir to produce the location for the error message file. See Section 10.12, “Setting the Error Message Language”.

  • --lc-messages-dir=dir_name

    Property Value
    Command-Line Format --lc-messages-dir=dir_name
    System Variable lc_messages_dir
    Scope Global
    Dynamic No
    Type Directory name

    The directory where error messages are located. The server uses the value together with the value of --lc-messages to produce the location for the error message file. See Section 10.12, “Setting the Error Message Language”.

  • --local-service

    Property Value
    Command-Line Format --local-service

    (Windows only) A --local-service option following the service name causes the server to run using the LocalService Windows account that has limited system privileges. If both --defaults-file and --local-service are given following the service name, they can be in any order. See Section 2.3.4.8, “Starting MySQL as a Windows Service”.

  • --log-error[=file_name]

    Property Value
    Command-Line Format --log-error[=file_name]
    System Variable log_error
    Scope Global
    Dynamic No
    Type File name

    Write the error log and startup messages to this file. See Section 5.4.2, “The Error Log”.

    If the option names no file, the error log file name on Unix and Unix-like systems is host_name.err in the data directory. The file name on Windows is the same, unless the --pid-file option is specified. In that case, the file name is the PID file base name with a suffix of .err in the data directory.

    If the option names a file, the error log file has that name (with an .err suffix added if the name has no suffix), located under the data directory unless an absolute path name is given to specify a different location.

    On Windows, --console takes precedence over --log-error if both are given. In this case, the server writes the error log to the console rather than to a file. (In MySQL 5.5 and 5.6, this is reversed: --log-error takes precedence over --console if both are given.)

  • --log-isam[=file_name]

    Property Value
    Command-Line Format --log-isam[=file_name]
    Type File name

    Log all MyISAM changes to this file (used only when debugging MyISAM).

  • --log-raw

    Property Value
    Command-Line Format --log-raw[={OFF|ON}]
    Type Boolean
    Default Value OFF

    Passwords in certain statements written to the general query log, slow query log, and binary log are rewritten by the server not to occur literally in plain text. Password rewriting can be suppressed for the general query log by starting the server with the --log-raw option. This option may be useful for diagnostic purposes, to see the exact text of statements as received by the server, but for security reasons is not recommended for production use.

    If a query rewrite plugin is installed, the --log-raw option affects statement logging as follows:

    • Without --log-raw, the server logs the statement returned by the query rewrite plugin. This may differ from the statement as received.

    • With --log-raw, the server logs the original statement as received.

    For more information, see Section 6.1.2.3, “Passwords and Logging”.

  • --log-short-format

    Property Value
    Command-Line Format --log-short-format[={OFF|ON}]
    Type Boolean
    Default Value OFF

    Log less information to the slow query log, if it has been activated.

  • --log-tc=file_name

    Property Value
    Command-Line Format --log-tc=file_name
    Type File name
    Default Value tc.log

    The name of the memory-mapped transaction coordinator log file (for XA transactions that affect multiple storage engines when the binary log is disabled). The default name is tc.log. The file is created under the data directory if not given as a full path name. This option is unused.

  • --log-tc-size=size

    Property Value
    Command-Line Format --log-tc-size=#
    Type Integer
    Default Value (64-bit platforms, >= 5.7.21) 6 * page size
    Default Value (64-bit platforms, <= 5.7.20) 24576
    Default Value (32-bit platforms, >= 5.7.21) 6 * page size
    Default Value (32-bit platforms, <= 5.7.20) 24576
    Minimum Value 6 * page size
    Maximum Value (64-bit platforms) 18446744073709551615
    Maximum Value (32-bit platforms) 4294967295

    The size in bytes of the memory-mapped transaction coordinator log. The default and minimum values are 6 times the page size, and the value must be a multiple of the page size. (Before MySQL 5.7.21, the default size is 24KB.)

  • --log-warnings[=level], -W [level]

    Property Value
    Command-Line Format --log-warnings[=#]
    Deprecated Yes
    System Variable log_warnings
    Scope Global
    Dynamic Yes
    Type Integer
    Default Value 2
    Minimum Value 0
    Maximum Value (64-bit platforms) 18446744073709551615
    Maximum Value (32-bit platforms) 4294967295
    Note

    The log_error_verbosity system variable is preferred over, and should be used instead of, the --log-warnings option or log_warnings system variable. For more information, see the descriptions of log_error_verbosity and log_warnings. The --log-warnings command-line option and log_warnings system variable are deprecated and will be removed in a future MySQL release.

    Whether to produce additional warning messages to the error log. This option is enabled by default. To disable it, use --log-warnings=0. Specifying the option without a level value increments the current value by 1. The server logs messages about statements that are unsafe for statement-based logging if the value is greater than 0. Aborted connections and access-denied errors for new connection attempts are logged if the value is greater than 1. See Section B.4.2.10, “Communication Errors and Aborted Connections”.

  • --memlock

    Property Value
    Command-Line Format --memlock[={OFF|ON}]
    Type Boolean
    Default Value OFF

    Lock the mysqld process in memory. This option might help if you have a problem where the operating system is causing mysqld to swap to disk.

    --memlock works on systems that support the mlockall() system call; this includes Solaris, most Linux distributions that use a 2.4 or higher kernel, and perhaps other Unix systems. On Linux systems, you can tell whether or not mlockall() (and thus this option) is supported by checking to see whether or not it is defined in the system mman.h file, like this:

    shell> grep mlockall /usr/include/sys/mman.h
    

    If mlockall() is supported, you should see in the output of the previous command something like the following:

    extern int mlockall (int __flags) __THROW;
    Important

    Use of this option may require you to run the server as root, which, for reasons of security, is normally not a good idea. See Section 6.1.5, “How to Run MySQL as a Normal User”.

    On Linux and perhaps other systems, you can avoid the need to run the server as root by changing the limits.conf file. See the notes regarding the memlock limit in Section 8.12.4.2, “Enabling Large Page Support”.

    You must not try to use this option on a system that does not support the mlockall() system call; if you do so, mysqld will very likely crash as soon as you try to start it.

  • --myisam-block-size=N

    Property Value
    Command-Line Format --myisam-block-size=#
    Type Integer
    Default Value 1024
    Minimum Value 1024
    Maximum Value 16384

    The block size to be used for MyISAM index pages.

  • --no-defaults

    Do not read any option files. If program startup fails due to reading unknown options from an option file, --no-defaults can be used to prevent them from being read. This must be the first option on the command line if it is used.

    For additional information about this and other option-file options, see Section 4.2.2.3, “Command-Line Options that Affect Option-File Handling”.

  • --old-style-user-limits

    Property Value
    Command-Line Format --old-style-user-limits[={OFF|ON}]
    Type Boolean
    Default Value OFF

    Enable old-style user limits. (Before MySQL 5.0.3, account resource limits were counted separately for each host from which a user connected rather than per account row in the user table.) See Section 6.2.16, “Setting Account Resource Limits”.

  • --partition[=value]

    Property Value
    Command-Line Format --partition[={OFF|ON}]
    Deprecated 5.7.16
    Disabled by skip-partition
    Type Boolean
    Default Value ON

    Enables or disables user-defined partitioning support in the MySQL Server.

    This option is deprecated in MySQL 5.7.16, and is removed from MySQL 8.0 because in MySQL 8.0, the partitioning engine is replaced by native partitioning, which cannot be disabled.

  • --performance-schema-xxx

    Configure a Performance Schema option. For details, see Section 25.14, “Performance Schema Command Options”.

  • --plugin-load=plugin_list

    Property Value
    Command-Line Format --plugin-load=plugin_list
    System Variable plugin_load
    Scope Global
    Dynamic No
    Type String

    This option tells the server to load the named plugins at startup. If multiple --plugin-load options are given, only the last one is used. Additional plugins to load may be specified using --plugin-load-add options.

    The option value is a semicolon-separated list of name=plugin_library and plugin_library values. Each name is the name of a plugin to load, and plugin_library is the name of the library file that contains the plugin code. If a plugin library is named without any preceding plugin name, the server loads all plugins in the library. The server looks for plugin library files in the directory named by the plugin_dir system variable.

    For example, if plugins named myplug1 and myplug2 have library files myplug1.so and myplug2.so, use this option to perform an early plugin load:

    shell> mysqld --plugin-load="myplug1=myplug1.so;myplug2=myplug2.so"
    

    Quotes are used around the argument value here because otherwise semicolon (;) is interpreted as a special character by some command interpreters. (Unix shells treat it as a command terminator, for example.)

    Each named plugin is loaded for a single invocation of mysqld only. After a restart, the plugin is not loaded unless --plugin-load is used again. This is in contrast to INSTALL PLUGIN, which adds an entry to the mysql.plugins table to cause the plugin to be loaded for every normal server startup.

    Under normal startup, the server determines which plugins to load by reading the mysql.plugins system table. If the server is started with the --skip-grant-tables option, it does not consult the mysql.plugins table and does not load plugins listed there. --plugin-load enables plugins to be loaded even when --skip-grant-tables is given. --plugin-load also enables plugins to be loaded at startup that cannot be loaded at runtime.

    For additional information about plugin loading, see Section 5.5.1, “Installing and Uninstalling Plugins”.

  • --plugin-load-add=plugin_list

    Property Value
    Command-Line Format --plugin-load-add=plugin_list
    System Variable plugin_load_add
    Scope Global
    Dynamic No
    Type String

    This option complements the --plugin-load option. --plugin-load-add adds a plugin or plugins to the set of plugins to be loaded at startup. The argument format is the same as for --plugin-load. --plugin-load-add can be used to avoid specifying a large set of plugins as a single long unwieldy --plugin-load argument.

    --plugin-load-add can be given in the absence of --plugin-load, but any instance of --plugin-load-add that appears before --plugin-load. has no effect because --plugin-load resets the set of plugins to load. In other words, these options:

    --plugin-load=x --plugin-load-add=y

    are equivalent to this option:

    --plugin-load="x;y"

    But these options:

    --plugin-load-add=y --plugin-load=x

    are equivalent to this option:

    --plugin-load=x

    For additional information about plugin loading, see Section 5.5.1, “Installing and Uninstalling Plugins”.

  • --plugin-xxx

    Specifies an option that pertains to a server plugin. For example, many storage engines can be built as plugins, and for such engines, options for them can be specified with a --plugin prefix. Thus, the --innodb-file-per-table option for InnoDB can be specified as --plugin-innodb-file-per-table.

    For boolean options that can be enabled or disabled, the --skip prefix and other alternative formats are supported as well (see Section 4.2.2.4, “Program Option Modifiers”). For example, --skip-plugin-innodb-file-per-table disables innodb-file-per-table.

    The rationale for the --plugin prefix is that it enables plugin options to be specified unambiguously if there is a name conflict with a built-in server option. For example, were a plugin writer to name a plugin sql and implement a mode option, the option name might be --sql-mode, which would conflict with the built-in option of the same name. In such cases, references to the conflicting name are resolved in favor of the built-in option. To avoid the ambiguity, users can specify the plugin option as --plugin-sql-mode. Use of the --plugin prefix for plugin options is recommended to avoid any question of ambiguity.

  • --port=port_num, -P port_num

    Property Value
    Command-Line Format --port=port_num
    System Variable port
    Scope Global
    Dynamic No
    Type Integer
    Default Value 3306
    Minimum Value 0
    Maximum Value 65535

    The port number to use when listening for TCP/IP connections. On Unix and Unix-like systems, the port number must be 1024 or higher unless the server is started by the root operating system user. Setting this option to 0 causes the default value to be used.

  • --port-open-timeout=num

    Property Value
    Command-Line Format --port-open-timeout=#
    Type Integer
    Default Value 0

    On some systems, when the server is stopped, the TCP/IP port might not become available immediately. If the server is restarted quickly afterward, its attempt to reopen the port can fail. This option indicates how many seconds the server should wait for the TCP/IP port to become free if it cannot be opened. The default is not to wait.

  • --print-defaults

    Print the program name and all options that it gets from option files. Password values are masked. This must be the first option on the command line if it is used, except that it may be used immediately after --defaults-file or --defaults-extra-file.

    For additional information about this and other option-file options, see Section 4.2.2.3, “Command-Line Options that Affect Option-File Handling”.

  • --remove [service_name]

    Property Value
    Command-Line Format --remove [service_name]
    Platform Specific Windows

    (Windows only) Remove a MySQL Windows service. The default service name is MySQL if no service_name value is given. For more information, see Section 2.3.4.8, “Starting MySQL as a Windows Service”.

  • --safe-user-create

    Property Value
    Command-Line Format --safe-user-create[={OFF|ON}]
    Type Boolean
    Default Value OFF

    If this option is enabled, a user cannot create new MySQL users by using the GRANT statement unless the user has the INSERT privilege for the mysql.user system table or any column in the table. If you want a user to have the ability to create new users that have those privileges that the user has the right to grant, you should grant the user the following privilege:

    GRANT INSERT(user) ON mysql.user TO 'user_name'@'host_name';
    

    This ensures that the user cannot change any privilege columns directly, but has to use the GRANT statement to give privileges to other users.

  • --skip-grant-tables

    Property Value
    Command-Line Format --skip-grant-tables[={OFF|ON}]
    Type Boolean
    Default Value OFF

    This option affects the server startup sequence:

    • --skip-grant-tables causes the server not to read the grant tables in the mysql system database, and thus to start without using the privilege system at all. This gives anyone with access to the server unrestricted access to all databases.

      To cause a server started with --skip-grant-tables to load the grant tables at runtime, perform a privilege-flushing operation, which can be done in these ways:

      Privilege flushing might also occur implicitly as a result of other actions performed after startup, thus causing the server to start using the grant tables. For example, mysql_upgrade flushes the privileges during the upgrade procedure.

    • In addition to causing the startup sequence not to load the grant tables, --skip-grant-tables causes the server not to load certain other objects stored in the mysql system database: plugins that were installed with the INSTALL PLUGIN statement, scheduled events, and user-defined functions (UDFs). To cause plugins to be loaded anyway, use the --plugin-load or --plugin-load-add option.

    • --skip-grant-tables causes the disabled_storage_engines system variable to have no effect.

  • --skip-host-cache

    Property Value
    Command-Line Format --skip-host-cache

    Disable use of the internal host cache for faster name-to-IP resolution. With the cache disabled, the server performs a DNS lookup every time a client connects.

    Use of --skip-host-cache is similar to setting the host_cache_size system variable to 0, but host_cache_size is more flexible because it can also be used to resize, enable, or disable the host cache at runtime, not just at server startup.

    If you start the server with --skip-host-cache, that does not prevent changes to the value of host_cache_size, but such changes have no effect and the cache is not re-enabled even if host_cache_size is set larger than 0.

    For more information about how the host cache works, see Section 8.12.5.2, “DNS Lookup Optimization and the Host Cache”.

  • --skip-innodb

    Disable the InnoDB storage engine. In this case, because the default storage engine is InnoDB, the server will not start unless you also use --default-storage-engine and --default-tmp-storage-engine to set the default to some other engine for both permanent and TEMPORARY tables.

    The InnoDB storage engine cannot be disabled, and the --skip-innodb option is deprecated and has no effect. Its use results in a warning. This option will be removed in a future MySQL release.

  • --skip-new

    Property Value
    Command-Line Format --skip-new

    This option disables (what used to be considered) new, possibly unsafe behaviors. It results in these settings: delay_key_write=OFF, concurrent_insert=NEVER, automatic_sp_privileges=OFF. It also causes OPTIMIZE TABLE to be mapped to ALTER TABLE for storage engines for which OPTIMIZE TABLE is not supported.

  • --skip-partition

    Property Value
    Command-Line Format

    --skip-partition

    --disable-partition

    Deprecated 5.7.16

    Disables user-defined partitioning. Partitioned tables can be seen using SHOW TABLES or by querying the INFORMATION_SCHEMA.TABLES table, but cannot be created or modified, nor can data in such tables be accessed. All partition-specific columns in the INFORMATION_SCHEMA.PARTITIONS table display NULL.

    Since DROP TABLE removes table definition (.frm) files, this statement works on partitioned tables even when partitioning is disabled using the option. The statement, however, does not remove partition definitions associated with partitioned tables in such cases. For this reason, you should avoid dropping partitioned tables with partitioning disabled, or take action to remove orphaned .par files manually (if present).

    Note

    In MySQL 5.7, partition definition (.par) files are no longer created for partitioned InnoDB tables. Instead, partition definitions are stored in the InnoDB internal data dictionary. Partition definition (.par) files continue to be used for partitioned MyISAM tables.

    This option is deprecated in MySQL 5.7.16, and is removed from MySQL 8.0 because in MySQL 8.0, the partitioning engine is replaced by native partitioning, which cannot be disabled.

  • --skip-show-database

    Property Value
    Command-Line Format --skip-show-database
    System Variable skip_show_database
    Scope Global
    Dynamic No

    This option sets the skip_show_database system variable that controls who is permitted to use the SHOW DATABASES statement. See Section 5.1.7, “Server System Variables”.

  • --skip-stack-trace

    Property Value
    Command-Line Format --skip-stack-trace

    Do not write stack traces. This option is useful when you are running mysqld under a debugger. On some systems, you also must use this option to get a core file. See Section 28.5, “Debugging and Porting MySQL”.

  • --slow-start-timeout=timeout

    Property Value
    Command-Line Format --slow-start-timeout=#
    Type Integer
    Default Value 15000

    This option controls the Windows service control manager's service start timeout. The value is the maximum number of milliseconds that the service control manager waits before trying to kill the windows service during startup. The default value is 15000 (15 seconds). If the MySQL service takes too long to start, you may need to increase this value. A value of 0 means there is no timeout.

  • --socket=path

    Property Value
    Command-Line Format --socket={file_name|pipe_name}
    System Variable socket
    Scope Global
    Dynamic No
    Type String
    Default Value (Other) /tmp/mysql.sock
    Default Value (Windows) MySQL

    On Unix, this option specifies the Unix socket file to use when listening for local connections. The default value is /tmp/mysql.sock. If this option is given, the server creates the file in the data directory unless an absolute path name is given to specify a different directory. On Windows, the option specifies the pipe name to use when listening for local connections that use a named pipe. The default value is MySQL (not case-sensitive).

  • --sql-mode=value[,value[,value...]]

    Property Value
    Command-Line Format --sql-mode=name
    System Variable sql_mode
    Scope Global, Session
    Dynamic Yes
    Type Set
    Default Value ONLY_FULL_GROUP_BY STRICT_TRANS_TABLES NO_ZERO_IN_DATE NO_ZERO_DATE ERROR_FOR_DIVISION_BY_ZERO NO_AUTO_CREATE_USER NO_ENGINE_SUBSTITUTION
    Valid Values

    ALLOW_INVALID_DATES

    ANSI_QUOTES

    ERROR_FOR_DIVISION_BY_ZERO

    HIGH_NOT_PRECEDENCE

    IGNORE_SPACE

    NO_AUTO_CREATE_USER

    NO_AUTO_VALUE_ON_ZERO

    NO_BACKSLASH_ESCAPES

    NO_DIR_IN_CREATE

    NO_ENGINE_SUBSTITUTION

    NO_FIELD_OPTIONS

    NO_KEY_OPTIONS

    NO_TABLE_OPTIONS

    NO_UNSIGNED_SUBTRACTION

    NO_ZERO_DATE

    NO_ZERO_IN_DATE

    ONLY_FULL_GROUP_BY

    PAD_CHAR_TO_FULL_LENGTH

    PIPES_AS_CONCAT

    REAL_AS_FLOAT

    STRICT_ALL_TABLES

    STRICT_TRANS_TABLES

    Set the SQL mode. See Section 5.1.10, “Server SQL Modes”.

    Note

    MySQL installation programs may configure the SQL mode during the installation process. If the SQL mode differs from the default or from what you expect, check for a setting in an option file that the server reads at startup.

  • --ssl*

    Options that begin with --ssl specify whether to permit clients to connect using SSL and indicate where to find SSL keys and certificates. See Command Options for Encrypted Connections.

  • --standalone

    Property Value
    Command-Line Format --standalone
    Platform Specific Windows

    Available on Windows only; instructs the MySQL server not to run as a service.

  • --super-large-pages

    Property Value
    Command-Line Format --super-large-pages[={OFF|ON}]
    Platform Specific Solaris
    Type Boolean
    Default Value OFF

    Standard use of large pages in MySQL attempts to use the largest size supported, up to 4MB. Under Solaris, a super large pages feature enables uses of pages up to 256MB. This feature is available for recent SPARC platforms. It can be enabled or disabled by using the --super-large-pages or --skip-super-large-pages option.

  • --symbolic-links, --skip-symbolic-links

    Property Value
    Command-Line Format --symbolic-links[={OFF|ON}]
    Type Boolean
    Default Value ON

    Enable or disable symbolic link support. On Unix, enabling symbolic links means that you can link a MyISAM index file or data file to another directory with the INDEX DIRECTORY or DATA DIRECTORY option of the CREATE TABLE statement. If you delete or rename the table, the files that its symbolic links point to also are deleted or renamed. See Section 8.12.3.2, “Using Symbolic Links for MyISAM Tables on Unix”.

    This option has no meaning on Windows.

  • --sysdate-is-now

    Property Value
    Command-Line Format --sysdate-is-now[={OFF|ON}]
    Type Boolean
    Default Value OFF

    SYSDATE() by default returns the time at which it executes, not the time at which the statement in which it occurs begins executing. This differs from the behavior of NOW(). This option causes SYSDATE() to be an alias for NOW(). For information about the implications for binary logging and replication, see the description for SYSDATE() in Section 12.6, “Date and Time Functions” and for SET TIMESTAMP in Section 5.1.7, “Server System Variables”.

  • --tc-heuristic-recover={COMMIT|ROLLBACK}

    Property Value
    Command-Line Format --tc-heuristic-recover=name
    Type Enumeration
    Default Value COMMIT
    Valid Values

    COMMIT

    ROLLBACK

    The type of decision to use in the heuristic recovery process. To use this option, two or more storage engines that support XA transactions must be installed.

  • --temp-pool

    Property Value
    Command-Line Format --temp-pool[={OFF|ON}]
    Deprecated 5.7.18
    Type Boolean
    Default Value (Other) OFF
    Default Value (Linux) ON

    This option is ignored except on Linux. On Linux, it causes most temporary files created by the server to use a small set of names, rather than a unique name for each new file. This works around a problem in the Linux kernel dealing with creating many new files with different names. With the old behavior, Linux seems to leak memory, because it is being allocated to the directory entry cache rather than to the disk cache.

    As of MySQL 5.7.18, this option is deprecated and is removed in MySQL 8.0.

  • --transaction-isolation=level

    Property Value
    Command-Line Format --transaction-isolation=name
    System Variable (>= 5.7.20) transaction_isolation
    Scope (>= 5.7.20) Global, Session
    Dynamic (>= 5.7.20) Yes
    Type Enumeration
    Default Value REPEATABLE-READ
    Valid Values

    READ-UNCOMMITTED

    READ-COMMITTED

    REPEATABLE-READ

    SERIALIZABLE

    Sets the default transaction isolation level. The level value can be READ-UNCOMMITTED, READ-COMMITTED, REPEATABLE-READ, or SERIALIZABLE. See Section 13.3.6, “SET TRANSACTION Statement”.

    The default transaction isolation level can also be set at runtime using the SET TRANSACTION statement or by setting the tx_isolation (or, as of MySQL 5.7.20, transaction_isolation) system variable.

  • --transaction-read-only

    Property Value
    Command-Line Format --transaction-read-only[={OFF|ON}]
    System Variable (>= 5.7.20) transaction_read_only
    Scope (>= 5.7.20) Global, Session
    Dynamic (>= 5.7.20) Yes
    Type Boolean
    Default Value OFF

    Sets the default transaction access mode. By default, read-only mode is disabled, so the mode is read/write.

    To set the default transaction access mode at runtime, use the SET TRANSACTION statement or set the tx_read_only (or, as of MySQL 5.7.20, transaction_read_only) system variable. See Section 13.3.6, “SET TRANSACTION Statement”.

  • --tmpdir=dir_name, -t dir_name

    Property Value
    Command-Line Format --tmpdir=dir_name
    System Variable tmpdir
    Scope Global
    Dynamic No
    Type Directory name

    The path of the directory to use for creating temporary files. It might be useful if your default /tmp directory resides on a partition that is too small to hold temporary tables. This option accepts several paths that are used in round-robin fashion. Paths should be separated by colon characters (:) on Unix and semicolon characters (;) on Windows.

    --tmpdir can be a non-permanent location, such as a directory on a memory-based file system or a directory that is cleared when the server host restarts. If the MySQL server is acting as a replication slave, and you are using a non-permanent location for --tmpdir, consider setting a different temporary directory for the slave using the slave_load_tmpdir system variable. For a replication slave, the temporary files used to replicate LOAD DATA statements are stored in this directory, so with a permanent location they can survive machine restarts, although replication can now continue after a restart if the temporary files have been removed.

    For more information about the storage location of temporary files, see Section B.4.3.5, “Where MySQL Stores Temporary Files”.

  • --user={user_name|user_id}, -u {user_name|user_id}

    Property Value
    Command-Line Format --user=name
    Type String

    Run the mysqld server as the user having the name user_name or the numeric user ID user_id. (User in this context refers to a system login account, not a MySQL user listed in the grant tables.)

    This option is mandatory when starting mysqld as root. The server changes its user ID during its startup sequence, causing it to run as that particular user rather than as root. See Section 6.1.1, “Security Guidelines”.

    To avoid a possible security hole where a user adds a --user=root option to a my.cnf file (thus causing the server to run as root), mysqld uses only the first --user option specified and produces a warning if there are multiple --user options. Options in /etc/my.cnf and $MYSQL_HOME/my.cnf are processed before command-line options, so it is recommended that you put a --user option in /etc/my.cnf and specify a value other than root. The option in /etc/my.cnf is found before any other --user options, which ensures that the server runs as a user other than root, and that a warning results if any other --user option is found.

  • --verbose, -v

    Use this option with the --help option for detailed help.

  • --version, -V

    Display version information and exit.

5.1.7 Server System Variables

The MySQL server maintains many system variables that configure its operation. Each system variable has a default value. System variables can be set at server startup using options on the command line or in an option file. Most of them can be changed dynamically at runtime using the SET statement, which enables you to modify operation of the server without having to stop and restart it. You can also use system variable values in expressions.

At runtime, setting a global system variable value requires the SUPER privilege. Setting a session system variable value normally requires no special privileges and can be done by any user, although there are exceptions. For more information, see Section 5.1.8.1, “System Variable Privileges”

There are several ways to see the names and values of system variables:

  • To see the values that a server will use based on its compiled-in defaults and any option files that it reads, use this command:

    mysqld --verbose --help
  • To see the values that a server will use based only on its compiled-in defaults, ignoring the settings in any option files, use this command:

    mysqld --no-defaults --verbose --help
  • To see the current values used by a running server, use the SHOW VARIABLES statement or the Performance Schema system variable tables. See Section 25.12.13, “Performance Schema System Variable Tables”.

This section provides a description of each system variable. For a system variable summary table, see Section 5.1.4, “Server System Variable Reference”. For more information about manipulation of system variables, see Section 5.1.8, “Using System Variables”.

For additional system variable information, see these sections:

Note

Some of the following variable descriptions refer to enabling or disabling a variable. These variables can be enabled with the SET statement by setting them to ON or 1, or disabled by setting them to OFF or 0. Boolean variables can be set at startup to the values ON, TRUE, OFF, and FALSE (not case-sensitive), as well as 1 and 0. See Section 4.2.2.4, “Program Option Modifiers”.

Some system variables control the size of buffers or caches. For a given buffer, the server might need to allocate internal data structures. These structures typically are allocated from the total memory allocated to the buffer, and the amount of space required might be platform dependent. This means that when you assign a value to a system variable that controls a buffer size, the amount of space actually available might differ from the value assigned. In some cases, the amount might be less than the value assigned. It is also possible that the server will adjust a value upward. For example, if you assign a value of 0 to a variable for which the minimal value is 1024, the server will set the value to 1024.

Values for buffer sizes, lengths, and stack sizes are given in bytes unless otherwise specified.

Some system variables take file name values. Unless otherwise specified, the default file location is the data directory if the value is a relative path name. To specify the location explicitly, use an absolute path name. Suppose that the data directory is /var/mysql/data. If a file-valued variable is given as a relative path name, it will be located under /var/mysql/data. If the value is an absolute path name, its location is as given by the path name.

  • authentication_windows_log_level

    Property Value
    Command-Line Format --authentication-windows-log-level=#
    System Variable authentication_windows_log_level
    Scope Global
    Dynamic No
    Type Integer
    Default Value 2
    Minimum Value 0
    Maximum Value 4

    This variable is available only if the authentication_windows Windows authentication plugin is enabled and debugging code is enabled. See Section 6.4.1.8, “Windows Pluggable Authentication”.

    This variable sets the logging level for the Windows authentication plugin. The following table shows the permitted values.

    Value Description
    0 No logging
    1 Log only error messages
    2 Log level 1 messages and warning messages
    3 Log level 2 messages and information notes
    4 Log level 3 messages and debug messages
  • authentication_windows_use_principal_name

    Property Value
    Command-Line Format --authentication-windows-use-principal-name[={OFF|ON}]
    System Variable authentication_windows_use_principal_name
    Scope Global
    Dynamic No
    Type Boolean
    Default Value ON

    This variable is available only if the authentication_windows Windows authentication plugin is enabled. See Section 6.4.1.8, “Windows Pluggable Authentication”.

    A client that authenticates using the InitSecurityContext() function should provide a string identifying the service to which it connects (targetName). MySQL uses the principal name (UPN) of the account under which the server is running. The UPN has the form user_id@computer_name and need not be registered anywhere to be used. This UPN is sent by the server at the beginning of authentication handshake.

    This variable controls whether the server sends the UPN in the initial challenge. By default, the variable is enabled. For security reasons, it can be disabled to avoid sending the server's account name to a client as cleartext. If the variable is disabled, the server always sends a 0x00 byte in the first challenge, the client does not specify targetName, and as a result, NTLM authentication is used.

    If the server fails to obtain its UPN (which will happen primarily in environments that do not support Kerberos authentication), the UPN is not sent by the server and NTLM authentication is used.

  • autocommit

    Property Value
    Command-Line Format --autocommit[={OFF|ON}]
    System Variable autocommit
    Scope Global, Session
    Dynamic Yes
    Type Boolean
    Default Value ON

    The autocommit mode. If set to 1, all changes to a table take effect immediately. If set to 0, you must use COMMIT to accept a transaction or ROLLBACK to cancel it. If autocommit is 0 and you change it to 1, MySQL performs an automatic COMMIT of any open transaction. Another way to begin a transaction is to use a START TRANSACTION or BEGIN statement. See Section 13.3.1, “START TRANSACTION, COMMIT, and ROLLBACK Statements”.

    By default, client connections begin with autocommit set to 1. To cause clients to begin with a default of 0, set the global autocommit value by starting the server with the --autocommit=0 option. To set the variable using an option file, include these lines:

    [mysqld]
    autocommit=0
  • automatic_sp_privileges

    Property Value
    Command-Line Format --automatic-sp-privileges[={OFF|ON}]
    System Variable automatic_sp_privileges
    Scope Global
    Dynamic Yes
    Type Boolean
    Default Value ON

    When this variable has a value of 1 (the default), the server automatically grants the EXECUTE and ALTER ROUTINE privileges to the creator of a stored routine, if the user cannot already execute and alter or drop the routine. (The ALTER ROUTINE privilege is required to drop the routine.) The server also automatically drops those privileges from the creator when the routine is dropped. If automatic_sp_privileges is 0, the server does not automatically add or drop these privileges.

    The creator of a routine is the account used to execute the CREATE statement for it. This might not be the same as the account named as the DEFINER in the routine definition.

    If you start mysqld with --skip-new, automatic_sp_privileges is set to OFF.

    See also Section 23.2.2, “Stored Routines and MySQL Privileges”.

  • auto_generate_certs

    Property Value
    Command-Line Format --auto-generate-certs[={OFF|ON}]
    System Variable auto_generate_certs
    Scope Global
    Dynamic No
    Type Boolean
    Default Value ON

    This variable is available if the server was compiled using OpenSSL (see Section 6.3.4, “SSL Library-Dependent Capabilities”). It controls whether the server autogenerates SSL key and certificate files in the data directory, if they do not already exist.

    At startup, the server automatically generates server-side and client-side SSL certificate and key files in the data directory if the auto_generate_certs system variable is enabled, no SSL options other than --ssl are specified, and the server-side SSL files are missing from the data directory. These files enable secure client connections using SSL; see Section 6.3.1, “Configuring MySQL to Use Encrypted Connections”.

    For more information about SSL file autogeneration, including file names and characteristics, see Section 6.3.3.1, “Creating SSL and RSA Certificates and Keys using MySQL”

    The sha256_password_auto_generate_rsa_keys system variable is related but controls autogeneration of RSA key-pair files needed for secure password exchange using RSA over unencypted connections.

  • avoid_temporal_upgrade

    Property Value
    Command-Line Format --avoid-temporal-upgrade[={OFF|ON}]
    Deprecated Yes
    System Variable avoid_temporal_upgrade
    Scope Global
    Dynamic Yes
    Type Boolean
    Default Value OFF

    This variable controls whether ALTER TABLE implicitly upgrades temporal columns found to be in pre-5.6.4 format (TIME, DATETIME, and TIMESTAMP columns without support for fractional seconds precision). Upgrading such columns requires a table rebuild, which prevents any use of fast alterations that might otherwise apply to the operation to be performed.

    This variable is disabled by default. Enabling it causes ALTER TABLE not to rebuild temporal columns and thereby be able to take advantage of possible fast alterations.

    This variable is deprecated and will be removed in a future MySQL release.

  • back_log

    Property Value
    Command-Line Format --back-log=#
    System Variable back_log
    Scope Global
    Dynamic No
    Type Integer
    Default Value -1 (signifies autosizing; do not assign this literal value)
    Minimum Value 1
    Maximum Value 65535

    The number of outstanding connection requests MySQL can have. This comes into play when the main MySQL thread gets very many connection requests in a very short time. It then takes some time (although very little) for the main thread to check the connection and start a new thread. The back_log value indicates how many requests can be stacked during this short time before MySQL momentarily stops answering new requests. You need to increase this only if you expect a large number of connections in a short period of time.

    In other words, this value is the size of the listen queue for incoming TCP/IP connections. Your operating system has its own limit on the size of this queue. The manual page for the Unix listen() system call should have more details. Check your OS documentation for the maximum value for this variable. back_log cannot be set higher than your operating system limit.

    The default value is based on the following formula, capped to a limit of 900:

    50 + (max_connections / 5)
  • basedir

    Property Value
    Command-Line Format --basedir=dir_name
    System Variable basedir
    Scope Global
    Dynamic No
    Type Directory name
    Default Value configuration-dependent default

    The path to the MySQL installation base directory.

  • big_tables

    Property Value
    Command-Line Format --big-tables[={OFF|ON}]
    System Variable big_tables
    Scope Global, Session
    Dynamic Yes
    Type Boolean
    Default Value OFF

    If enabled, the server stores all temporary tables on disk rather than in memory. This prevents most The table tbl_name is full errors for SELECT operations that require a large temporary table, but also slows down queries for which in-memory tables would suffice.

    The default value for new connection is OFF (use in-memory temporary tables). Normally, it should never be necessary to enable this variable because the server is able to handle large result sets automatically by using memory for small temporary tables and switching to disk-based tables as required.

  • bind_address

    Property Value
    Command-Line Format --bind-address=addr
    System Variable bind_address
    Scope Global
    Dynamic No
    Type String
    Default Value *

    The MySQL server listens on a single network socket for TCP/IP connections. This socket is bound to a single address, but it is possible for an address to map onto multiple network interfaces. To specify an address, set bind_address=addr at server startup, where addr is an IPv4 or IPv6 address or a host name. If addr is a host name, the server resolves the name to an IP address and binds to that address. If a host name resolves to multiple IP addresses, the server uses the first IPv4 address if there are any, or the first IPv6 address otherwise.

    The server treats different types of addresses as follows:

    • If the address is *, the server accepts TCP/IP connections on all server host IPv4 interfaces, and, if the server host supports IPv6, on all IPv6 interfaces. Use this address to permit both IPv4 and IPv6 connections on all server interfaces. This value is the default.

    • If the address is 0.0.0.0, the server accepts TCP/IP connections on all server host IPv4 interfaces.

    • If the address is ::, the server accepts TCP/IP connections on all server host IPv4 and IPv6 interfaces.

    • If the address is an IPv4-mapped address, the server accepts TCP/IP connections for that address, in either IPv4 or IPv6 format. For example, if the server is bound to ::ffff:127.0.0.1, clients can connect using --host=127.0.0.1 or --host=::ffff:127.0.0.1.

    • If the address is a regular IPv4 or IPv6 address (such as 127.0.0.1 or ::1), the server accepts TCP/IP connections only for that IPv4 or IPv6 address.

    If binding to the address fails, the server produces an error and does not start.

    If you intend to bind the server to a specific address, be sure that the mysql.user system table contains an account with administrative privileges that you can use to connect to that address. Otherwise, you will not be able to shut down the server. For example, if you bind the server to *, you can connect to it using all existing accounts. But if you bind the server to ::1, it accepts connections only on that address. In that case, first make sure that the 'root'@'::1' account is present in the mysql.user table so you can still connect to the server to shut it down.

    This variable has no effect for the embedded server (libmysqld) and is not visible within the embedded server.

  • block_encryption_mode

    Property Value
    Command-Line Format --block-encryption-mode=#
    System Variable block_encryption_mode
    Scope Global, Session
    Dynamic Yes
    Type String
    Default Value aes-128-ecb

    This variable controls the block encryption mode for block-based algorithms such as AES. It affects encryption for AES_ENCRYPT() and AES_DECRYPT().

    block_encryption_mode takes a value in aes-keylen-mode format, where keylen is the key length in bits and mode is the encryption mode. The value is not case-sensitive. Permitted keylen values are 128, 192, and 256. Permitted encryption modes depend on whether MySQL was compiled using OpenSSL or yaSSL:

    • For OpenSSL, permitted mode values are: ECB, CBC, CFB1, CFB8, CFB128, OFB

    • For yaSSL, permitted mode values are: ECB, CBC

    For example, this statement causes the AES encryption functions to use a key length of 256 bits and the CBC mode:

    SET block_encryption_mode = 'aes-256-cbc';

    An error occurs for attempts to set block_encryption_mode to a value containing an unsupported key length or a mode that the SSL library does not support.

  • bulk_insert_buffer_size

    Property Value
    Command-Line Format --bulk-insert-buffer-size=#
    System Variable bulk_insert_buffer_size
    Scope Global, Session
    Dynamic Yes
    Type Integer
    Default Value 8388608
    Minimum Value 0
    Maximum Value (64-bit platforms) 18446744073709551615
    Maximum Value (32-bit platforms) 4294967295

    MyISAM uses a special tree-like cache to make bulk inserts faster for INSERT ... SELECT, INSERT ... VALUES (...), (...), ..., and LOAD DATA when adding data to nonempty tables. This variable limits the size of the cache tree in bytes per thread. Setting it to 0 disables this optimization. The default value is 8MB.

  • character_set_client

    Property Value
    System Variable character_set_client
    Scope Global, Session
    Dynamic Yes
    Type String
    Default Value utf8

    The character set for statements that arrive from the client. The session value of this variable is set using the character set requested by the client when the client connects to the server. (Many clients support a --default-character-set option to enable this character set to be specified explicitly. See also Section 10.4, “Connection Character Sets and Collations”.) The global value of the variable is used to set the session value in cases when the client-requested value is unknown or not available, or the server is configured to ignore client requests:

    • The client requests a character set not known to the server. For example, a Japanese-enabled client requests sjis when connecting to a server not configured with sjis support.

    • The client is from a version of MySQL older than MySQL 4.1, and thus does not request a character set.

    • mysqld was started with the --skip-character-set-client-handshake option, which causes it to ignore client character set configuration. This reproduces MySQL 4.0 behavior and is useful should you wish to upgrade the server without upgrading all the clients.

    Some character sets cannot be used as the client character set. Attempting to use them as the character_set_client value produces an error. See Impermissible Client Character Sets.

  • character_set_connection

    Property Value
    System Variable character_set_connection
    Scope Global, Session
    Dynamic Yes
    Type String
    Default Value utf8

    The character set used for literals specified without a character set introducer and for number-to-string conversion. For information about introducers, see Section 10.3.8, “Character Set Introducers”.

  • character_set_database

    Property Value
    System Variable character_set_database
    Scope Global, Session
    Dynamic Yes
    Type String
    Default Value latin1
    Footnote This option is dynamic, but only the server should set this information. You should not set the value of this variable manually.

    The character set used by the default database. The server sets this variable whenever the default database changes. If there is no default database, the variable has the same value as character_set_server.

    The global character_set_database and collation_database system variables are deprecated in MySQL 5.7 and will be removed in a future version of MySQL.

    Assigning a value to the session character_set_database and collation_database system variables is deprecated in MySQL 5.7 and assignments produce a warning. The session variables will become read only in a future version of MySQL and assignments will produce an error. It will remain possible to access the session variables to determine the database character set and collation for the default database.

  • character_set_filesystem

    Property Value
    Command-Line Format --character-set-filesystem=name
    System Variable character_set_filesystem
    Scope Global, Session
    Dynamic Yes
    Type String
    Default Value binary

    The file system character set. This variable is used to interpret string literals that refer to file names, such as in the LOAD DATA and SELECT ... INTO OUTFILE statements and the LOAD_FILE() function. Such file names are converted from character_set_client to character_set_filesystem before the file opening attempt occurs. The default value is binary, which means that no conversion occurs. For systems on which multibyte file names are permitted, a different value may be more appropriate. For example, if the system represents file names using UTF-8, set character_set_filesystem to 'utf8mb4'.

  • character_set_results

    Property Value
    System Variable character_set_results
    Scope Global, Session
    Dynamic Yes
    Type String
    Default Value utf8

    The character set used for returning query results to the client. This includes result data such as column values, result metadata such as column names, and error messages.

  • character_set_server

    Property Value
    Command-Line Format --character-set-server=name
    System Variable character_set_server
    Scope Global, Session
    Dynamic Yes
    Type String
    Default Value latin1

    The servers default character set. See Section 10.15, “Character Set Configuration”. If you set this variable, you should also set collation_server to specify the collation for the character set.

  • character_set_system

    Property Value
    System Variable character_set_system
    Scope Global
    Dynamic No
    Type String
    Default Value utf8

    The character set used by the server for storing identifiers. The value is always utf8.

  • character_sets_dir

    Property Value
    Command-Line Format --character-sets-dir=dir_name
    System Variable character_sets_dir
    Scope Global
    Dynamic No
    Type Directory name

    The directory where character sets are installed. See Section 10.15, “Character Set Configuration”.

  • check_proxy_users

    Property Value
    Command-Line Format --check-proxy-users[={OFF|ON}]
    System Variable check_proxy_users
    Scope Global
    Dynamic Yes
    Type Boolean
    Default Value OFF

    Some authentication plugins implement proxy user mapping for themselves (for example, the PAM and Windows authentication plugins). Other authentication plugins do not support proxy users by default. Of these, some can request that the MySQL server itself map proxy users according to granted proxy privileges: mysql_native_password, sha256_password.

    If the check_proxy_users system variable is enabled, the server performs proxy user mapping for any authentication plugins that make such a request. However, it may also be necessary to enable plugin-specific system variables to take advantage of server proxy user mapping support:

    For information about user proxying, see Section 6.2.14, “Proxy Users”.

  • collation_connection

    Property Value
    System Variable collation_connection
    Scope Global, Session
    Dynamic Yes
    Type String

    The collation of the connection character set. collation_connection is important for comparisons of literal strings. For comparisons of strings with column values, collation_connection does not matter because columns have their own collation, which has a higher collation precedence (see Section 10.8.4, “Collation Coercibility in Expressions”).

  • collation_database

    Property Value
    System Variable collation_database
    Scope Global, Session
    Dynamic Yes
    Type String
    Default Value latin1_swedish_ci
    Footnote This option is dynamic, but only the server should set this information. You should not set the value of this variable manually.

    The collation used by the default database. The server sets this variable whenever the default database changes. If there is no default database, the variable has the same value as collation_server.

    The global character_set_database and collation_database system variables are deprecated in MySQL 5.7 and will be removed in a future version of MySQL.

    Assigning a value to the session character_set_database and collation_database system variables is deprecated in MySQL 5.7 and assignments produce a warning. The session variables will become read only in a future version of MySQL and assignments will produce an error. It will remain possible to access the session variables to determine the database character set and collation for the default database.

  • collation_server

    Property Value
    Command-Line Format --collation-server=name
    System Variable collation_server
    Scope Global, Session
    Dynamic Yes
    Type String
    Default Value latin1_swedish_ci

    The server's default collation. See Section 10.15, “Character Set Configuration”.

  • completion_type

    Property Value
    Command-Line Format --completion-type=#
    System Variable completion_type
    Scope Global, Session
    Dynamic Yes
    Type Enumeration
    Default Value NO_CHAIN
    Valid Values

    NO_CHAIN

    CHAIN

    RELEASE

    0

    1

    2

    The transaction completion type. This variable can take the values shown in the following table. The variable can be assigned using either the name values or corresponding integer values.

    Value Description
    NO_CHAIN (or 0) COMMIT and ROLLBACK are unaffected. This is the default value.
    CHAIN (or 1) COMMIT and ROLLBACK are equivalent to COMMIT AND CHAIN and ROLLBACK AND CHAIN, respectively. (A new transaction starts immediately with the same isolation level as the just-terminated transaction.)
    RELEASE (or 2) COMMIT and ROLLBACK are equivalent to COMMIT RELEASE and ROLLBACK RELEASE, respectively. (The server disconnects after terminating the transaction.)

    completion_type affects transactions that begin with START TRANSACTION or BEGIN and end with COMMIT or ROLLBACK. It does not apply to implicit commits resulting from execution of the statements listed in Section 13.3.3, “Statements That Cause an Implicit Commit”. It also does not apply for XA COMMIT, XA ROLLBACK, or when autocommit=1.

  • concurrent_insert

    Property Value
    Command-Line Format --concurrent-insert[=value]
    System Variable concurrent_insert
    Scope Global
    Dynamic Yes
    Type Enumeration
    Default Value AUTO
    Valid Values

    NEVER

    AUTO

    ALWAYS

    0

    1

    2

    If AUTO (the default), MySQL permits INSERT and SELECT statements to run concurrently for MyISAM tables that have no free blocks in the middle of the data file.

    This variable can take the values shown in the following table. The variable can be assigned using either the name values or corresponding integer values.

    Value Description
    NEVER (or 0) Disables concurrent inserts
    AUTO (or 1) (Default) Enables concurrent insert for MyISAM tables that do not have holes
    ALWAYS (or 2) Enables concurrent inserts for all MyISAM tables, even those that have holes. For a table with a hole, new rows are inserted at the end of the table if it is in use by another thread. Otherwise, MySQL acquires a normal write lock and inserts the row into the hole.

    If you start mysqld with --skip-new, concurrent_insert is set to NEVER.

    See also Section 8.11.3, “Concurrent Inserts”.

  • connect_timeout

    Property Value
    Command-Line Format --connect-timeout=#
    System Variable connect_timeout
    Scope Global
    Dynamic Yes
    Type Integer
    Default Value 10
    Minimum Value 2
    Maximum Value 31536000

    The number of seconds that the mysqld server waits for a connect packet before responding with Bad handshake. The default value is 10 seconds.

    Increasing the connect_timeout value might help if clients frequently encounter errors of the form Lost connection to MySQL server at 'XXX', system error: errno.

  • core_file

    Property Value
    System Variable core_file
    Scope Global
    Dynamic No
    Type Boolean
    Default Value OFF

    Whether to write a core file if the server crashes. This variable is set by the --core-file option.

  • datadir

    Property Value
    Command-Line Format --datadir=dir_name
    System Variable datadir
    Scope Global
    Dynamic No
    Type Directory name

    The path to the MySQL server data directory. Relative paths are resolved with respect to the current directory. If the server will be started automatically (that is, in contexts for which you cannot assume what the current directory will be), it is best to specify the datadir value as an absolute path.

  • date_format

    This variable is unused. It is deprecated and is removed in MySQL 8.0.

  • datetime_format

    This variable is unused. It is deprecated and is removed in MySQL 8.0.

  • debug

    Property Value
    Command-Line Format --debug[=debug_options]
    System Variable debug
    Scope Global, Session
    Dynamic Yes
    Type String
    Default Value (Windows) d:t:i:O,\mysqld.trace
    Default Value (Unix) d:t:i:o,/tmp/mysqld.trace

    This variable indicates the current debugging settings. It is available only for servers built with debugging support. The initial value comes from the value of instances of the --debug option given at server startup. The global and session values may be set at runtime.

    Setting the session value of this system variable is a restricted operation. The session user must have privileges sufficient to set restricted session variables. See Section 5.1.8.1, “System Variable Privileges”.

    Assigning a value that begins with + or - cause the value to added to or subtracted from the current value:

    mysql> SET debug = 'T';
    mysql> SELECT @@debug;
    +---------+
    | @@debug |
    +---------+
    | T       |
    +---------+
    
    mysql> SET debug = '+P';
    mysql> SELECT @@debug;
    +---------+
    | @@debug |
    +---------+
    | P:T     |
    +---------+
    
    mysql> SET debug = '-P';
    mysql> SELECT @@debug;
    +---------+
    | @@debug |
    +---------+
    | T       |
    +---------+
    

    For more information, see Section 28.5.3, “The DBUG Package”.

  • debug_sync

    Property Value
    System Variable debug_sync
    Scope Session
    Dynamic Yes
    Type String

    This variable is the user interface to the Debug Sync facility. Use of Debug Sync requires that MySQL be configured with the -DENABLE_DEBUG_SYNC=1 CMake option (see Section 2.9.7, “MySQL Source-Configuration Options”). If Debug Sync is not compiled in, this system variable is not available.

    The global variable value is read only and indicates whether the facility is enabled. By default, Debug Sync is disabled and the value of debug_sync is OFF. If the server is started with --debug-sync-timeout=N, where N is a timeout value greater than 0, Debug Sync is enabled and the value of debug_sync is ON - current signal followed by the signal name. Also, N becomes the default timeout for individual synchronization points.

    The session value can be read by any user and will have the same value as the global variable. The session value can be set to control synchronization points.

    Setting the session value of this system variable is a restricted operation. The session user must have privileges sufficient to set restricted session variables. See Section 5.1.8.1, “System Variable Privileges”.

    For a description of the Debug Sync facility and how to use synchronization points, see MySQL Internals: Test Synchronization.

  • default_authentication_plugin

    Property Value
    Command-Line Format --default-authentication-plugin=plugin_name
    System Variable default_authentication_plugin
    Scope Global
    Dynamic No
    Type Enumeration
    Default Value mysql_native_password
    Valid Values

    mysql_native_password

    sha256_password

    The default authentication plugin. These values are permitted:

    Note

    If this variable has a value other than mysql_native_password, clients older than MySQL 5.5.7 cannot connect because, of the permitted default authentication plugins, they understand only the mysql_native_password authentication protocol.

    The default_authentication_plugin value affects these aspects of server operation:

    • It determines which authentication plugin the server assigns to new accounts created by CREATE USER and GRANT statements that do not explicitly specify an authentication plugin.

    • The old_passwords system variable affects password hashing for accounts that use the mysql_native_password or sha256_password authentication plugin. If the default authentication plugin is one of those plugins, the server sets old_passwords at startup to the value required by the plugin password hashing method.

    • For an account created with either of the following statements, the server associates the account with the default authentication plugin and assigns the account the given password, hashed as required by that plugin:

      CREATE USER ... IDENTIFIED BY 'cleartext password';
      GRANT ...  IDENTIFIED BY 'cleartext password';
      
    • For an account created with either of the following statements, the server associates the account with the default authentication plugin and assigns the account the given password hash, if the password hash has the format required by the plugin:

      CREATE USER ... IDENTIFIED BY PASSWORD 'encrypted password';
      GRANT ...  IDENTIFIED BY PASSWORD 'encrypted password';
      

      If the password hash is not in the format required by the default authentication plugin, the statement fails.

  • default_password_lifetime

    Property Value
    Command-Line Format --default-password-lifetime=#
    System Variable default_password_lifetime
    Scope Global
    Dynamic Yes
    Type Integer
    Default Value (>= 5.7.11) 0
    Default Value (<= 5.7.10) 360
    Minimum Value 0
    Maximum Value 65535

    This variable defines the global automatic password expiration policy. The default default_password_lifetime value is 0, which disables automatic password expiration. If the value of default_password_lifetime is a positive integer N, it indicates the permitted password lifetime; passwords must be changed every N days.

    The global password expiration policy can be overridden as desired for individual accounts using the password expiration options of the ALTER USER statement. See Section 6.2.11, “Password Management”.

    Note

    Prior to MySQL 5.7.11, the default default_password_lifetime value is 360 (passwords must be changed approximately once per year). For those versions, be aware that, if you make no changes to the default_password_lifetime variable or to individual user accounts, all user passwords will expire after 360 days, and all user accounts will start running in restricted mode when this happens. Clients (which are effectively users) connecting to the server will then get an error indicating that the password must be changed: ERROR 1820 (HY000): You must reset your password using ALTER USER statement before executing this statement.

    However, this is easy to miss for clients that automatically connect to the server, such as connections made from scripts. To avoid having such clients suddenly stop working due to a password expiring, make sure to change the password expiration settings for those clients, like this:

    ALTER USER 'script'@'localhost' PASSWORD EXPIRE NEVER

    Alternatively, set the default_password_lifetime variable to 0, thus disabling automatic password expiration for all users.

  • default_storage_engine

    Property Value
    Command-Line Format --default-storage-engine=name
    System Variable default_storage_engine
    Scope Global, Session
    Dynamic Yes
    Type Enumeration
    Default Value InnoDB

    The default storage engine for tables. See Chapter 15, Alternative Storage Engines. This variable sets the storage engine for permanent tables only. To set the storage engine for TEMPORARY tables, set the default_tmp_storage_engine system variable.

    To see which storage engines are available and enabled, use the SHOW ENGINES statement or query the INFORMATION_SCHEMA ENGINES table.

    If you disable the default storage engine at server startup, you must set the default engine for both permanent and TEMPORARY tables to a different engine or the server will not start.

  • default_tmp_storage_engine

    Property Value
    Command-Line Format --default-tmp-storage-engine=name
    System Variable default_tmp_storage_engine
    Scope Global, Session
    Dynamic Yes
    Type Enumeration
    Default Value InnoDB

    The default storage engine for TEMPORARY tables (created with CREATE TEMPORARY TABLE). To set the storage engine for permanent tables, set the default_storage_engine system variable. Also see the discussion of that variable regarding possible values.

    If you disable the default storage engine at server startup, you must set the default engine for both permanent and TEMPORARY tables to a different engine or the server will not start.

  • default_week_format

    Property Value
    Command-Line Format --default-week-format=#
    System Variable default_week_format
    Scope Global, Session
    Dynamic Yes
    Type Integer
    Default Value 0
    Minimum Value 0
    Maximum Value 7

    The default mode value to use for the WEEK() function. See Section 12.6, “Date and Time Functions”.

  • delay_key_write

    Property Value
    Command-Line Format --delay-key-write[={OFF|ON|ALL}]
    System Variable delay_key_write
    Scope Global
    Dynamic Yes
    Type Enumeration
    Default Value ON
    Valid Values

    ON

    OFF

    ALL

    This variable specifies how to use delayed key writes. It applies only to MyISAM tables. Delayed key writing causes key buffers not to be flushed between writes. See also Section 15.2.1, “MyISAM Startup Options”.

    This variable can have one of the following values to affect handling of the DELAY_KEY_WRITE table option that can be used in CREATE TABLE statements.

    Option Description
    OFF DELAY_KEY_WRITE is ignored.
    ON MySQL honors any DELAY_KEY_WRITE option specified in CREATE TABLE statements. This is the default value.
    ALL All new opened tables are treated as if they were created with the DELAY_KEY_WRITE option enabled.
    Note

    If you set this variable to ALL, you should not use MyISAM tables from within another program (such as another MySQL server or myisamchk) when the tables are in use. Doing so leads to index corruption.

    If DELAY_KEY_WRITE is enabled for a table, the key buffer is not flushed for the table on every index update, but only when the table is closed. This speeds up writes on keys a lot, but if you use this feature, you should add automatic checking of all MyISAM tables by starting the server with the myisam_recover_options system variable set (for example, myisam_recover_options='BACKUP,FORCE'). See Section 5.1.7, “Server System Variables”, and Section 15.2.1, “MyISAM Startup Options”.

    If you start mysqld with --skip-new, delay_key_write is set to OFF.

    Warning

    If you enable external locking with --external-locking, there is no protection against index corruption for tables that use delayed key writes.

  • delayed_insert_limit

    Property Value
    Command-Line Format --delayed-insert-limit=#
    Deprecated Yes
    System Variable delayed_insert_limit
    Scope Global
    Dynamic Yes
    Type Integer
    Default Value 100
    Minimum Value 1
    Maximum Value (64-bit platforms) 18446744073709551615
    Maximum Value (32-bit platforms) 4294967295

    This system variable is deprecated (because DELAYED inserts are not supported), and will be removed in a future release.

  • delayed_insert_timeout

    Property Value
    Command-Line Format --delayed-insert-timeout=#
    Deprecated Yes
    System Variable delayed_insert_timeout
    Scope Global
    Dynamic Yes
    Type Integer
    Default Value 300

    This system variable is deprecated (because DELAYED inserts are not supported), and will be removed in a future release.

  • delayed_queue_size

    Property Value
    Command-Line Format --delayed-queue-size=#
    Deprecated Yes
    System Variable delayed_queue_size
    Scope Global
    Dynamic Yes
    Type Integer
    Default Value 1000
    Minimum Value 1
    Maximum Value (64-bit platforms) 18446744073709551615
    Maximum Value (32-bit platforms) 4294967295

    This system variable is deprecated (because DELAYED inserts are not supported), and will be removed in a future release.

  • disabled_storage_engines

    Property Value
    Command-Line Format --disabled-storage-engines=engine[,engine]...
    System Variable disabled_storage_engines
    Scope Global
    Dynamic No
    Type String
    Default Value empty string

    This variable indicates which storage engines cannot be used to create tables or tablespaces. For example, to prevent new MyISAM or FEDERATED tables from being created, start the server with these lines in the server option file:

    [mysqld]
    disabled_storage_engines="MyISAM,FEDERATED"

    By default, disabled_storage_engines is empty (no engines disabled), but it can be set to a comma-separated list of one or more engines (not case-sensitive). Any engine named in the value cannot be used to create tables or tablespaces with CREATE TABLE or CREATE TABLESPACE, and cannot be used with ALTER TABLE ... ENGINE or ALTER TABLESPACE ... ENGINE to change the storage engine of existing tables or tablespaces. Attempts to do so result in an ER_DISABLED_STORAGE_ENGINE error.

    disabled_storage_engines does not restrict other DDL statements for existing tables, such as CREATE INDEX, TRUNCATE TABLE, ANALYZE TABLE, DROP TABLE, or DROP TABLESPACE. This permits a smooth transition so that existing tables or tablespaces that use a disabled engine can be migrated to a permitted engine by means such as ALTER TABLE ... ENGINE permitted_engine.

    It is permitted to set the default_storage_engine or default_tmp_storage_engine system variable to a storage engine that is disabled. This could cause applications to behave erratically or fail, although that might be a useful technique in a development environment for identifying applications that use disabled engines, so that they can be modified.

    disabled_storage_engines is disabled and has no effect if the server is started with any of these options: --bootstrap, --initialize, --initialize-insecure, --skip-grant-tables.

  • disconnect_on_expired_password

    Property Value
    Command-Line Format --disconnect-on-expired-password[={OFF|ON}]
    System Variable disconnect_on_expired_password
    Scope Global
    Dynamic No
    Type Boolean
    Default Value ON

    This variable controls how the server handles clients with expired passwords:

    For more information about the interaction of client and server settings relating to expired-password handling, see Section 6.2.12, “Server Handling of Expired Passwords”.

  • div_precision_increment

    Property Value
    Command-Line Format --div-precision-increment=#
    System Variable div_precision_increment
    Scope Global, Session
    Dynamic Yes
    Type Integer
    Default Value 4
    Minimum Value 0
    Maximum Value 30

    This variable indicates the number of digits by which to increase the scale of the result of division operations performed with the / operator. The default value is 4. The minimum and maximum values are 0 and 30, respectively. The following example illustrates the effect of increasing the default value.

    mysql> SELECT 1/7;
    +--------+
    | 1/7    |
    +--------+
    | 0.1429 |
    +--------+
    mysql> SET div_precision_increment = 12;
    mysql> SELECT 1/7;
    +----------------+
    | 1/7            |
    +----------------+
    | 0.142857142857 |
    +----------------+
    
  • end_markers_in_json

    Property Value
    Command-Line Format --end-markers-in-json[={OFF|ON}]
    System Variable end_markers_in_json
    Scope Global, Session
    Dynamic Yes
    Type Boolean
    Default Value OFF

    Whether optimizer JSON output should add end markers. See MySQL Internals: The end_markers_in_json System Variable.

  • eq_range_index_dive_limit

    Property Value
    Command-Line Format --eq-range-index-dive-limit=#
    System Variable eq_range_index_dive_limit
    Scope Global, Session
    Dynamic Yes
    Type Integer
    Default Value 200
    Minimum Value 0
    Maximum Value 4294967295

    This variable indicates the number of equality ranges in an equality comparison condition when the optimizer should switch from using index dives to index statistics in estimating the number of qualifying rows. It applies to evaluation of expressions that have either of these equivalent forms, where the optimizer uses a nonunique index to look up col_name values:

    col_name IN(val1, ..., valN)
    col_name = val1 OR ... OR col_name = valN
    

    In both cases, the expression contains N equality ranges. The optimizer can make row estimates using index dives or index statistics. If eq_range_index_dive_limit is greater than 0, the optimizer uses existing index statistics instead of index dives if there are eq_range_index_dive_limit or more equality ranges. Thus, to permit use of index dives for up to N equality ranges, set eq_range_index_dive_limit to N + 1. To disable use of index statistics and always use index dives regardless of N, set eq_range_index_dive_limit to 0.

    For more information, see Equality Range Optimization of Many-Valued Comparisons.

    To update table index statistics for best estimates, use ANALYZE TABLE.

  • error_count

    The number of errors that resulted from the last statement that generated messages. This variable is read only. See Section 13.7.5.17, “SHOW ERRORS Statement”.

  • event_scheduler

    Property Value
    Command-Line Format --event-scheduler[=value]
    System Variable event_scheduler
    Scope Global
    Dynamic Yes
    Type Enumeration
    Default Value OFF
    Valid Values

    ON

    OFF

    DISABLED

    This variable enables or disables, and starts or stops, the Event Scheduler. The possible status values are ON, OFF, and DISABLED, with the default being OFF. Turning the Event Scheduler OFF is not the same as disabling the Event Scheduler, which requires setting the status to DISABLED. This variable and its effects on the Event Scheduler's operation are discussed in greater detail in Section 23.4.2, “Event Scheduler Configuration”

  • explicit_defaults_for_timestamp

    Property Value
    Command-Line Format --explicit-defaults-for-timestamp[={OFF|ON}]
    Deprecated Yes
    System Variable explicit_defaults_for_timestamp
    Scope Global, Session
    Dynamic Yes
    Type Boolean
    Default Value OFF

    This system variable determines whether the server enables certain nonstandard behaviors for default values and NULL-value handling in TIMESTAMP columns. By default, explicit_defaults_for_timestamp is disabled, which enables the nonstandard behaviors.

    If explicit_defaults_for_timestamp is disabled, the server enables the nonstandard behaviors and handles TIMESTAMP columns as follows:

    • TIMESTAMP columns not explicitly declared with the NULL attribute are automatically declared with the NOT NULL attribute. Assigning such a column a value of NULL is permitted and sets the column to the current timestamp.

    • The first TIMESTAMP column in a table, if not explicitly declared with the NULL attribute or an explicit DEFAULT or ON UPDATE attribute, is automatically declared with the DEFAULT CURRENT_TIMESTAMP and ON UPDATE CURRENT_TIMESTAMP attributes.

    • TIMESTAMP columns following the first one, if not explicitly declared with the NULL attribute or an explicit DEFAULT attribute, are automatically declared as DEFAULT '0000-00-00 00:00:00' (the zero timestamp). For inserted rows that specify no explicit value for such a column, the column is assigned '0000-00-00 00:00:00' and no warning occurs.

      Depending on whether strict SQL mode or the NO_ZERO_DATE SQL mode is enabled, a default value of '0000-00-00 00:00:00' may be invalid. Be aware that the TRADITIONAL SQL mode includes strict mode and NO_ZERO_DATE. See Section 5.1.10, “Server SQL Modes”.

    The nonstandard behaviors just described are deprecated and will be removed in a future MySQL release.

    If explicit_defaults_for_timestamp is enabled, the server disables the nonstandard behaviors and handles TIMESTAMP columns as follows:

    • It is not possible to assign a TIMESTAMP column a value of NULL to set it to the current timestamp. To assign the current timestamp, set the column to CURRENT_TIMESTAMP or a synonym such as NOW().

    • TIMESTAMP columns not explicitly declared with the NOT NULL attribute are automatically declared with the NULL attribute and permit NULL values. Assigning such a column a value of NULL sets it to NULL, not the current timestamp.

    • TIMESTAMP columns declared with the NOT NULL attribute do not permit NULL values. For inserts that specify NULL for such a column, the result is either an error for a single-row insert or if strict SQL mode is enabled, or '0000-00-00 00:00:00' is inserted for multiple-row inserts with strict SQL mode disabled. In no case does assigning the column a value of NULL set it to the current timestamp.

    • TIMESTAMP columns explicitly declared with the NOT NULL attribute and without an explicit DEFAULT attribute are treated as having no default value. For inserted rows that specify no explicit value for such a column, the result depends on the SQL mode. If strict SQL mode is enabled, an error occurs. If strict SQL mode is not enabled, the column is declared with the implicit default of '0000-00-00 00:00:00' and a warning occurs. This is similar to how MySQL treats other temporal types such as DATETIME.

    • No TIMESTAMP column is automatically declared with the DEFAULT CURRENT_TIMESTAMP or ON UPDATE CURRENT_TIMESTAMP attributes. Those attributes must be explicitly specified.

    • The first TIMESTAMP column in a table is not handled differently from TIMESTAMP columns following the first one.

    If explicit_defaults_for_timestamp is disabled at server startup, this warning appears in the error log:

    [Warning] TIMESTAMP with implicit DEFAULT value is deprecated.
    Please use --explicit_defaults_for_timestamp server option (see
    documentation for more details).

    As indicated by the warning, to disable the deprecated nonstandard behaviors, enable the explicit_defaults_for_timestamp system variable at server startup.

    Note

    explicit_defaults_for_timestamp is itself deprecated because its only purpose is to permit control over deprecated TIMESTAMP behaviors that are to be removed in a future MySQL release. When removal of those behaviors occurs, explicit_defaults_for_timestamp will have no purpose and will be removed as well.

    For additional information, see Section 11.2.6, “Automatic Initialization and Updating for TIMESTAMP and DATETIME”.

  • external_user

    Property Value
    System Variable external_user
    Scope Session
    Dynamic No
    Type String

    The external user name used during the authentication process, as set by the plugin used to authenticate the client. With native (built-in) MySQL authentication, or if the plugin does not set the value, this variable is NULL. See Section 6.2.14, “Proxy Users”.

  • flush

    Property Value
    Command-Line Format --flush[={OFF|ON}]
    System Variable flush
    Scope Global
    Dynamic Yes
    Type Boolean
    Default Value OFF

    If ON, the server flushes (synchronizes) all changes to disk after each SQL statement. Normally, MySQL does a write of all changes to disk only after each SQL statement and lets the operating system handle the synchronizing to disk. See Section B.4.3.3, “What to Do If MySQL Keeps Crashing”. This variable is set to ON if you start mysqld with the --flush option.

    Note

    If flush is enabled, the value of flush_time does not matter and changes to flush_time have no effect on flush behavior.

  • flush_time

    Property Value
    Command-Line Format --flush-time=#
    System Variable flush_time
    Scope Global
    Dynamic Yes
    Type Integer
    Default Value 0
    Minimum Value 0

    If this is set to a nonzero value, all tables are closed every flush_time seconds to free up resources and synchronize unflushed data to disk. This option is best used only on systems with minimal resources.

    Note

    If flush is enabled, the value of flush_time does not matter and changes to flush_time have no effect on flush behavior.

  • foreign_key_checks

    Property Value
    System Variable foreign_key_checks
    Scope Global, Session
    Dynamic Yes
    Type Boolean
    Default Value ON

    If set to 1 (the default), foreign key constraints are checked. If set to 0, foreign key constraints are ignored, with a couple of exceptions. When re-creating a table that was dropped, an error is returned if the table definition does not conform to the foreign key constraints referencing the table. Likewise, an ALTER TABLE operation returns an error if a foreign key definition is incorrectly formed. For more information, see Section 13.1.18.6, “FOREIGN KEY Constraints”.

    Setting this variable has the same effect on NDB tables as it does for InnoDB tables. Typically you leave this setting enabled during normal operation, to enforce referential integrity. Disabling foreign key checking can be useful for reloading InnoDB tables in an order different from that required by their parent/child relationships. See Section 13.1.18.6, “FOREIGN KEY Constraints”.

    Setting foreign_key_checks to 0 also affects data definition statements: DROP SCHEMA drops a schema even if it contains tables that have foreign keys that are referred to by tables outside the schema, and DROP TABLE drops tables that have foreign keys that are referred to by other tables.

    Note

    Setting foreign_key_checks to 1 does not trigger a scan of the existing table data. Therefore, rows added to the table while foreign_key_checks=0 will not be verified for consistency.

    Dropping an index required by a foreign key constraint is not permitted, even with foreign_key_checks=0. The foreign key constraint must be removed before dropping the index (Bug #70260).

  • ft_boolean_syntax

    Property Value
    Command-Line Format --ft-boolean-syntax=name
    System Variable ft_boolean_syntax
    Scope Global
    Dynamic Yes
    Type String
    Default Value + -><()~*:""&|

    The list of operators supported by boolean full-text searches performed using IN BOOLEAN MODE. See Section 12.9.2, “Boolean Full-Text Searches”.

    The default variable value is '+ -><()~*:""&|'. The rules for changing the value are as follows:

    • Operator function is determined by position within the string.

    • The replacement value must be 14 characters.

    • Each character must be an ASCII nonalphanumeric character.

    • Either the first or second character must be a space.

    • No duplicates are permitted except the phrase quoting operators in positions 11 and 12. These two characters are not required to be the same, but they are the only two that may be.

    • Positions 10, 13, and 14 (which by default are set to :, &, and |) are reserved for future extensions.

  • ft_max_word_len

    Property Value
    Command-Line Format --ft-max-word-len=#
    System Variable ft_max_word_len
    Scope Global
    Dynamic No
    Type Integer
    Minimum Value 10

    The maximum length of the word to be included in a MyISAM FULLTEXT index.

    Note

    FULLTEXT indexes on MyISAM tables must be rebuilt after changing this variable. Use REPAIR TABLE tbl_name QUICK.

  • ft_min_word_len

    Property Value
    Command-Line Format --ft-min-word-len=#
    System Variable ft_min_word_len
    Scope Global
    Dynamic No
    Type Integer
    Default Value 4
    Minimum Value 1

    The minimum length of the word to be included in a MyISAM FULLTEXT index.

    Note

    FULLTEXT indexes on MyISAM tables must be rebuilt after changing this variable. Use REPAIR TABLE tbl_name QUICK.

  • ft_query_expansion_limit

    Property Value
    Command-Line Format --ft-query-expansion-limit=#
    System Variable ft_query_expansion_limit
    Scope Global
    Dynamic No
    Type Integer
    Default Value 20
    Minimum Value 0
    Maximum Value 1000

    The number of top matches to use for full-text searches performed using WITH QUERY EXPANSION.

  • ft_stopword_file

    Property Value
    Command-Line Format --ft-stopword-file=file_name
    System Variable ft_stopword_file
    Scope Global
    Dynamic No
    Type File name

    The file from which to read the list of stopwords for full-text searches on MyISAM tables. The server looks for the file in the data directory unless an absolute path name is given to specify a different directory. All the words from the file are used; comments are not honored. By default, a built-in list of stopwords is used (as defined in the storage/myisam/ft_static.c file). Setting this variable to the empty string ('') disables stopword filtering. See also Section 12.9.4, “Full-Text Stopwords”.

    Note

    FULLTEXT indexes on MyISAM tables must be rebuilt after changing this variable or the contents of the stopword file. Use REPAIR TABLE tbl_name QUICK.

  • general_log

    Property Value
    Command-Line Format --general-log[={OFF|ON}]
    System Variable general_log
    Scope Global
    Dynamic Yes
    Type Boolean
    Default Value OFF

    Whether the general query log is enabled. The value can be 0 (or OFF) to disable the log or 1 (or ON) to enable the log. The destination for log output is controlled by the log_output system variable; if that value is NONE, no log entries are written even if the log is enabled.

  • general_log_file

    Property Value
    Command-Line Format --general-log-file=file_name
    System Variable general_log_file
    Scope Global
    Dynamic Yes
    Type File name
    Default Value host_name.log

    The name of the general query log file. The default value is host_name.log, but the initial value can be changed with the --general_log_file option.

  • group_concat_max_len

    Property Value
    Command-Line Format --group-concat-max-len=#
    System Variable group_concat_max_len
    Scope Global, Session
    Dynamic Yes
    Type Integer
    Default Value 1024
    Minimum Value 4
    Maximum Value (64-bit platforms) 18446744073709551615
    Maximum Value (32-bit platforms) 4294967295

    The maximum permitted result length in bytes for the GROUP_CONCAT() function. The default is 1024.

  • have_compress

    YES if the zlib compression library is available to the server, NO if not. If not, the COMPRESS() and UNCOMPRESS() functions cannot be used.

  • have_crypt

    YES if the crypt() system call is available to the server, NO if not. If not, the ENCRYPT() function cannot be used.

    Note

    The ENCRYPT() function is deprecated in MySQL 5.7, will be removed in a future MySQL release, and should no longer be used. (For one-way hashing, consider using SHA2() instead.) Consequently, have_crypt also is deprecated and will be removed.

  • have_dynamic_loading

    YES if mysqld supports dynamic loading of plugins, NO if not. If the value is NO, you cannot use options such as --plugin-load to load plugins at server startup, or the INSTALL PLUGIN statement to load plugins at runtime.

  • have_geometry

    YES if the server supports spatial data types, NO if not.

  • have_openssl

    This variable is an alias for have_ssl.

  • have_profiling

    YES if statement profiling capability is present, NO if not. If present, the profiling system variable controls whether this capability is enabled or disabled. See Section 13.7.5.31, “SHOW PROFILES Statement”.

    This variable is deprecated and will be removed in a future MySQL release.

  • have_query_cache

    YES if mysqld supports the query cache, NO if not.

    Note

    The query cache is deprecated as of MySQL 5.7.20, and is removed in MySQL 8.0. Deprecation includes have_query_cache.

  • have_rtree_keys

    YES if RTREE indexes are available, NO if not. (These are used for spatial indexes in MyISAM tables.)

  • have_ssl

    Property Value
    System Variable have_ssl
    Scope Global
    Dynamic No
    Type String
    Valid Values

    YES (SSL support available)

    DISABLED (SSL support was compiled into the server, but the server was not started with the necessary options to enable it)

    YES if mysqld supports SSL connections, DISABLED if the server was compiled with SSL support, but was not started with the appropriate --ssl-xxx options. For more information, see Section 2.9.6, “Configuring SSL Library Support”.

  • have_statement_timeout

    Property Value
    System Variable have_statement_timeout
    Scope Global
    Dynamic No
    Type Boolean

    Whether the statement execution timeout feature is available (see Statement Execution Time Optimizer Hints). The value can be NO if the background thread used by this feature could not be initialized.

  • have_symlink

    YES if symbolic link support is enabled, NO if not. This is required on Unix for support of the DATA DIRECTORY and INDEX DIRECTORY table options. If the server is started with the --skip-symbolic-links option, the value is DISABLED.

    This variable has no meaning on Windows.

  • host_cache_size

    Property Value
    Command-Line Format --host-cache-size=#
    System Variable host_cache_size
    Scope Global
    Dynamic Yes
    Type Integer
    Default Value -1 (signifies autosizing; do not assign this literal value)
    Minimum Value 0
    Maximum Value 65536

    This variable controls the size of the host cache, as well as the size of the Performance Schema host_cache table that exposes the cache contents. Setting the size to 0 disables the host cache. Changing the cache size at runtime causes an implicit FLUSH HOSTS operation that clears the host cache, truncates the host_cache table, and unblocks any blocked hosts.

    The default value is autosized to 128, plus 1 for a value of max_connections up to 500, plus 1 for every increment of 20 over 500 in the max_connections value, capped to a limit of 2000.

    Using the --skip-host-cache option is similar to setting the host_cache_size system variable to 0, but host_cache_size is more flexible because it can also be used to resize, enable, and disable the host cache at runtime, not just at server startup. Starting the server with --skip-host-cache does not prevent changes to the value of host_cache_size, but such changes have no effect and the cache is not re-enabled even if host_cache_size is set larger than 0 at runtime.

    For more information about how the host cache works, see Section 8.12.5.2, “DNS Lookup Optimization and the Host Cache”.

  • hostname

    Property Value
    System Variable hostname
    Scope Global
    Dynamic No
    Type String

    The server sets this variable to the server host name at startup.

  • identity

    This variable is a synonym for the last_insert_id variable. It exists for compatibility with other database systems. You can read its value with SELECT @@identity, and set it using SET identity.

  • ignore_db_dirs

    Property Value
    Deprecated 5.7.16
    System Variable ignore_db_dirs
    Scope Global
    Dynamic No
    Type String

    A comma-separated list of names that are not considered as database directories in the data directory. The value is set from any instances of --ignore-db-dir given at server startup.

    As of MySQL 5.7.11, --ignore-db-dir can be used at data directory initialization time with mysqld --initialize to specify directories that the server should ignore for purposes of assessing whether an existing data directory is considered empty. See Section 2.10.1, “Initializing the Data Directory”.

    This system variable is deprecated in MySQL 5.7. With the introduction of the data dictionary in MySQL 8.0, it became superfluous and was removed in that version.

  • init_connect

    Property Value
    Command-Line Format --init-connect=name
    System Variable init_connect
    Scope Global
    Dynamic Yes
    Type String

    A string to be executed by the server for each client that connects. The string consists of one or more SQL statements, separated by semicolon characters.

    For users that have the SUPER privilege, the content of init_connect is not executed. This is done so that an erroneous value for init_connect does not prevent all clients from connecting. For example, the value might contain a statement that has a syntax error, thus causing client connections to fail. Not executing init_connect for users that have the SUPER privilege enables them to open a connection and fix the init_connect value.

    As of MySQL 5.7.22, init_connect execution is skipped for any client user with an expired password. This is done because such a user cannot execute arbitrary statements, and thus init_connect execution will fail, leaving the client unable to connect. Skipping init_connect execution enables the user to connect and change password.

    The server discards any result sets produced by statements in the value of of init_connect.

  • init_file

    Property Value
    Command-Line Format --init-file=file_name
    System Variable init_file
    Scope Global
    Dynamic No
    Type File name

    If specified, this variable names a file containing SQL statements to be read and executed during the startup process. Each statement must be on a single line and should not include comments.

    If the server is started with any of the --bootstrap, --initialize, or --initialize-insecure options, it operates in bootstap mode and some functionality is unavailable that limits the statements permitted in the file. These include statements that relate to account management (such as CREATE USER or GRANT), replication, and global transaction identifiers. See Section 16.1.3, “Replication with Global Transaction Identifiers”.

  • innodb_xxx

    InnoDB system variables are listed in Section 14.15, “InnoDB Startup Options and System Variables”. These variables control many aspects of storage, memory use, and I/O patterns for InnoDB tables, and are especially important now that InnoDB is the default storage engine.

  • insert_id

    The value to be used by the following INSERT or ALTER TABLE statement when inserting an AUTO_INCREMENT value. This is mainly used with the binary log.

  • interactive_timeout

    Property Value
    Command-Line Format --interactive-timeout=#
    System Variable interactive_timeout
    Scope Global, Session
    Dynamic Yes
    Type Integer
    Default Value 28800
    Minimum Value 1

    The number of seconds the server waits for activity on an interactive connection before closing it. An interactive client is defined as a client that uses the CLIENT_INTERACTIVE option to mysql_real_connect(). See also wait_timeout.

  • internal_tmp_disk_storage_engine

    Property Value
    Command-Line Format --internal-tmp-disk-storage-engine=#
    System Variable internal_tmp_disk_storage_engine
    Scope Global
    Dynamic Yes
    Type Enumeration
    Default Value INNODB
    Valid Values

    MYISAM

    INNODB

    The storage engine for on-disk internal temporary tables (see Section 8.4.4, “Internal Temporary Table Use in MySQL”). Permitted values are MYISAM and INNODB (the default).

    The optimizer uses the storage engine defined by internal_tmp_disk_storage_engine for on-disk internal temporary tables.

    When using internal_tmp_disk_storage_engine=INNODB (the default), queries that generate on-disk internal temporary tables that exceed InnoDB row or column limits will return Row size too large or Too many columns errors. The workaround is to set internal_tmp_disk_storage_engine to MYISAM.

  • join_buffer_size

    Property Value
    Command-Line Format --join-buffer-size=#
    System Variable join_buffer_size
    Scope Global, Session
    Dynamic Yes
    Type Integer
    Default Value 262144
    Minimum Value 128
    Maximum Value (Other, 64-bit platforms) 18446744073709547520
    Maximum Value (Other, 32-bit platforms) 4294967295
    Maximum Value (Windows) 4294967295

    The minimum size of the buffer that is used for plain index scans, range index scans, and joins that do not use indexes and thus perform full table scans. Normally, the best way to get fast joins is to add indexes. Increase the value of join_buffer_size to get a faster full join when adding indexes is not possible. One join buffer is allocated for each full join between two tables. For a complex join between several tables for which indexes are not used, multiple join buffers might be necessary.

    Unless a Block Nested-Loop or Batched Key Access algorithm is used, there is no gain from setting the buffer larger than required to hold each matching row, and all joins allocate at least the minimum size, so use caution in setting this variable to a large value globally. It is better to keep the global setting small and change the session setting to a larger value only in sessions that are doing large joins. Memory allocation time can cause substantial performance drops if the global size is larger than needed by most queries that use it.

    When Block Nested-Loop is used, a larger join buffer can be beneficial up to the point where all required columns from all rows in the first table are stored in the join buffer. This depends on the query; the optimal size may be smaller than holding all rows from the first tables.

    When Batched Key Access is used, the value of join_buffer_size defines how large the batch of keys is in each request to the storage engine. The larger the buffer, the more sequential access will be to the right hand table of a join operation, which can significantly improve performance.

    The default is 256KB. The maximum permissible setting for join_buffer_size is 4GB−1. Larger values are permitted for 64-bit platforms (except 64-bit Windows, for which large values are truncated to 4GB−1 with a warning).

    For additional information about join buffering, see Section 8.2.1.6, “Nested-Loop Join Algorithms”. For information about Batched Key Access, see Section 8.2.1.11, “Block Nested-Loop and Batched Key Access Joins”.

  • keep_files_on_create

    Property Value
    Command-Line Format --keep-files-on-create[={OFF|ON}]
    System Variable keep_files_on_create
    Scope Global, Session
    Dynamic Yes
    Type Boolean
    Default Value OFF

    If a MyISAM table is created with no DATA DIRECTORY option, the .MYD file is created in the database directory. By default, if MyISAM finds an existing .MYD file in this case, it overwrites it. The same applies to .MYI files for tables created with no INDEX DIRECTORY option. To suppress this behavior, set the keep_files_on_create variable to ON (1), in which case MyISAM will not overwrite existing files and returns an error instead. The default value is OFF (0).

    If a MyISAM table is created with a DATA DIRECTORY or INDEX DIRECTORY option and an existing .MYD or .MYI file is found, MyISAM always returns an error. It will not overwrite a file in the specified directory.

  • key_buffer_size

    Property Value
    Command-Line Format --key-buffer-size=#
    System Variable key_buffer_size
    Scope Global
    Dynamic Yes
    Type Integer
    Default Value 8388608
    Minimum Value 8
    Maximum Value (64-bit platforms) OS_PER_PROCESS_LIMIT
    Maximum Value (32-bit platforms) 4294967295

    Index blocks for MyISAM tables are buffered and are shared by all threads. key_buffer_size is the size of the buffer used for index blocks. The key buffer is also known as the key cache.

    The maximum permissible setting for key_buffer_size is 4GB−1 on 32-bit platforms. Larger values are permitted for 64-bit platforms. The effective maximum size might be less, depending on your available physical RAM and per-process RAM limits imposed by your operating system or hardware platform. The value of this variable indicates the amount of memory requested. Internally, the server allocates as much memory as possible up to this amount, but the actual allocation might be less.

    You can increase the value to get better index handling for all reads and multiple writes; on a system whose primary function is to run MySQL using the MyISAM storage engine, 25% of the machine's total memory is an acceptable value for this variable. However, you should be aware that, if you make the value too large (for example, more than 50% of the machine's total memory), your system might start to page and become extremely slow. This is because MySQL relies on the operating system to perform file system caching for data reads, so you must leave some room for the file system cache. You should also consider the memory requirements of any other storage engines that you may be using in addition to MyISAM.

    For even more speed when writing many rows at the same time, use LOCK TABLES. See Section 8.2.4.1, “Optimizing INSERT Statements”.

    You can check the performance of the key buffer by issuing a SHOW STATUS statement and examining the Key_read_requests, Key_reads, Key_write_requests, and Key_writes status variables. (See Section 13.7.5, “SHOW Statements”.) The Key_reads/Key_read_requests ratio should normally be less than 0.01. The Key_writes/Key_write_requests ratio is usually near 1 if you are using mostly updates and deletes, but might be much smaller if you tend to do updates that affect many rows at the same time or if you are using the DELAY_KEY_WRITE table option.

    The fraction of the key buffer in use can be determined using key_buffer_size in conjunction with the Key_blocks_unused status variable and the buffer block size, which is available from the key_cache_block_size system variable:

    1 - ((Key_blocks_unused * key_cache_block_size) / key_buffer_size)

    This value is an approximation because some space in the key buffer is allocated internally for administrative structures. Factors that influence the amount of overhead for these structures include block size and pointer size. As block size increases, the percentage of the key buffer lost to overhead tends to decrease. Larger blocks results in a smaller number of read operations (because more keys are obtained per read), but conversely an increase in reads of keys that are not examined (if not all keys in a block are relevant to a query).

    It is possible to create multiple MyISAM key caches. The size limit of 4GB applies to each cache individually, not as a group. See Section 8.10.2, “The MyISAM Key Cache”.

  • key_cache_age_threshold

    Property Value
    Command-Line Format --key-cache-age-threshold=#
    System Variable key_cache_age_threshold
    Scope Global
    Dynamic Yes
    Type Integer
    Default Value 300
    Minimum Value 100
    Maximum Value (64-bit platforms) 18446744073709551615
    Maximum Value (32-bit platforms) 4294967295

    This value controls the demotion of buffers from the hot sublist of a key cache to the warm sublist. Lower values cause demotion to happen more quickly. The minimum value is 100. The default value is 300. See Section 8.10.2, “The MyISAM Key Cache”.

  • key_cache_block_size

    Property Value
    Command-Line Format --key-cache-block-size=#
    System Variable key_cache_block_size
    Scope Global
    Dynamic Yes
    Type Integer
    Default Value 1024
    Minimum Value 512
    Maximum Value 16384

    The size in bytes of blocks in the key cache. The default value is 1024. See Section 8.10.2, “The MyISAM Key Cache”.

  • key_cache_division_limit

    Property Value
    Command-Line Format --key-cache-division-limit=#
    System Variable key_cache_division_limit
    Scope Global
    Dynamic Yes
    Type Integer
    Default Value 100
    Minimum Value 1
    Maximum Value 100

    The division point between the hot and warm sublists of the key cache buffer list. The value is the percentage of the buffer list to use for the warm sublist. Permissible values range from 1 to 100. The default value is 100. See Section 8.10.2, “The MyISAM Key Cache”.

  • large_files_support

    Property Value
    System Variable large_files_support
    Scope Global
    Dynamic No

    Whether mysqld was compiled with options for large file support.

  • large_pages

    Property Value
    Command-Line Format --large-pages[={OFF|ON}]
    System Variable large_pages
    Scope Global
    Dynamic No
    Platform Specific Linux
    Type Boolean
    Default Value OFF

    Whether large page support is enabled (via the --large-pages option). See Section 8.12.4.2, “Enabling Large Page Support”.

  • large_page_size

    Property Value
    System Variable large_page_size
    Scope Global
    Dynamic No
    Type Integer
    Default Value 0

    If large page support is enabled, this shows the size of memory pages. Large memory pages are supported only on Linux; on other platforms, the value of this variable is always 0. See Section 8.12.4.2, “Enabling Large Page Support”.

  • last_insert_id

    The value to be returned from LAST_INSERT_ID(). This is stored in the binary log when you use LAST_INSERT_ID() in a statement that updates a table. Setting this variable does not update the value returned by the mysql_insert_id() C API function.

  • lc_messages

    Property Value
    Command-Line Format --lc-messages=name
    System Variable lc_messages
    Scope Global, Session
    Dynamic Yes
    Type String
    Default Value en_US

    The locale to use for error messages. The default is en_US. The server converts the argument to a language name and combines it with the value of lc_messages_dir to produce the location for the error message file. See Section 10.12, “Setting the Error Message Language”.

  • lc_messages_dir

    Property Value
    Command-Line Format --lc-messages-dir=dir_name
    System Variable lc_messages_dir
    Scope Global
    Dynamic No
    Type Directory name

    The directory where error messages are located. The server uses the value together with the value of lc_messages to produce the location for the error message file. See Section 10.12, “Setting the Error Message Language”.

  • lc_time_names

    Property Value
    Command-Line Format --lc-time-names=value
    System Variable lc_time_names
    Scope Global, Session
    Dynamic Yes
    Type String

    This variable specifies the locale that controls the language used to display day and month names and abbreviations. This variable affects the output from the DATE_FORMAT(), DAYNAME() and MONTHNAME() functions. Locale names are POSIX-style values such as 'ja_JP' or 'pt_BR'. The default value is 'en_US' regardless of your system's locale setting. For further information, see Section 10.16, “MySQL Server Locale Support”.

  • license

    Property Value
    System Variable license
    Scope Global
    Dynamic No
    Type String
    Default Value GPL

    The type of license the server has.

  • local_infile

    Property Value
    Command-Line Format --local-infile[={OFF|ON}]
    System Variable local_infile
    Scope Global
    Dynamic Yes
    Type Boolean
    Default Value ON

    This variable controls server-side LOCAL capability for LOAD DATA statements. Depending on the local_infile setting, the server refuses or permits local data loading by clients that have LOCAL enabled on the client side.

    To explicitly cause the server to refuse or permit LOAD DATA LOCAL statements (regardless of how client programs and libraries are configured at build time or runtime), start mysqld with local_infile disabled or enabled, respectively. local_infile can also be set at runtime. For more information, see Section 6.1.6, “Security Issues with LOAD DATA LOCAL”.

  • lock_wait_timeout

    Property Value
    Command-Line Format --lock-wait-timeout=#
    System Variable lock_wait_timeout
    Scope Global, Session
    Dynamic Yes
    Type Integer
    Default Value 31536000
    Minimum Value 1
    Maximum Value 31536000

    This variable specifies the timeout in seconds for attempts to acquire metadata locks. The permissible values range from 1 to 31536000 (1 year). The default is 31536000.

    This timeout applies to all statements that use metadata locks. These include DML and DDL operations on tables, views, stored procedures, and stored functions, as well as LOCK TABLES, FLUSH TABLES WITH READ LOCK, and HANDLER statements.

    This timeout does not apply to implicit accesses to system tables in the mysql database, such as grant tables modified by GRANT or REVOKE statements or table logging statements. The timeout does apply to system tables accessed directly, such as with SELECT or UPDATE.

    The timeout value applies separately for each metadata lock attempt. A given statement can require more than one lock, so it is possible for the statement to block for longer than the lock_wait_timeout value before reporting a timeout error. When lock timeout occurs, ER_LOCK_WAIT_TIMEOUT is reported.

    lock_wait_timeout does not apply to delayed inserts, which always execute with a timeout of 1 year. This is done to avoid unnecessary timeouts because a session that issues a delayed insert receives no notification of delayed insert timeouts.

  • locked_in_memory

    Property Value
    System Variable locked_in_memory
    Scope Global
    Dynamic No

    Whether mysqld was locked in memory with --memlock.

  • log_error

    Property Value
    Command-Line Format --log-error[=file_name]
    System Variable log_error
    Scope Global
    Dynamic No
    Type File name

    The error log output destination. If the destination is the console, the value is stderr. Otherwise, the destination is a file and the log_error value is the file name. See Section 5.4.2, “The Error Log”.

  • log_error_verbosity

    Property Value
    Command-Line Format --log-error-verbosity=#
    System Variable log_error_verbosity
    Scope Global
    Dynamic Yes
    Type Integer
    Default Value 3
    Minimum Value 1
    Maximum Value 3

    The verbosity of the server in writing error, warning, and note messages to the error log. The following table shows the permitted values. The default is 3.

    Desired Log Filtering log_error_verbosity Value
    Error messages 1
    Error and warning messages 2
    Error, warning, and information messages 3

    log_error_verbosity was added in MySQL 5.7.2. It is preferred over, and should be used instead of, the older log_warnings system variable. See the description of log_warnings for information about how that variable relates to log_error_verbosity. In particular, assigning a value to log_warnings assigns a value to log_error_verbosity and vice versa.

  • log_output

    Property Value
    Command-Line Format --log-output=name
    System Variable log_output
    Scope Global
    Dynamic Yes
    Type Set
    Default Value FILE
    Valid Values

    TABLE

    FILE

    NONE

    The destination or destinations for general query log and slow query log output. The value is a list one or more comma-separated words chosen from TABLE, FILE, and NONE. TABLE selects logging to the general_log and slow_log tables in the mysql system database. FILE selects logging to log files. NONE disables logging. If NONE is present in the value, it takes precedence over any other words that are present. TABLE and FILE can both be given to select both log output destinations.

    This variable selects log output destinations, but does not enable log output. To do that, enable the general_log and slow_query_log system variables. For FILE logging, the general_log_file and slow_query_log_file system variables determine the log file locations. For more information, see Section 5.4.1, “Selecting General Query Log and Slow Query Log Output Destinations”.

  • log_queries_not_using_indexes

    Property Value
    Command-Line Format --log-queries-not-using-indexes[={OFF|ON}]
    System Variable log_queries_not_using_indexes
    Scope Global
    Dynamic Yes
    Type Boolean
    Default Value OFF

    If you enable this variable with the slow query log enabled, queries that are expected to retrieve all rows are logged. See Section 5.4.5, “The Slow Query Log”. This option does not necessarily mean that no index is used. For example, a query that uses a full index scan uses an index but would be logged because the index would not limit the number of rows.

  • log_slow_admin_statements

    Property Value
    Command-Line Format --log-slow-admin-statements[={OFF|ON}]
    System Variable log_slow_admin_statements
    Scope Global
    Dynamic Yes
    Type Boolean
    Default Value OFF

    Include slow administrative statements in the statements written to the slow query log. Administrative statements include ALTER TABLE, ANALYZE TABLE, CHECK TABLE, CREATE INDEX, DROP INDEX, OPTIMIZE TABLE, and REPAIR TABLE.

  • log_syslog

    Property Value
    Command-Line Format --log-syslog[={OFF|ON}]
    System Variable log_syslog
    Scope Global
    Dynamic Yes
    Type Boolean
    Default Value (Windows) ON
    Default Value (Unix) OFF

    Whether to write error log output to the system log. This is the Event Log on Windows, and syslog on Unix and Unix-like systems. The default value is platform specific:

    • On Windows, Event Log output is enabled by default.

    • On Unix and Unix-like systems, syslog output is disabled by default.

    Regardless of the default, log_syslog can be set explicitly to control output on any supported platform.

    System log output control is distinct from sending error output to a file or the console. Error output can be directed to a file or the console in addition to or instead of the system log as desired. See Section 5.4.2, “The Error Log”.

  • log_syslog_facility

    Property Value
    Command-Line Format --log-syslog-facility=value
    System Variable log_syslog_facility
    Scope Global
    Dynamic Yes
    Type String
    Default Value daemon

    The facility for error log output written to syslog (what type of program is sending the message). This variable has no effect unless the log_syslog system variable is enabled. See Section 5.4.2.3, “Error Logging to the System Log”.

    The permitted values can vary per operating system; consult your system syslog documentation.

    This variable does not exist on Windows.

  • log_syslog_include_pid

    Property Value
    Command-Line Format --log-syslog-include-pid[={OFF|ON}]
    System Variable log_syslog_include_pid
    Scope Global
    Dynamic Yes
    Type Boolean
    Default Value ON

    Whether to include the server process ID in each line of error log output written to syslog. This variable has no effect unless the log_syslog system variable is enabled. See Section 5.4.2.3, “Error Logging to the System Log”.

    This variable does not exist on Windows.

  • log_syslog_tag

    Property Value
    Command-Line Format --log-syslog-tag=tag
    System Variable log_syslog_tag
    Scope Global
    Dynamic Yes
    Type String
    Default Value empty string

    The tag to be added to the server identifier in error log output written to syslog. This variable has no effect unless the log_syslog system variable is enabled. See Section 5.4.2.3, “Error Logging to the System Log”.

    By default, the server identifier is mysqld with no tag. If a tag value of tag is specified, it is appended to the server identifier with a leading hyphen, resulting in an identifier of mysqld-tag.

    On Windows, to use a tag that does not already exist, the server must be run from an account with Administrator privileges, to permit creation of a registry entry for the tag. Elevated privileges are not required if the tag already exists.

  • log_timestamps

    Property Value
    Command-Line Format --log-timestamps=#
    System Variable log_timestamps
    Scope Global
    Dynamic Yes
    Type Enumeration
    Default Value UTC
    Valid Values

    UTC

    SYSTEM

    This variable controls the time zone of timestamps in messages written to the error log, and in general query log and slow query log messages written to files. It does not affect the time zone of general query log and slow query log messages written to tables (mysql.general_log, mysql.slow_log). Rows retrieved from those tables can be converted from the local system time zone to any desired time zone with CONVERT_TZ() or by setting the session time_zone system variable.

    Permitted log_timestamps values are UTC (the default) and SYSTEM (local system time zone).

    Timestamps are written using ISO 8601 / RFC 3339 format: YYYY-MM-DDThh:mm:ss.uuuuuu plus a tail value of Z signifying Zulu time (UTC) or ±hh:mm (an offset from UTC).

  • log_throttle_queries_not_using_indexes

    Property Value
    Command-Line Format --log-throttle-queries-not-using-indexes=#
    System Variable log_throttle_queries_not_using_indexes
    Scope Global
    Dynamic Yes
    Type Integer
    Default Value 0

    If log_queries_not_using_indexes is enabled, the log_throttle_queries_not_using_indexes variable limits the number of such queries per minute that can be written to the slow query log. A value of 0 (the default) means no limit. For more information, see Section 5.4.5, “The Slow Query Log”.

  • log_warnings

    Property Value
    Command-Line Format --log-warnings[=#]
    Deprecated Yes
    System Variable log_warnings
    Scope Global
    Dynamic Yes
    Type Integer
    Default Value 2
    Minimum Value 0
    Maximum Value (64-bit platforms) 18446744073709551615
    Maximum Value (32-bit platforms) 4294967295

    Whether to produce additional warning messages to the error log. As of MySQL 5.7.2, information items previously governed by log_warnings are governed by log_error_verbosity, which is preferred over, and should be used instead of, the older log_warnings system variable. (The log_warnings system variable and --log-warnings command-line option are deprecated and will be removed in a future MySQL release.)

    log_warnings is enabled by default (the default is 1 before MySQL 5.7.2, 2 as of 5.7.2). To disable it, set it to 0. If the value is greater than 0, the server logs messages about statements that are unsafe for statement-based logging. If the value is greater than 1, the server logs aborted connections and access-denied errors for new connection attempts. See Section B.4.2.10, “Communication Errors and Aborted Connections”.

    If you use replication, enabling this variable by setting it greater than 0 is recommended, to get more information about what is happening, such as messages about network failures and reconnections.

    If a slave server is started with log_warnings enabled, the slave prints messages to the error log to provide information about its status, such as the binary log and relay log coordinates where it starts its job, when it is switching to another relay log, when it reconnects after a disconnect, and so forth.

    Assigning a value to log_warnings assigns a value to log_error_verbosity and vice versa. The variables are related as follows:

    As of MySQL 5.7.2, the default log level is controlled by log_error_verbosity, which has a default of 3. In addition, the default for log_warnings changes from 1 to 2, which corresponds to log_error_verbosity=3. To achieve a logging level similar to the previous default, set log_error_verbosity=2.

    In MySQL 5.7.2 and higher, use of log_warnings is still permitted but maps onto use of log_error_verbosity as follows:

  • long_query_time

    Property Value
    Command-Line Format --long-query-time=#
    System Variable long_query_time
    Scope Global, Session
    Dynamic Yes
    Type Numeric
    Default Value 10
    Minimum Value 0

    If a query takes longer than this many seconds, the server increments the Slow_queries status variable. If the slow query log is enabled, the query is logged to the slow query log file. This value is measured in real time, not CPU time, so a query that is under the threshold on a lightly loaded system might be above the threshold on a heavily loaded one. The minimum and default values of long_query_time are 0 and 10, respectively. The value can be specified to a resolution of microseconds. See Section 5.4.5, “The Slow Query Log”.

  • low_priority_updates

    Property Value
    Command-Line Format --low-priority-updates[={OFF|ON}]
    System Variable low_priority_updates
    Scope Global, Session
    Dynamic Yes
    Type Boolean
    Default Value OFF

    If set to 1, all INSERT, UPDATE, DELETE, and LOCK TABLE WRITE statements wait until there is no pending SELECT or LOCK TABLE READ on the affected table. The same effect can be obtained using {INSERT | REPLACE | DELETE | UPDATE} LOW_PRIORITY ... to lower the priority of only one query. This variable affects only storage engines that use only table-level locking (such as MyISAM, MEMORY, and MERGE). See Section 8.11.2, “Table Locking Issues”.

  • lower_case_file_system

    Property Value
    System Variable lower_case_file_system
    Scope Global
    Dynamic No
    Type Boolean

    This variable describes the case sensitivity of file names on the file system where the data directory is located. OFF means file names are case-sensitive, ON means they are not case-sensitive. This variable is read only because it reflects a file system attribute and setting it would have no effect on the file system.

  • lower_case_table_names

    Property Value
    Command-Line Format --lower-case-table-names[=#]
    System Variable lower_case_table_names
    Scope Global
    Dynamic No
    Type Integer
    Default Value 0
    Minimum Value 0
    Maximum Value 2

    If set to 0, table names are stored as specified and comparisons are case-sensitive. If set to 1, table names are stored in lowercase on disk and comparisons are not case-sensitive. If set to 2, table names are stored as given but compared in lowercase. This option also applies to database names and table aliases. For additional details, see Section 9.2.3, “Identifier Case Sensitivity”.

    On Windows the default value is 1. On macOS, the default value is 2. On Linux, a value of 2 is not supported; the server forces the value to 0 instead.

    You should not set lower_case_table_names to 0 if you are running MySQL on a system where the data directory resides on a case-insensitive file system (such as on Windows or macOS). It is an unsupported combination that could result in a hang condition when running an INSERT INTO ... SELECT ... FROM tbl_name operation with the wrong tbl_name lettercase. With MyISAM, accessing table names using different lettercases could cause index corruption.

    An error message is printed and the server exits if you attempt to start the server with --lower_case_table_names=0 on a case-insensitive file system.

    If you are using InnoDB tables, you should set this variable to 1 on all platforms to force names to be converted to lowercase.

    The setting of this variable affects the behavior of replication filtering options with regard to case sensitivity. For more information, see Section 16.2.5, “How Servers Evaluate Replication Filtering Rules”.

  • max_allowed_packet

    Property Value
    Command-Line Format --max-allowed-packet=#
    System Variable max_allowed_packet
    Scope Global, Session
    Dynamic Yes
    Type Integer
    Default Value 4194304
    Minimum Value 1024
    Maximum Value 1073741824

    The maximum size of one packet or any generated/intermediate string, or any parameter sent by the mysql_stmt_send_long_data() C API function. The default is 4MB.

    The packet message buffer is initialized to net_buffer_length bytes, but can grow up to max_allowed_packet bytes when needed. This value by default is small, to catch large (possibly incorrect) packets.

    You must increase this value if you are using large BLOB columns or long strings. It should be as big as the largest BLOB you want to use. The protocol limit for max_allowed_packet is 1GB. The value should be a multiple of 1024; nonmultiples are rounded down to the nearest multiple.

    When you change the message buffer size by changing the value of the max_allowed_packet variable, you should also change the buffer size on the client side if your client program permits it. The default max_allowed_packet value built in to the client library is 1GB, but individual client programs might override this. For example, mysql and mysqldump have defaults of 16MB and 24MB, respectively. They also enable you to change the client-side value by setting max_allowed_packet on the command line or in an option file.

    The session value of this variable is read only. The client can receive up to as many bytes as the session value. However, the server will not send to the client more bytes than the current global max_allowed_packet value. (The global value could be less than the session value if the global value is changed after the client connects.)

  • max_connect_errors

    Property Value
    Command-Line Format --max-connect-errors=#
    System Variable max_connect_errors
    Scope Global
    Dynamic Yes
    Type Integer
    Default Value 100
    Minimum Value 1
    Maximum Value (64-bit platforms) 18446744073709551615
    Maximum Value (32-bit platforms) 4294967295

    After max_connect_errors successive connection requests from a host are interrupted without a successful connection, the server blocks that host from further connections. If a connection from a host is established successfully within fewer than max_connect_errors attempts after a previous connection was interrupted, the error count for the host is cleared to zero. However, once a host is blocked, flushing the host cache is the only way to unblock it. To flush the host cache, execute a FLUSH HOSTS statement, a TRUNCATE TABLE statement that truncates the Performance Schema host_cache table, or a mysqladmin flush-hosts command.

    For more information about how the host cache works, see Section 8.12.5.2, “DNS Lookup Optimization and the Host Cache”.

  • max_connections

    Property Value
    Command-Line Format --max-connections=#
    System Variable max_connections
    Scope Global
    Dynamic Yes
    Type Integer
    Default Value 151
    Minimum Value 1
    Maximum Value 100000

    The maximum permitted number of simultaneous client connections. For more information, see Section 8.12.5.1, “How MySQL Handles Client Connections”.

  • max_delayed_threads

    Property Value
    Command-Line Format --max-delayed-threads=#
    Deprecated Yes
    System Variable max_delayed_threads
    Scope Global, Session
    Dynamic Yes
    Type Integer
    Default Value 20
    Minimum Value 0
    Maximum Value 16384

    This system variable is deprecated (because DELAYED inserts are not supported), and will be removed in a future release.

  • max_digest_length

    Property Value
    Command-Line Format --max-digest-length=#
    System Variable max_digest_length
    Scope Global
    Dynamic No
    Type Integer
    Default Value 1024
    Minimum Value 0
    Maximum Value 1048576

    The maximum number of bytes of memory reserved per session for computation of normalized statement digests. Once that amount of space is used during digest computation, truncation occurs: no further tokens from a parsed statement are collected or figure into its digest value. Statements that differ only after that many bytes of parsed tokens produce the same normalized statement digest and are considered identical if compared or if aggregated for digest statistics.

    Decreasing the max_digest_length value reduces memory use but causes the digest value of more statements to become indistinguishable if they differ only at the end. Increasing the value permits longer statements to be distinguished but increases memory use, particularly for workloads that involve large numbers of simultaneous sessions (the server allocates max_digest_length bytes per session).

    The parser uses this system variable as a limit on the maximum length of normalized statement digests that it computes. The Performance Schema, if it tracks statement digests, makes a copy of the digest value, using the performance_schema_max_digest_length. system variable as a limit on the maximum length of digests that it stores. Consequently, if performance_schema_max_digest_length is less than max_digest_length, digest values stored in the Performance Schema are truncated relative to the original digest values.

    For more information about statement digesting, see Section 25.10, “Performance Schema Statement Digests”.

  • max_error_count

    Property Value
    Command-Line Format --max-error-count=#
    System Variable max_error_count
    Scope Global, Session
    Dynamic Yes
    Type Integer
    Default Value 64
    Minimum Value 0
    Maximum Value 65535

    The maximum number of error, warning, and information messages to be stored for display by the SHOW ERRORS and SHOW WARNINGS statements. This is the same as the number of condition areas in the diagnostics area, and thus the number of conditions that can be inspected by GET DIAGNOSTICS.

  • max_execution_time

    Property Value
    Command-Line Format --max-execution-time=#
    System Variable max_execution_time
    Scope Global, Session
    Dynamic Yes
    Type Integer
    Default Value 0

    The execution timeout for SELECT statements, in milliseconds. If the value is 0, timeouts are not enabled.

    max_execution_time applies as follows:

    • The global max_execution_time value provides the default for the session value for new connections. The session value applies to SELECT executions executed within the session that include no MAX_EXECUTION_TIME(N) optimizer hint or for which N is 0.

    • max_execution_time applies to read-only SELECT statements. Statements that are not read only are those that invoke a stored function that modifies data as a side effect.

    • max_execution_time is ignored for SELECT statements in stored programs.

  • max_heap_table_size

    Property Value
    Command-Line Format --max-heap-table-size=#
    System Variable max_heap_table_size
    Scope Global, Session
    Dynamic Yes
    Type Integer
    Default Value 16777216
    Minimum Value 16384
    Maximum Value (64-bit platforms) 1844674407370954752
    Maximum Value (32-bit platforms) 4294967295

    This variable sets the maximum size to which user-created MEMORY tables are permitted to grow. The value of the variable is used to calculate MEMORY table MAX_ROWS values. Setting this variable has no effect on any existing MEMORY table, unless the table is re-created with a statement such as CREATE TABLE or altered with ALTER TABLE or TRUNCATE TABLE. A server restart also sets the maximum size of existing MEMORY tables to the global max_heap_table_size value.

    This variable is also used in conjunction with tmp_table_size to limit the size of internal in-memory tables. See Section 8.4.4, “Internal Temporary Table Use in MySQL”.

    max_heap_table_size is not replicated. See Section 16.4.1.20, “Replication and MEMORY Tables”, and Section 16.4.1.37, “Replication and Variables”, for more information.

  • max_insert_delayed_threads

    Property Value
    Deprecated Yes
    System Variable max_insert_delayed_threads
    Scope Global, Session
    Dynamic Yes
    Type Integer

    This variable is a synonym for max_delayed_threads.

    This system variable is deprecated (because DELAYED inserts are not supported), and will be removed in a future release.

  • max_join_size

    Property Value
    Command-Line Format --max-join-size=#
    System Variable max_join_size
    Scope Global, Session
    Dynamic Yes
    Type Integer
    Default Value 18446744073709551615
    Minimum Value 1
    Maximum Value 18446744073709551615

    Do not permit statements that probably need to examine more than max_join_size rows (for single-table statements) or row combinations (for multiple-table statements) or that are likely to do more than max_join_size disk seeks. By setting this value, you can catch statements where keys are not used properly and that would probably take a long time. Set it if your users tend to perform joins that lack a WHERE clause, that take a long time, or that return millions of rows. For more information, see Using Safe-Updates Mode (--safe-updates).

    Setting this variable to a value other than DEFAULT resets the value of sql_big_selects to 0. If you set the sql_big_selects value again, the max_join_size variable is ignored.

    If a query result is in the query cache, no result size check is performed, because the result has previously been computed and it does not burden the server to send it to the client.

  • max_length_for_sort_data

    Property Value
    Command-Line Format --max-length-for-sort-data=#
    System Variable max_length_for_sort_data
    Scope Global, Session
    Dynamic Yes
    Type Integer
    Default Value 1024
    Minimum Value 4
    Maximum Value 8388608

    The cutoff on the size of index values that determines which filesort algorithm to use. See Section 8.2.1.14, “ORDER BY Optimization”.

  • max_points_in_geometry

    Property Value
    Command-Line Format --max-points-in-geometry=#
    System Variable max_points_in_geometry
    Scope Global, Session
    Dynamic Yes
    Type Integer
    Default Value 65536
    Minimum Value 3
    Maximum Value 1048576

    The maximum value of the points_per_circle argument to the ST_Buffer_Strategy() function.

  • max_prepared_stmt_count

    Property Value
    Command-Line Format --max-prepared-stmt-count=#
    System Variable max_prepared_stmt_count
    Scope Global
    Dynamic Yes
    Type Integer
    Default Value 16382
    Minimum Value 0
    Maximum Value 1048576

    This variable limits the total number of prepared statements in the server. It can be used in environments where there is the potential for denial-of-service attacks based on running the server out of memory by preparing huge numbers of statements. If the value is set lower than the current number of prepared statements, existing statements are not affected and can be used, but no new statements can be prepared until the current number drops below the limit. Setting the value to 0 disables prepared statements.

  • max_seeks_for_key

    Property Value
    Command-Line Format --max-seeks-for-key=#
    System Variable max_seeks_for_key
    Scope Global, Session
    Dynamic Yes
    Type Integer
    Default Value (64-bit platforms) 18446744073709551615
    Default Value (32-bit platforms) 4294967295
    Minimum Value 1
    Maximum Value (64-bit platforms) 18446744073709551615
    Maximum Value (32-bit platforms) 4294967295

    Limit the assumed maximum number of seeks when looking up rows based on a key. The MySQL optimizer assumes that no more than this number of key seeks are required when searching for matching rows in a table by scanning an index, regardless of the actual cardinality of the index (see Section 13.7.5.22, “SHOW INDEX Statement”). By setting this to a low value (say, 100), you can force MySQL to prefer indexes instead of table scans.

  • max_sort_length

    Property Value
    Command-Line Format --max-sort-length=#
    System Variable max_sort_length
    Scope Global, Session
    Dynamic Yes
    Type Integer
    Default Value 1024
    Minimum Value 4
    Maximum Value 8388608

    The number of bytes to use when sorting data values. The server uses only the first max_sort_length bytes of each value and ignores the rest. Consequently, values that differ only after the first max_sort_length bytes compare as equal for GROUP BY, ORDER BY, and DISTINCT operations.

    Increasing the value of max_sort_length may require increasing the value of sort_buffer_size as well. For details, see Section 8.2.1.14, “ORDER BY Optimization”

  • max_sp_recursion_depth

    Property Value
    Command-Line Format --max-sp-recursion-depth[=#]
    System Variable max_sp_recursion_depth
    Scope Global, Session
    Dynamic Yes
    Type Integer
    Default Value 0
    Maximum Value 255

    The number of times that any given stored procedure may be called recursively. The default value for this option is 0, which completely disables recursion in stored procedures. The maximum value is 255.

    Stored procedure recursion increases the demand on thread stack space. If you increase the value of max_sp_recursion_depth, it may be necessary to increase thread stack size by increasing the value of thread_stack at server startup.

  • max_tmp_tables

    This variable is unused. It is deprecated and is removed in MySQL 8.0.

  • max_user_connections

    Property Value
    Command-Line Format --max-user-connections=#
    System Variable max_user_connections
    Scope Global, Session
    Dynamic Yes
    Type Integer
    Default Value 0
    Minimum Value 0
    Maximum Value 4294967295

    The maximum number of simultaneous connections permitted to any given MySQL user account. A value of 0 (the default) means no limit.

    This variable has a global value that can be set at server startup or runtime. It also has a read-only session value that indicates the effective simultaneous-connection limit that applies to the account associated with the current session. The session value is initialized as follows:

    • If the user account has a nonzero MAX_USER_CONNECTIONS resource limit, the session max_user_connections value is set to that limit.

    • Otherwise, the session max_user_connections value is set to the global value.

    Account resource limits are specified using the CREATE USER or ALTER USER statement. See Section 6.2.16, “Setting Account Resource Limits”.

  • max_write_lock_count

    Property Value
    Command-Line Format --max-write-lock-count=#
    System Variable max_write_lock_count
    Scope Global
    Dynamic Yes
    Type Integer
    Default Value (64-bit platforms) 18446744073709551615
    Default Value (32-bit platforms) 4294967295
    Minimum Value 1
    Maximum Value (64-bit platforms) 18446744073709551615
    Maximum Value (32-bit platforms) 4294967295

    After this many write locks, permit some pending read lock requests to be processed in between. Write lock requests have higher priority than read lock requests. However, if max_write_lock_count is set to some low value (say, 10), read lock requests may be preferred over pending write lock requests if the read lock requests have already been passed over in favor of 10 write lock requests. Normally this behavior does not occur because max_write_lock_count by default has a very large value.

  • mecab_rc_file

    Property Value
    Command-Line Format --mecab-rc-file=file_name
    System Variable mecab_rc_file
    Scope Global
    Dynamic No
    Type File name

    The mecab_rc_file option is used when setting up the MeCab full-text parser.

    The mecab_rc_file option defines the path to the mecabrc configuration file, which is the configuration file for MeCab. The option is read-only and can only be set at startup. The mecabrc configuration file is required to initialize MeCab.

    For information about the MeCab full-text parser, see Section 12.9.9, “MeCab Full-Text Parser Plugin”.

    For information about options that can be specified in the MeCab mecabrc configuration file, refer to the MeCab Documentation on the Google Developers site.

  • metadata_locks_cache_size

    Property Value
    Command-Line Format --metadata-locks-cache-size=#
    Deprecated Yes
    System Variable metadata_locks_cache_size
    Scope Global
    Dynamic No
    Type Integer
    Default Value 1024
    Minimum Value 1
    Maximum Value 1048576

    The size of the metadata locks cache. The server uses this cache to avoid creation and destruction of synchronization objects. This is particularly helpful on systems where such operations are expensive, such as Windows XP.

    In MySQL 5.7.4, metadata locking implementation changes make this variable unnecessary, so it is deprecated and will be removed in a future MySQL release.

  • metadata_locks_hash_instances

    Property Value
    Command-Line Format --metadata-locks-hash-instances=#
    Deprecated Yes
    System Variable metadata_locks_hash_instances
    Scope Global
    Dynamic No
    Type Integer
    Default Value 8
    Minimum Value 1
    Maximum Value 1024

    The set of metadata locks can be partitioned into separate hashes to permit connections accessing different objects to use different locking hashes and reduce contention. The metadata_locks_hash_instances system variable specifies the number of hashes (default 8).

    In MySQL 5.7.4, metadata locking implementation changes make this variable unnecessary, so it is deprecated and will be removed in a future MySQL release.

  • min_examined_row_limit

    Property Value
    Command-Line Format --min-examined-row-limit=#
    System Variable min_examined_row_limit
    Scope Global, Session
    Dynamic Yes
    Type Integer
    Default Value 0
    Minimum Value 0
    Maximum Value (64-bit platforms) 18446744073709551615
    Maximum Value (32-bit platforms) 4294967295

    Queries that examine fewer than this number of rows are not logged to the slow query log.

  • multi_range_count

    Property Value
    Command-Line Format --multi-range-count=#
    Deprecated Yes
    System Variable multi_range_count
    Scope Global, Session
    Dynamic Yes
    Type Integer
    Default Value 256
    Minimum Value 1
    Maximum Value 4294967295

    This variable has no effect. It is deprecated and is removed in MySQL 8.0.

  • myisam_data_pointer_size

    Property Value
    Command-Line Format --myisam-data-pointer-size=#
    System Variable myisam_data_pointer_size
    Scope Global
    Dynamic Yes
    Type Integer
    Default Value 6
    Minimum Value 2
    Maximum Value 7

    The default pointer size in bytes, to be used by CREATE TABLE for MyISAM tables when no MAX_ROWS option is specified. This variable cannot be less than 2 or larger than 7. The default value is 6. See Section B.4.2.11, “The table is full”.

  • myisam_max_sort_file_size

    Property Value
    Command-Line Format --myisam-max-sort-file-size=#
    System Variable myisam_max_sort_file_size
    Scope Global
    Dynamic Yes
    Type Integer
    Default Value (64-bit platforms) 9223372036854775807
    Default Value (32-bit platforms) 2147483648

    The maximum size of the temporary file that MySQL is permitted to use while re-creating a MyISAM index (during REPAIR TABLE, ALTER TABLE, or LOAD DATA). If the file size would be larger than this value, the index is created using the key cache instead, which is slower. The value is given in bytes.

    If MyISAM index files exceed this size and disk space is available, increasing the value may help performance. The space must be available in the file system containing the directory where the original index file is located.

  • myisam_mmap_size

    Property Value
    Command-Line Format --myisam-mmap-size=#
    System Variable myisam_mmap_size
    Scope Global
    Dynamic No
    Type Integer
    Default Value (64-bit platforms) 18446744073709551615
    Default Value (32-bit platforms) 4294967295
    Minimum Value 7
    Maximum Value (64-bit platforms) 18446744073709551615
    Maximum Value (32-bit platforms) 4294967295

    The maximum amount of memory to use for memory mapping compressed MyISAM files. If many compressed MyISAM tables are used, the value can be decreased to reduce the likelihood of memory-swapping problems.

  • myisam_recover_options

    Property Value
    Command-Line Format --myisam-recover-options[=list]
    System Variable myisam_recover_options
    Scope Global
    Dynamic No
    Type Enumeration
    Default Value OFF
    Valid Values

    OFF

    DEFAULT

    BACKUP

    FORCE

    QUICK

    Set the MyISAM storage engine recovery mode. The variable value is any combination of the values of OFF, DEFAULT, BACKUP, FORCE, or QUICK. If you specify multiple values, separate them by commas. Specifying the variable with no value at server startup is the same as specifying DEFAULT, and specifying with an explicit value of "" disables recovery (same as a value of OFF). If recovery is enabled, each time mysqld opens a MyISAM table, it checks whether the table is marked as crashed or was not closed properly. (The last option works only if you are running with external locking disabled.) If this is the case, mysqld runs a check on the table. If the table was corrupted, mysqld attempts to repair it.

    The following options affect how the repair works.

    Option Description
    OFF No recovery.
    DEFAULT Recovery without backup, forcing, or quick checking.
    BACKUP If the data file was changed during recovery, save a backup of the tbl_name.MYD file as tbl_name-datetime.BAK.
    FORCE Run recovery even if we would lose more than one row from the .MYD file.
    QUICK Do not check the rows in the table if there are not any delete blocks.

    Before the server automatically repairs a table, it writes a note about the repair to the error log. If you want to be able to recover from most problems without user intervention, you should use the options BACKUP,FORCE. This forces a repair of a table even if some rows would be deleted, but it keeps the old data file as a backup so that you can later examine what happened.

    See Section 15.2.1, “MyISAM Startup Options”.

  • myisam_repair_threads

    Property Value
    Command-Line Format --myisam-repair-threads=#
    System Variable myisam_repair_threads
    Scope Global, Session
    Dynamic Yes
    Type Integer
    Default Value 1
    Minimum Value 1
    Maximum Value (64-bit platforms) 18446744073709551615
    Maximum Value (32-bit platforms) 4294967295

    If this value is greater than 1, MyISAM table indexes are created in parallel (each index in its own thread) during the Repair by sorting process. The default value is 1.

    Note

    Multithreaded repair is still beta-quality code.

  • myisam_sort_buffer_size

    Property Value
    Command-Line Format --myisam-sort-buffer-size=#
    System Variable myisam_sort_buffer_size
    Scope Global, Session
    Dynamic Yes
    Type Integer
    Default Value 8388608
    Minimum Value 4096
    Maximum Value (Other, 64-bit platforms) 18446744073709551615
    Maximum Value (Other, 32-bit platforms) 4294967295
    Maximum Value (Windows, 64-bit platforms) 18446744073709551615
    Maximum Value (Windows, 32-bit platforms) 4294967295

    The size of the buffer that is allocated when sorting MyISAM indexes during a REPAIR TABLE or when creating indexes with CREATE INDEX or ALTER TABLE.

  • myisam_stats_method

    Property Value
    Command-Line Format --myisam-stats-method=name
    System Variable myisam_stats_method
    Scope Global, Session
    Dynamic Yes
    Type Enumeration
    Default Value nulls_unequal
    Valid Values

    nulls_equal

    nulls_unequal

    nulls_ignored

    How the server treats NULL values when collecting statistics about the distribution of index values for MyISAM tables. This variable has three possible values, nulls_equal, nulls_unequal, and nulls_ignored. For nulls_equal, all NULL index values are considered equal and form a single value group that has a size equal to the number of NULL values. For nulls_unequal, NULL values are considered unequal, and each NULL forms a distinct value group of size 1. For nulls_ignored, NULL values are ignored.

    The method that is used for generating table statistics influences how the optimizer chooses indexes for query execution, as described in Section 8.3.7, “InnoDB and MyISAM Index Statistics Collection”.

  • myisam_use_mmap

    Property Value
    Command-Line Format --myisam-use-mmap[={OFF|ON}]
    System Variable myisam_use_mmap
    Scope Global
    Dynamic Yes
    Type Boolean
    Default Value OFF

    Use memory mapping for reading and writing MyISAM tables.

  • mysql_native_password_proxy_users

    Property Value
    Command-Line Format --mysql-native-password-proxy-users[={OFF|ON}]
    System Variable mysql_native_password_proxy_users
    Scope Global
    Dynamic Yes
    Type Boolean
    Default Value OFF

    This variable controls whether the mysql_native_password built-in authentication plugin supports proxy users. It has no effect unless the check_proxy_users system variable is enabled. For information about user proxying, see Section 6.2.14, “Proxy Users”.

  • named_pipe

    Property Value
    Command-Line Format --named-pipe[={OFF|ON}]
    System Variable named_pipe
    Scope Global
    Dynamic No
    Platform Specific Windows
    Type Boolean
    Default Value OFF

    (Windows only.) Indicates whether the server supports connections over named pipes.

  • named_pipe_full_access_group

    Property Value
    Command-Line Format --named-pipe-full-access-group=value
    Introduced 5.7.25
    System Variable named_pipe_full_access_group
    Scope Global
    Dynamic No
    Platform Specific Windows
    Type String
    Default Value *everyone*
    Valid Values

    *everyone*

    empty string

    (Windows only.) The access control granted to clients on the named pipe created by the MySQL server is set to the minimum necessary for successful communication when the named_pipe system variable is enabled to support named-pipe connections. Newer MySQL client software can open named pipe connections without any additional configuration, however, older client software may still require full access to open a named pipe connection.

    This variable sets the name of a Windows local group whose members are granted sufficient access by the MySQL server to use older named-pipe clients. Initially, the value is set to '*everyone*' by default, which permits users of the Everyone group on Windows to continue using older clients until the older clients are upgraded. In contrast, setting the value to an empty string means that no Windows user will be granted full access to the named pipe. The default value '*everyone*' provides a language-independent way of referring to the Everyone group on Windows.

    Ideally, a new Windows local group name (for example, mysql_old_client_users) should be created in Windows and then used to replace the default value for this variable when access to older client software is absolutely necessary. In this case, limit the membership of the group to as few users as possible, removing users from the group when their client software is upgraded. A non-member of the group who attempts to open a connection to MySQL with the older named-pipe client is denied access until the user is added to the group by a Windows administrator, and then the user logs out and logs in (required by Windows).

  • net_buffer_length

    Property Value
    Command-Line Format --net-buffer-length=#
    System Variable net_buffer_length
    Scope Global, Session
    Dynamic Yes
    Type Integer
    Default Value 16384
    Minimum Value 1024
    Maximum Value 1048576

    Each client thread is associated with a connection buffer and result buffer. Both begin with a size given by net_buffer_length but are dynamically enlarged up to max_allowed_packet bytes as needed. The result buffer shrinks to net_buffer_length after each SQL statement.

    This variable should not normally be changed, but if you have very little memory, you can set it to the expected length of statements sent by clients. If statements exceed this length, the connection buffer is automatically enlarged. The maximum value to which net_buffer_length can be set is 1MB.

    The session value of this variable is read only.

  • net_read_timeout

    Property Value
    Command-Line Format --net-read-timeout=#
    System Variable net_read_timeout
    Scope Global, Session
    Dynamic Yes
    Type Integer
    Default Value 30
    Minimum Value 1

    The number of seconds to wait for more data from a connection before aborting the read. When the server is reading from the client, net_read_timeout is the timeout value controlling when to abort. When the server is writing to the client, net_write_timeout is the timeout value controlling when to abort. See also slave_net_timeout.

  • net_retry_count

    Property Value
    Command-Line Format --net-retry-count=#
    System Variable net_retry_count
    Scope Global, Session
    Dynamic Yes
    Type Integer
    Default Value 10
    Minimum Value 1
    Maximum Value (64-bit platforms) 18446744073709551615
    Maximum Value (32-bit platforms) 4294967295

    If a read or write on a communication port is interrupted, retry this many times before giving up. This value should be set quite high on FreeBSD because internal interrupts are sent to all threads.

  • net_write_timeout

    Property Value
    Command-Line Format --net-write-timeout=#
    System Variable net_write_timeout
    Scope Global, Session
    Dynamic Yes
    Type Integer
    Default Value 60
    Minimum Value 1

    The number of seconds to wait for a block to be written to a connection before aborting the write. See also net_read_timeout.

  • new

    Property Value
    Command-Line Format --new[={OFF|ON}]
    System Variable new
    Scope Global, Session
    Dynamic Yes
    Disabled by skip-new
    Type Boolean
    Default Value OFF

    This variable was used in MySQL 4.0 to turn on some 4.1 behaviors, and is retained for backward compatibility. Its value is always OFF.

    In NDB Cluster, setting this variable to ON makes it possible to employ partitioning types other than KEY or LINEAR KEY with NDB tables. This feature is experimental only, and not supported in production. For additional information, see User-defined partitioning and the NDB storage engine (NDB Cluster).

  • ngram_token_size

    Property Value
    Command-Line Format --ngram-token-size=#
    System Variable ngram_token_size
    Scope Global
    Dynamic No
    Type Integer
    Default Value 2
    Minimum Value 1
    Maximum Value 10

    Defines the n-gram token size for the n-gram full-text parser. The ngram_token_size option is read-only and can only be modified at startup. The default value is 2 (bigram). The maximum value is 10.

    For more information about how to configure this variable, see Section 12.9.8, “ngram Full-Text Parser”.

  • offline_mode

    Property Value
    Command-Line Format --offline-mode[={OFF|ON}]
    System Variable offline_mode
    Scope Global
    Dynamic Yes
    Type Boolean
    Default Value OFF

    Whether the server is in offline mode, which has these characteristics:

    • Connected client users who do not have the SUPER privilege are disconnected on the next request, with an appropriate error. Disconnection includes terminating running statements and releasing locks. Such clients also cannot initiate new connections, and receive an appropriate error.

    • Connected client users who have the SUPER privilege are not disconnected, and can initiate new connections to manage the server.

    • Replication slave threads are permitted to keep applying data to the server.

    Only users who have the SUPER privilege can control offline mode. To put a server in offline mode, change the value of the offline_mode system variable from OFF to ON. To resume normal operations, change offline_mode from ON to OFF. In offline mode, clients that are refused access receive an ER_SERVER_OFFLINE_MODE error.

  • old

    Property Value
    Command-Line Format --old[={OFF|ON}]
    System Variable old
    Scope Global
    Dynamic No
    Type Boolean
    Default Value OFF

    old is a compatibility variable. It is disabled by default, but can be enabled at startup to revert the server to behaviors present in older versions.

    When old is enabled, it changes the default scope of index hints to that used prior to MySQL 5.1.17. That is, index hints with no FOR clause apply only to how indexes are used for row retrieval and not to resolution of ORDER BY or GROUP BY clauses. (See Section 8.9.4, “Index Hints”.) Take care about enabling this in a replication setup. With statement-based binary logging, having different modes for the master and slaves might lead to replication errors.

  • old_alter_table

    Property Value
    Command-Line Format --old-alter-table[={OFF|ON}]
    System Variable old_alter_table
    Scope Global, Session
    Dynamic Yes
    Type Boolean
    Default Value OFF

    When this variable is enabled, the server does not use the optimized method of processing an ALTER TABLE operation. It reverts to using a temporary table, copying over the data, and then renaming the temporary table to the original, as used by MySQL 5.0 and earlier. For more information on the operation of ALTER TABLE, see Section 13.1.8, “ALTER TABLE Statement”.

  • old_passwords

    Property Value
    Command-Line Format --old-passwords=value
    Deprecated Yes
    System Variable old_passwords
    Scope Global, Session
    Dynamic Yes
    Type Enumeration
    Default Value 0
    Valid Values

    0

    2

    Note

    This system variable is deprecated in MySQL 5.7 and will be removed in a future MySQL release.

    This variable controls the password hashing method used by the PASSWORD() function. It also influences password hashing performed by CREATE USER and GRANT statements that specify a password using an IDENTIFIED BY clause.

    The following table shows, for each password hashing method, the permitted value of old_passwords and which authentication plugins use the hashing method.

    Password Hashing Method old_passwords Value Associated Authentication Plugin
    MySQL 4.1 native hashing 0 mysql_native_password
    SHA-256 hashing 2 sha256_password

    If you set old_passwords=2, follow the instructions for using the sha256_password plugin at Section 6.4.1.5, “SHA-256 Pluggable Authentication”.

    The server sets the global old_passwords value during startup to be consistent with the password hashing method required by the authentication plugin indicated by the default_authentication_plugin system variable.

    When a client successfully connects to the server, the server sets the session old_passwords value appropriately for the account authentication method. For example, if the account uses the sha256_password authentication plugin, the server sets old_passwords=2.

    For additional information about authentication plugins and hashing formats, see Section 6.2.13, “Pluggable Authentication”, and Section 6.1.2.4, “Password Hashing in MySQL”.

  • open_files_limit

    Property Value
    Command-Line Format --open-files-limit=#
    System Variable open_files_limit
    Scope Global
    Dynamic No
    Type Integer
    Default Value 5000, with possible adjustment
    Minimum Value 0
    Maximum Value platform dependent

    The number of file descriptors available to mysqld from the operating system. mysqld reserves descriptors with setrlimit(), using the value requested at startup by setting this variable directly or by using the --open-files-limit option to mysqld_safe. If mysqld produces the error Too many open files, try increasing the open_files_limit value. Internally, the maximum value for this variable is the maximum unsigned integer value, but the actual maximum is platform dependent.

    The value of open_files_limit at runtime indicates the number of file descriptors actually permitted to mysqld by the operating system, which might differ from the value requested at startup. If the number of file descriptors requested during startup cannot be allocated, mysqld writes a warning to the error log.

    The effective open_files_limit value is based on the value specified at system startup (if any) and the values of max_connections and table_open_cache, using these formulas:

    • 10 + max_connections + (table_open_cache * 2)

    • max_connections * 5

    • Operating system limit if that limit is positive but not Infinity

    • If operating system limit is Infinity: open_files_limit value if specified at startup, 5000 if not

    The server attempts to obtain the number of file descriptors using the maximum of those values. If that many descriptors cannot be obtained, the server attempts to obtain as many as the system will permit.

    The effective value is 0 on systems where MySQL cannot change the number of open files.

    On Unix, the value cannot be set greater than ulimit -n.

  • optimizer_prune_level

    Property Value
    Command-Line Format --optimizer-prune-level=#
    System Variable optimizer_prune_level
    Scope Global, Session
    Dynamic Yes
    Type Integer
    Default Value 1
    Minimum Value 0
    Maximum Value 1

    Controls the heuristics applied during query optimization to prune less-promising partial plans from the optimizer search space. A value of 0 disables heuristics so that the optimizer performs an exhaustive search. A value of 1 causes the optimizer to prune plans based on the number of rows retrieved by intermediate plans.

  • optimizer_search_depth

    Property Value
    Command-Line Format --optimizer-search-depth=#
    System Variable optimizer_search_depth
    Scope Global, Session
    Dynamic Yes
    Type Integer
    Default Value 62
    Minimum Value 0
    Maximum Value 62

    The maximum depth of search performed by the query optimizer. Values larger than the number of relations in a query result in better query plans, but take longer to generate an execution plan for a query. Values smaller than the number of relations in a query return an execution plan quicker, but the resulting plan may be far from being optimal. If set to 0, the system automatically picks a reasonable value.

  • optimizer_switch

    Property Value
    Command-Line Format --optimizer-switch=value
    System Variable optimizer_switch
    Scope Global, Session
    Dynamic Yes
    Type Set
    Valid Values

    batched_key_access={on|off}

    block_nested_loop={on|off}

    condition_fanout_filter={on|off}

    derived_merge={on|off}

    duplicateweedout={on|off}

    engine_condition_pushdown={on|off}

    firstmatch={on|off}

    index_condition_pushdown={on|off}

    index_merge={on|off}

    index_merge_intersection={on|off}

    index_merge_sort_union={on|off}

    index_merge_union={on|off}

    loosescan={on|off}

    materialization={on|off}

    mrr={on|off}

    mrr_cost_based={on|off}

    semijoin={on|off}

    subquery_materialization_cost_based={on|off}

    use_index_extensions={on|off}

    The optimizer_switch system variable enables control over optimizer behavior. The value of this variable is a set of flags, each of which has a value of on or off to indicate whether the corresponding optimizer behavior is enabled or disabled. This variable has global and session values and can be changed at runtime. The global default can be set at server startup.

    To see the current set of optimizer flags, select the variable value:

    mysql> SELECT @@optimizer_switch\G
    *************************** 1. row ***************************
    @@optimizer_switch: index_merge=on,index_merge_union=on,
                        index_merge_sort_union=on,
                        index_merge_intersection=on,
                        engine_condition_pushdown=on,
                        index_condition_pushdown=on,
                        mrr=on,mrr_cost_based=on,
                        block_nested_loop=on,batched_key_access=off,
                        materialization=on,semijoin=on,loosescan=on,
                        firstmatch=on,duplicateweedout=on,
                        subquery_materialization_cost_based=on,
                        use_index_extensions=on,
                        condition_fanout_filter=on,derived_merge=on
    

    For more information about the syntax of this variable and the optimizer behaviors that it controls, see Section 8.9.2, “Switchable Optimizations”.

  • optimizer_trace

    Property Value
    Command-Line Format --optimizer-trace=value
    System Variable optimizer_trace
    Scope Global, Session
    Dynamic Yes
    Type String

    This variable controls optimizer tracing. For details, see MySQL Internals: Tracing the Optimizer.

  • optimizer_trace_features

    Property Value
    Command-Line Format --optimizer-trace-features=value
    System Variable optimizer_trace_features
    Scope Global, Session
    Dynamic Yes
    Type String

    This variable enables or disables selected optimizer tracing features. For details, see MySQL Internals: Tracing the Optimizer.

  • optimizer_trace_limit

    Property Value
    Command-Line Format --optimizer-trace-limit=#
    System Variable optimizer_trace_limit
    Scope Global, Session
    Dynamic Yes
    Type Integer
    Default Value 1

    The maximum number of optimizer traces to display. For details, see MySQL Internals: Tracing the Optimizer.

  • optimizer_trace_max_mem_size

    Property Value
    Command-Line Format --optimizer-trace-max-mem-size=#
    System Variable optimizer_trace_max_mem_size
    Scope Global, Session
    Dynamic Yes
    Type Integer
    Default Value 16384

    The maximum cumulative size of stored optimizer traces. For details, see MySQL Internals: Tracing the Optimizer.

  • optimizer_trace_offset

    Property Value
    Command-Line Format --optimizer-trace-offset=#
    System Variable optimizer_trace_offset
    Scope Global, Session
    Dynamic Yes
    Type Integer
    Default Value -1

    The offset of optimizer traces to display. For details, see MySQL Internals: Tracing the Optimizer.

  • performance_schema_xxx

    Performance Schema system variables are listed in Section 25.15, “Performance Schema System Variables”. These variables may be used to configure Performance Schema operation.

  • parser_max_mem_size

    Property Value
    Command-Line Format --parser-max-mem-size=#
    Introduced 5.7.12
    System Variable parser_max_mem_size
    Scope Global, Session
    Dynamic Yes
    Type Integer
    Default Value (64-bit platforms) 18446744073709551615
    Default Value (32-bit platforms) 4294967295
    Minimum Value 10000000
    Maximum Value (64-bit platforms) 18446744073709551615
    Maximum Value (32-bit platforms) 4294967295

    The maximum amount of memory available to the parser. The default value places no limit on memory available. The value can be reduced to protect against out-of-memory situations caused by parsing long or complex SQL statements.

  • pid_file

    Property Value
    Command-Line Format --pid-file=file_name
    System Variable pid_file
    Scope Global
    Dynamic No
    Type File name

    The path name of the file in which the server writes its process ID. The server creates the file in the data directory unless an absolute path name is given to specify a different directory. If you specify this variable, you must specify a value. If you do not specify this variable, MySQL uses a default value of host_name.pid, where host_name is the name of the host machine.

    The process ID file is used by other programs such as mysqld_safe to determine the server's process ID. On Windows, this variable also affects the default error log file name. See Section 5.4.2, “The Error Log”.

  • plugin_dir

    Property Value
    Command-Line Format --plugin-dir=dir_name
    System Variable plugin_dir
    Scope Global
    Dynamic No
    Type Directory name
    Default Value BASEDIR/lib/plugin

    The path name of the plugin directory.

    If the plugin directory is writable by the server, it may be possible for a user to write executable code to a file in the directory using SELECT ... INTO DUMPFILE. This can be prevented by making plugin_dir read only to the server or by setting secure_file_priv to a directory where SELECT writes can be made safely.

  • port

    Property Value
    Command-Line Format --port=port_num
    System Variable port
    Scope Global
    Dynamic No
    Type Integer
    Default Value 3306
    Minimum Value 0
    Maximum Value 65535

    The number of the port on which the server listens for TCP/IP connections. This variable can be set with the --port option.

  • preload_buffer_size

    Property Value
    Command-Line Format --preload-buffer-size=#
    System Variable preload_buffer_size
    Scope Global, Session
    Dynamic Yes
    Type Integer
    Default Value 32768
    Minimum Value 1024
    Maximum Value 1073741824

    The size of the buffer that is allocated when preloading indexes.

  • profiling

    If set to 0 or OFF (the default), statement profiling is disabled. If set to 1 or ON, statement profiling is enabled and the SHOW PROFILE and SHOW PROFILES statements provide access to profiling information. See Section 13.7.5.31, “SHOW PROFILES Statement”.

    This variable is deprecated and will be removed in a future MySQL release.

  • profiling_history_size

    The number of statements for which to maintain profiling information if profiling is enabled. The default value is 15. The maximum value is 100. Setting the value to 0 effectively disables profiling. See Section 13.7.5.31, “SHOW PROFILES Statement”.

    This variable is deprecated and will be removed in a future MySQL release.

  • protocol_version

    Property Value
    System Variable protocol_version
    Scope Global
    Dynamic No
    Type Integer

    The version of the client/server protocol used by the MySQL server.

  • proxy_user

    Property Value
    System Variable proxy_user
    Scope Session
    Dynamic No
    Type String

    If the current client is a proxy for another user, this variable is the proxy user account name. Otherwise, this variable is NULL. See Section 6.2.14, “Proxy Users”.

  • pseudo_slave_mode

    Property Value
    System Variable pseudo_slave_mode
    Scope Session
    Dynamic Yes
    Type Integer

    This system variable is for internal server use. pseudo_slave_mode assists with the correct handling of transactions that originated on older or newer servers than the server currently processing them. mysqlbinlog sets the value of pseudo_slave_mode to true before executing any SQL statements.

    pseudo_slave_mode has the following effects on the handling of prepared XA transactions, which can be attached to or detached from the handling session (by default, the session that issues XA START):

    • If true, and the handling session has executed an internal-use BINLOG statement, XA transactions are automatically detached from the session as soon as the first part of the transaction up to XA PREPARE finishes, so they can be committed or rolled back by any session that has the XA_RECOVER_ADMIN privilege.

    • If false, XA transactions remain attached to the handling session as long as that session is alive, during which time no other session can commit the transaction. The prepared transaction is only detached if the session disconnects or the server restarts.

  • pseudo_thread_id

    Property Value
    System Variable pseudo_thread_id
    Scope Session
    Dynamic Yes
    Type Integer

    This variable is for internal server use.

  • query_alloc_block_size

    Property Value
    Command-Line Format --query-alloc-block-size=#
    System Variable query_alloc_block_size
    Scope Global, Session
    Dynamic Yes
    Type Integer
    Default Value 8192
    Minimum Value 1024
    Maximum Value 4294967295
    Block Size 1024

    The allocation size of memory blocks that are allocated for objects created during statement parsing and execution. If you have problems with memory fragmentation, it might help to increase this parameter.

  • query_cache_limit

    Property Value
    Command-Line Format --query-cache-limit=#
    Deprecated 5.7.20
    System Variable query_cache_limit
    Scope Global
    Dynamic Yes
    Type Integer
    Default Value 1048576
    Minimum Value 0
    Maximum Value (64-bit platforms) 18446744073709551615
    Maximum Value (32-bit platforms) 4294967295

    Do not cache results that are larger than this number of bytes. The default value is 1MB.

    Note

    The query cache is deprecated as of MySQL 5.7.20, and is removed in MySQL 8.0. Deprecation includes query_cache_limit.

  • query_cache_min_res_unit

    Property Value
    Command-Line Format --query-cache-min-res-unit=#
    Deprecated 5.7.20
    System Variable query_cache_min_res_unit
    Scope Global
    Dynamic Yes
    Type Integer
    Default Value 4096
    Minimum Value 512
    Maximum Value (64-bit platforms) 18446744073709551615
    Maximum Value (32-bit platforms) 4294967295

    The minimum size (in bytes) for blocks allocated by the query cache. The default value is 4096 (4KB). Tuning information for this variable is given in Section 8.10.3.3, “Query Cache Configuration”.

    Note

    The query cache is deprecated as of MySQL 5.7.20, and is removed in MySQL 8.0. Deprecation includes query_cache_min_res_unit.

  • query_cache_size

    Property Value
    Command-Line Format --query-cache-size=#
    Deprecated 5.7.20
    System Variable query_cache_size
    Scope Global
    Dynamic Yes
    Type Integer
    Default Value 1048576
    Minimum Value 0
    Maximum Value (64-bit platforms) 18446744073709551615
    Maximum Value (32-bit platforms) 4294967295

    The amount of memory allocated for caching query results. By default, the query cache is disabled. This is achieved using a default value of 1M, with a default for query_cache_type of 0. (To reduce overhead significantly if you set the size to 0, you should also start the server with query_cache_type=0.

    The permissible values are multiples of 1024; other values are rounded down to the nearest multiple. For nonzero values of query_cache_size, that many bytes of memory are allocated even if query_cache_type=0. See Section 8.10.3.3, “Query Cache Configuration”, for more information.

    The query cache needs a minimum size of about 40KB to allocate its structures. (The exact size depends on system architecture.) If you set the value of query_cache_size too small, a warning will occur, as described in Section 8.10.3.3, “Query Cache Configuration”.

    Note

    The query cache is deprecated as of MySQL 5.7.20, and is removed in MySQL 8.0. Deprecation includes query_cache_size.

  • query_cache_type

    Property Value
    Command-Line Format --query-cache-type=#
    Deprecated 5.7.20
    System Variable query_cache_type
    Scope Global, Session
    Dynamic Yes
    Type Enumeration
    Default Value 0
    Valid Values

    0

    1

    2

    Set the query cache type. Setting the GLOBAL value sets the type for all clients that connect thereafter. Individual clients can set the SESSION value to affect their own use of the query cache. Possible values are shown in the following table.

    Option Description
    0 or OFF Do not cache results in or retrieve results from the query cache. Note that this does not deallocate the query cache buffer. To do that, you should set query_cache_size to 0.
    1 or ON Cache all cacheable query results except for those that begin with SELECT SQL_NO_CACHE.
    2 or DEMAND Cache results only for cacheable queries that begin with SELECT SQL_CACHE.

    This variable defaults to OFF.

    If the server is started with query_cache_type set to 0, it does not acquire the query cache mutex at all, which means that the query cache cannot be enabled at runtime and there is reduced overhead in query execution.

    Note

    The query cache is deprecated as of MySQL 5.7.20, and is removed in MySQL 8.0. Deprecation includes query_cache_type.

  • query_cache_wlock_invalidate

    Property Value
    Command-Line Format --query-cache-wlock-invalidate[={OFF|ON}]
    Deprecated 5.7.20
    System Variable query_cache_wlock_invalidate
    Scope Global, Session
    Dynamic Yes
    Type Boolean
    Default Value OFF

    Normally, when one client acquires a WRITE lock on a MyISAM table, other clients are not blocked from issuing statements that read from the table if the query results are present in the query cache. Setting this variable to 1 causes acquisition of a WRITE lock for a table to invalidate any queries in the query cache that refer to the table. This forces other clients that attempt to access the table to wait while the lock is in effect.

    Note

    The query cache is deprecated as of MySQL 5.7.20, and is removed in MySQL 8.0. Deprecation includes query_cache_wlock_invalidate.

  • query_prealloc_size

    Property Value
    Command-Line Format --query-prealloc-size=#
    System Variable query_prealloc_size
    Scope Global, Session
    Dynamic Yes
    Type Integer
    Default Value 8192
    Minimum Value 8192
    Maximum Value (64-bit platforms) 18446744073709551615
    Maximum Value (32-bit platforms) 4294967295
    Block Size 1024

    The size of the persistent buffer used for statement parsing and execution. This buffer is not freed between statements. If you are running complex queries, a larger query_prealloc_size value might be helpful in improving performance, because it can reduce the need for the server to perform memory allocation during query execution operations.

  • rand_seed1

    Property Value
    System Variable rand_seed1
    Scope Session
    Dynamic Yes
    Type Integer

    The rand_seed1 and rand_seed2 variables exist as session variables only, and can be set but not read. The variables—but not their values—are shown in the output of SHOW VARIABLES.

    The purpose of these variables is to support replication of the RAND() function. For statements that invoke RAND(), the master passes two values to the slave, where they are used to seed the random number generator. The slave uses these values to set the session variables rand_seed1 and rand_seed2 so that RAND() on the slave generates the same value as on the master.

  • rand_seed2

    See the description for rand_seed1.

  • range_alloc_block_size

    Property Value
    Command-Line Format --range-alloc-block-size=#
    System Variable range_alloc_block_size
    Scope Global, Session
    Dynamic Yes
    Type Integer
    Default Value 4096
    Minimum Value 4096
    Maximum Value (64-bit platforms) 18446744073709547520
    Maximum Value 4294967295
    Block Size 1024

    The size of blocks that are allocated when doing range optimization.

  • range_optimizer_max_mem_size

    Property Value
    Command-Line Format --range-optimizer-max-mem-size=#
    System Variable range_optimizer_max_mem_size
    Scope Global, Session
    Dynamic Yes
    Type Integer
    Default Value (>= 5.7.12) 8388608
    Default Value (<= 5.7.11) 1536000
    Minimum Value 0
    Maximum Value 18446744073709551615

    The limit on memory consumption for the range optimizer. A value of 0 means no limit. If an execution plan considered by the optimizer uses the range access method but the optimizer estimates that the amount of memory needed for this method would exceed the limit, it abandons the plan and considers other plans. For more information, see Limiting Memory Use for Range Optimization.

  • rbr_exec_mode

    Property Value
    System Variable rbr_exec_mode
    Scope Global, Session
    Dynamic Yes
    Type Enumeration
    Default Value STRICT
    Valid Values

    IDEMPOTENT

    STRICT

    For internal use by mysqlbinlog. This variable switches the server between IDEMPOTENT mode and STRICT mode. IDEMPOTENT mode causes suppression of duplicate-key and no-key-found errors in BINLOG statements generated by mysqlbinlog. This mode is useful when replaying a row-based binary log on a server that causes conflicts with existing data. mysqlbinlog sets this mode when you specify the --idempotent option by writing the following to the output:

    SET SESSION RBR_EXEC_MODE=IDEMPOTENT;
  • read_buffer_size

    Property Value
    Command-Line Format --read-buffer-size=#
    System Variable read_buffer_size
    Scope Global, Session
    Dynamic Yes
    Type Integer
    Default Value 131072
    Minimum Value 8200
    Maximum Value 2147479552

    Each thread that does a sequential scan for a MyISAM table allocates a buffer of this size (in bytes) for each table it scans. If you do many sequential scans, you might want to increase this value, which defaults to 131072. The value of this variable should be a multiple of 4KB. If it is set to a value that is not a multiple of 4KB, its value will be rounded down to the nearest multiple of 4KB.

    This option is also used in the following context for all storage engines:

    • For caching the indexes in a temporary file (not a temporary table), when sorting rows for ORDER BY.

    • For bulk insert into partitions.

    • For caching results of nested queries.

    read_buffer_size is also used in one other storage engine-specific way: to determine the memory block size for MEMORY tables.

    For more information about memory use during different operations, see Section 8.12.4.1, “How MySQL Uses Memory”.

  • read_only

    Property Value
    Command-Line Format --read-only[={OFF|ON}]
    System Variable read_only
    Scope Global
    Dynamic Yes
    Type Boolean
    Default Value OFF

    When the read_only system variable is enabled, the server permits no client updates except from users who have the SUPER privilege. This variable is disabled by default.

    The server also supports a super_read_only system variable (disabled by default), which has these effects:

    Even with read_only enabled, the server permits these operations:

    Changes to read_only on a master server are not replicated to slave servers. The value can be set on a slave server independent of the setting on the master.

    The following conditions apply to attempts to enable read_only (including implicit attempts resulting from enabling super_read_only):

    • The attempt fails and an error occurs if you have any explicit locks (acquired with LOCK TABLES) or have a pending transaction.

    • The attempt blocks while other clients have any ongoing statement, active LOCK TABLES WRITE, or ongoing commit, until the locks are released and the statements and transactions end. While the attempt to enable read_only is pending, requests by other clients for table locks or to begin transactions also block until read_only has been set.

    • The attempt blocks if there are active transactions that hold metadata locks, until those transactions end.

    • read_only can be enabled while you hold a global read lock (acquired with FLUSH TABLES WITH READ LOCK) because that does not involve table locks.

  • read_rnd_buffer_size

    Property Value
    Command-Line Format --read-rnd-buffer-size=#
    System Variable read_rnd_buffer_size
    Scope Global, Session
    Dynamic Yes
    Type Integer
    Default Value 262144
    Minimum Value 1
    Maximum Value 2147483647

    This variable is used for reads from MyISAM tables, and, for any storage engine, for Multi-Range Read optimization.

    When reading rows from a MyISAM table in sorted order following a key-sorting operation, the rows are read through this buffer to avoid disk seeks. See Section 8.2.1.14, “ORDER BY Optimization”. Setting the variable to a large value can improve ORDER BY performance by a lot. However, this is a buffer allocated for each client, so you should not set the global variable to a large value. Instead, change the session variable only from within those clients that need to run large queries.

    For more information about memory use during different operations, see Section 8.12.4.1, “How MySQL Uses Memory”. For information about Multi-Range Read optimization, see Section 8.2.1.10, “Multi-Range Read Optimization”.

  • require_secure_transport

    Property Value
    Command-Line Format --require-secure-transport[={OFF|ON}]
    System Variable require_secure_transport
    Scope Global
    Dynamic Yes
    Type Boolean
    Default Value OFF

    Whether client connections to the server are required to use some form of secure transport. When this variable is enabled, the server permits only TCP/IP connections that use SSL, or connections that use a socket file (on Unix) or shared memory (on Windows). The server rejects nonsecure connection attempts, which fail with an ER_SECURE_TRANSPORT_REQUIRED error.

    This capability supplements per-account SSL requirements, which take precedence. For example, if an account is defined with REQUIRE SSL, enabling require_secure_transport does not make it possible to use the account to connect using a Unix socket file.

    It is possible for a server to have no secure transports available. For example, a server on Windows supports no secure transports if started without specifying any SSL certificate or key files and with the shared_memory system variable disabled. Under these conditions, attempts to enable require_secure_transport at startup cause the server to write a message to the error log and exit. Attempts to enable the variable at runtime fail with an ER_NO_SECURE_TRANSPORTS_CONFIGURED error.

    See also Configuring Encrypted Connections as Mandatory.

  • secure_auth

    Property Value
    Command-Line Format --secure-auth[={OFF|ON}]
    Deprecated Yes
    System Variable secure_auth
    Scope Global
    Dynamic Yes
    Type Boolean
    Default Value ON
    Valid Values ON

    If this variable is enabled, the server blocks connections by clients that attempt to use accounts that have passwords stored in the old (pre-4.1) format. Enable this variable to prevent all use of passwords employing the old format (and hence insecure communication over the network).

    This variable is deprecated and will be removed in a future MySQL release. It is always enabled and attempting to disable it produces an error.

    Server startup fails with an error if this variable is enabled and the privilege tables are in pre-4.1 format. See Section 6.4.1.3, “Migrating Away from Pre-4.1 Password Hashing and the mysql_old_password Plugin”.

    Note

    Passwords that use the pre-4.1 hashing method are less secure than passwords that use the native password hashing method and should be avoided. Pre-4.1 passwords are deprecated and support for them is removed in MySQL 5.7.5. For account upgrade instructions, see Section 6.4.1.3, “Migrating Away from Pre-4.1 Password Hashing and the mysql_old_password Plugin”.

  • secure_file_priv

    Property Value
    Command-Line Format --secure-file-priv=dir_name
    System Variable secure_file_priv
    Scope Global
    Dynamic No
    Type String
    Default Value platform specific
    Valid Values

    empty string

    dirname

    NULL

    This variable is used to limit the effect of data import and export operations, such as those performed by the LOAD DATA and SELECT ... INTO OUTFILE statements and the LOAD_FILE() function. These operations are permitted only to users who have the FILE privilege.

    secure_file_priv may be set as follows:

    • If empty, the variable has no effect. This is not a secure setting.

    • If set to the name of a directory, the server limits import and export operations to work only with files in that directory. The directory must exist; the server will not create it.

    • If set to NULL, the server disables import and export operations.

    The default value is platform specific and depends on the value of the INSTALL_LAYOUT CMake option, as shown in the following table. To specify the default secure_file_priv value explicitly if you are building from source, use the INSTALL_SECURE_FILE_PRIVDIR CMake option.

    INSTALL_LAYOUT Value Default secure_file_priv Value
    STANDALONE, WIN NULL (>= MySQL 5.7.16), empty (< MySQL 5.7.16)
    DEB, RPM, SLES, SVR4 /var/lib/mysql-files
    Otherwise mysql-files under the CMAKE_INSTALL_PREFIX value

    To set the default secure_file_priv value for the libmysqld embedded server, use the INSTALL_SECURE_FILE_PRIV_EMBEDDEDDIR CMake option. The default value for this option is NULL.

    The server checks the value of secure_file_priv at startup and writes a warning to the error log if the value is insecure. A non-NULL value is considered insecure if it is empty, or the value is the data directory or a subdirectory of it, or a directory that is accessible by all users. If secure_file_priv is set to a nonexistent path, the server writes an error message to the error log and exits.

  • session_track_gtids

    Property Value
    Command-Line Format --session-track-gtids=value
    System Variable session_track_gtids
    Scope Global, Session
    Dynamic Yes
    Type Enumeration
    Default Value OFF
    Valid Values

    OFF

    OWN_GTID

    ALL_GTIDS

    Controls whether the server tracks GTIDs within the current session and returns them to the client. Depending on the variable value, at the end of executing each transaction, the server GTIDs are captured by the tracker and returned to the client. These session_track_gtids values are permitted:

    • OFF: The tracker collects no GTIDs. This is the default.

    • OWN_GTID: The tracker collects GTIDs generated by successfully committed read/write transactions.

    • ALL_GTIDS: The tracker collects all GTIDs in the gtid_executed system variable at the time the current transaction commits, regardless of whether the transaction is read/write or read only.

    session_track_gtids cannot be set within transactional context.

    For more information about session state tracking, see Section 5.1.14, “Server Tracking of Client Session State Changes”.

  • session_track_schema

    Property Value
    Command-Line Format --session-track-schema[={OFF|ON}]
    System Variable session_track_schema
    Scope Global, Session
    Dynamic Yes
    Type Boolean
    Default Value ON

    Controls whether the server tracks when the default schema (database) is set within the current session and notifies the client to make the schema name available.

    If the schema name tracker is enabled, name notification occurs each time the default schema is set, even if the new schema name is the same as the old.

    For more information about session state tracking, see Section 5.1.14, “Server Tracking of Client Session State Changes”.

  • session_track_state_change

    Property Value
    Command-Line Format --session-track-state-change[={OFF|ON}]
    System Variable session_track_state_change
    Scope Global, Session
    Dynamic Yes
    Type Boolean
    Default Value OFF

    Controls whether the server tracks changes to the state of the current session and notifies the client when state changes occur. Changes can be reported for these attributes of client session state:

    • The default schema (database).

    • Session-specific values for system variables.

    • User-defined variables.

    • Temporary tables.

    • Prepared statements.

    If the session state tracker is enabled, notification occurs for each change that involves tracked session attributes, even if the new attribute values are the same as the old. For example, setting a user-defined variable to its current value results in a notification.

    The session_track_state_change variable controls only notification of when changes occur, not what the changes are. For example, state-change notifications occur when the default schema is set or tracked session system variables are assigned, but the notification does not include the schema name or variable values. To receive notification of the schema name or session system variable values, use the session_track_schema or session_track_system_variables system variable, respectively.

    Note

    Assigning a value to session_track_state_change itself is not considered a state change and is not reported as such. However, if its name listed in the value of session_track_system_variables, any assignments to it do result in notification of the new value.

    For more information about session state tracking, see Section 5.1.14, “Server Tracking of Client Session State Changes”.

  • session_track_system_variables

    Property Value
    Command-Line Format --session-track-system-variables=#
    System Variable session_track_system_variables
    Scope Global, Session
    Dynamic Yes
    Type String
    Default Value time_zone, autocommit, character_set_client, character_set_results, character_set_connection

    Controls whether the server tracks assignments to session system variables and notifies the client of the name and value of each assigned variable. The variable value is a comma-separated list of variables for which to track assignments. By default, notification is enabled for time_zone, autocommit, character_set_client, character_set_results, and character_set_connection. (The latter three variables are those affected by SET NAMES.)

    The special value * causes the server to track assignments to all session variables. If given, this value must be specified by itself without specific system variable names.

    To disable notification of session variable assignments, set session_track_system_variables to the empty string.

    If session system variable tracking is enabled, notification occurs for all assignments to tracked session variables, even if the new values are the same as the old.

    For more information about session state tracking, see Section 5.1.14, “Server Tracking of Client Session State Changes”.

  • session_track_transaction_info

    Property Value
    Command-Line Format --session-track-transaction-info=value
    System Variable session_track_transaction_info
    Scope Global, Session
    Dynamic Yes
    Type Enumeration
    Default Value OFF
    Valid Values

    OFF

    STATE

    CHARACTERISTICS

    Controls whether the server tracks the state and characteristics of transactions within the current session and notifies the client to make this information available. These session_track_transaction_info values are permitted:

    • OFF: Disable transaction state tracking. This is the default.

    • STATE: Enable transaction state tracking without characteristics tracking. State tracking enables the client to determine whether a transaction is in progress and whether it could be moved to a different session without being rolled back.

    • CHARACTERISTICS: Enable transaction state tracking, including characteristics tracking. Characteristics tracking enables the client to determine how to restart a transaction in another session so that it has the same characteristics as in the original session. The following characteristics are relevant for this purpose:

      ISOLATION LEVEL
      READ ONLY 
      READ WRITE
      WITH CONSISTENT SNAPSHOT

    For a client to safely relocate a transaction to another session, it must track not only transaction state but also transaction characteristics. In addition, the client must track the transaction_isolation and transaction_read_only system variables to correctly determine the session defaults. (To track these variables, list them in the value of the session_track_system_variables system variable.)

    For more information about session state tracking, see Section 5.1.14, “Server Tracking of Client Session State Changes”.

  • sha256_password_auto_generate_rsa_keys

    Property Value
    Command-Line Format --sha256-password-auto-generate-rsa-keys[={OFF|ON}]
    System Variable sha256_password_auto_generate_rsa_keys
    Scope Global
    Dynamic No
    Type Boolean
    Default Value ON

    This variable is available if the server was compiled using OpenSSL (see Section 6.3.4, “SSL Library-Dependent Capabilities”). It controls whether the server autogenerates RSA private/public key-pair files in the data directory, if they do not already exist.

    At startup, the server automatically generates RSA private/public key-pair files in the data directory if the sha256_password_auto_generate_rsa_keys system variable is enabled, no RSA options are specified, and the RSA files are missing from the data directory. These files enable secure password exchange using RSA over unencrypted connections for accounts authenticated by the sha256_password plugin; see Section 6.4.1.5, “SHA-256 Pluggable Authentication”.

    For more information about RSA file autogeneration, including file names and characteristics, see Section 6.3.3.1, “Creating SSL and RSA Certificates and Keys using MySQL”

    The auto_generate_certs system variable is related but controls autogeneration of SSL certificate and key files needed for secure connections using SSL.

  • sha256_password_private_key_path

    Property Value
    Command-Line Format --sha256-password-private-key-path=file_name
    System Variable sha256_password_private_key_path
    Scope Global
    Dynamic No
    Type File name
    Default Value private_key.pem

    This variable is available if MySQL was compiled using OpenSSL (see Section 6.3.4, “SSL Library-Dependent Capabilities”). Its value is the path name of the RSA private key file for the sha256_password authentication plugin. If the file is named as a relative path, it is interpreted relative to the server data directory. The file must be in PEM format.

    Important

    Because this file stores a private key, its access mode should be restricted so that only the MySQL server can read it.

    For information about sha256_password, see Section 6.4.1.5, “SHA-256 Pluggable Authentication”.

  • sha256_password_proxy_users

    Property Value
    Command-Line Format --sha256-password-proxy-users[={OFF|ON}]
    System Variable sha256_password_proxy_users
    Scope Global
    Dynamic Yes
    Type Boolean
    Default Value OFF

    This variable controls whether the sha256_password built-in authentication plugin supports proxy users. It has no effect unless the check_proxy_users system variable is enabled. For information about user proxying, see Section 6.2.14, “Proxy Users”.

  • sha256_password_public_key_path

    Property Value
    Command-Line Format --sha256-password-public-key-path=file_name
    System Variable sha256_password_public_key_path
    Scope Global
    Dynamic No
    Type File name
    Default Value public_key.pem

    This variable is available if MySQL was compiled using OpenSSL (see Section 6.3.4, “SSL Library-Dependent Capabilities”). Its value is the path name of the RSA public key file for the sha256_password authentication plugin. If the file is named as a relative path, it is interpreted relative to the server data directory. The file must be in PEM format. Because this file stores a public key, copies can be freely distributed to client users. (Clients that explicitly specify a public key when connecting to the server using RSA password encryption must use the same public key as that used by the server.)

    For information about sha256_password, including information about how clients specify the RSA public key, see Section 6.4.1.5, “SHA-256 Pluggable Authentication”.

  • shared_memory

    Property Value
    Command-Line Format --shared-memory[={OFF|ON}]
    System Variable shared_memory
    Scope Global
    Dynamic No
    Platform Specific Windows
    Type Boolean
    Default Value OFF

    (Windows only.) Whether the server permits shared-memory connections.

  • shared_memory_base_name

    Property Value
    Command-Line Format --shared-memory-base-name=name
    System Variable shared_memory_base_name
    Scope Global
    Dynamic No
    Platform Specific Windows
    Type String
    Default Value MYSQL

    (Windows only.) The name of shared memory to use for shared-memory connections. This is useful when running multiple MySQL instances on a single physical machine. The default name is MYSQL. The name is case-sensitive.

    This variable applies only if the server is started with the shared_memory system variable enabled to support shared-memory connections.

  • show_compatibility_56

    Property Value
    Command-Line Format --show-compatibility-56[={OFF|ON}]
    Deprecated Yes
    System Variable show_compatibility_56
    Scope Global
    Dynamic Yes
    Type Boolean
    Default Value OFF

    The INFORMATION_SCHEMA has tables that contain system and status variable information (see Section 24.11, “The INFORMATION_SCHEMA GLOBAL_VARIABLES and SESSION_VARIABLES Tables”, and Section 24.10, “The INFORMATION_SCHEMA GLOBAL_STATUS and SESSION_STATUS Tables”). As of MySQL 5.7.6, the Performance Schema also contains system and status variable tables (see Section 25.12.13, “Performance Schema System Variable Tables”, and Section 25.12.14, “Performance Schema Status Variable Tables”). The Performance Schema tables are intended to replace the INFORMATION_SCHEMA tables, which are deprecated as of MySQL 5.7.6 and will be removed in a future MySQL release.

    For advice on migrating away from the INFORMATION_SCHEMA tables to the Performance Schema tables, see Section 25.20, “Migrating to Performance Schema System and Status Variable Tables”. To assist in the migration, you can use the show_compatibility_56 system variable, which affects whether MySQL 5.6 compatibility is enabled with respect to how system and status variable information is provided by the INFORMATION_SCHEMA and Performance Schema tables, and also by the SHOW VARIABLES and SHOW STATUS statements.

    Note

    show_compatibility_56 is deprecated because its only purpose is to permit control over deprecated system and status variable information sources that will be removed in a future MySQL release. When those sources are removed, show_compatibility_56 will have no purpose and will be removed as well.

    The following discussion describes the effects of show_compatibility_56:

    For better understanding, it is strongly recommended that you also read these sections:

    Overview of show_compatibility_56 Effects

    The show_compatibility_56 system variable affects these aspects of server operation regarding system and status variables:

    • Information available from the SHOW VARIABLES and SHOW STATUS statements

    • Information available from the INFORMATION_SCHEMA tables that provide system and status variable information

    • Information available from the Performance Schema tables that provide system and status variable information

    • The effect of the FLUSH STATUS statement on status variables

    This list summarizes the effects of show_compatibility_56, with additional details given later:

    • When show_compatibility_56 is ON, compatibility with MySQL 5.6 is enabled. Older variable information sources (SHOW statements, INFORMATION_SCHEMA tables) produce the same output as in MySQL 5.6.

    • When show_compatibility_56 is OFF, compatibility with MySQL 5.6 is disabled. Selecting from the INFORMATION_SCHEMA tables produces an error because the Performance Schema tables are intended to replace them. The INFORMATION_SCHEMA tables are deprecated as of MySQL 5.7.6 and will be removed in a future MySQL release.

      To obtain system and status variable information When show_compatibility_56=OFF, use the Performance Schema tables or the SHOW statements.

      Note

      When show_compatibility_56=OFF, the SHOW VARIABLES and SHOW STATUS statements display rows from the Performance Schema global_variables, session_variables, global_status, and session_status tables.

      As of MySQL 5.7.9, those tables are world readable and accessible without the SELECT privilege, which means that SELECT is not needed to use the SHOW statements, either. Before MySQL 5.7.9, the SELECT privilege is required to access those Performance Schema tables, either directly, or indirectly through the SHOW statements.

    • Several Slave_xxx status variables are available from SHOW STATUS when show_compatibility_56 is ON. When show_compatibility_56 is OFF, some of those variables are not exposed to SHOW STATUS. The information they provide is available in replication-related Performance Schema tables, as described later.

    • show_compatibility_56 has no effect on system variable access using @@ notation: @@GLOBAL.var_name, @@SESSION.var_name, @@var_name.

    • show_compatibility_56 has no effect for the embedded server, which produces 5.6-compatible output in all cases.

    The following descriptions detail the effect of setting show_compatibility_56 to ON or OFF in the contexts in which this variable applies.

    Effect of show_compatibility_56 on SHOW Statements

    SHOW GLOBAL VARIABLES statement:

    • ON: MySQL 5.6 output.

    • OFF: Output displays rows from the Performance Schema global_variables table.

    SHOW [SESSION | LOCAL] VARIABLES statement:

    • ON: MySQL 5.6 output.

    • OFF: Output displays rows from the Performance Schema session_variables table. (In MySQL 5.7.6 and 5.7.7, OFF output does not fully reflect all system variable values in effect for the current session; it includes no rows for global variables that have no session counterpart. This is corrected in MySQL 5.7.8.)

    SHOW GLOBAL STATUS statement:

    • ON: MySQL 5.6 output.

    • OFF: Output displays rows from the Performance Schema global_status table, plus the Com_xxx statement execution counters.

      OFF output includes no rows for session variables that have no global counterpart, unlike ON output.

    SHOW [SESSION | LOCAL] STATUS statement:

    • ON: MySQL 5.6 output.

    • OFF: Output displays rows from the Performance Schema session_status table, plus the Com_xxx statement execution counters. (In MySQL 5.7.6 and 5.7.7, OFF output does not fully reflect all status variable values in effect for the current session; it includes no rows for global variables that have no session counterpart. This is corrected in MySQL 5.7.8.)

    In MySQL 5.7.6 and 5.7.7, for each of the SHOW statements just described, use of a WHERE clause produces a warning when show_compatibility_56=ON and an error when show_compatibility_56=OFF. (This applies to WHERE clauses that are not optimized away. For example, WHERE 1 is trivially true, is optimized away, and thus produces no warning or error.) This behavior does not occur as of MySQL 5.7.8; WHERE is supported as before 5.7.6.

    Effect of show_compatibility_56 on INFORMATION_SCHEMA Tables

    INFORMATION_SCHEMA tables (GLOBAL_VARIABLES, SESSION_VARIABLES, GLOBAL_STATUS, and SESSION_STATUS):

    • ON: MySQL 5.6 output, with a deprecation warning.

    • OFF: Selecting from these tables produces an error. (Before 5.7.9, selecting from these tables produces no output, with a deprecation warning.)

    Effect of show_compatibility_56 on Performance Schema Tables

    Performance Schema system variable tables:

    • OFF:

      • global_variables: Global system variables only.

      • session_variables: System variables in effect for the current session: A row for each session variable, and a row for each global variable that has no session counterpart.

      • variables_by_thread: Session system variables only, for each active session.

    • ON: Same output as for OFF. (Before 5.7.8, these tables produce no output.)

    Performance Schema status variable tables:

    Effect of show_compatibility_56 on Slave Status Variables

    Slave status variables:

    Effect of show_compatibility_56 on FLUSH STATUS

    FLUSH STATUS statement:

    • ON: This statement produces MySQL 5.6 behavior. It adds the current thread's session status variable values to the global values and resets the session values to zero. Some global variables may be reset to zero as well. It also resets the counters for key caches (default and named) to zero and sets Max_used_connections to the current number of open connections.

    • OFF: This statement adds the session status from all active sessions to the global status variables, resets the status of all active sessions, and resets account, host, and user status values aggregated from disconnected sessions.

  • show_create_table_verbosity

    Property Value
    Command-Line Format --show-create-table-verbosity[={OFF|ON}]
    Introduced 5.7.22
    System Variable show_create_table_verbosity
    Scope Global, Session
    Dynamic Yes
    Type Boolean
    Default Value OFF

    SHOW CREATE TABLE normally does not show the ROW_FORMAT table option if the row format is the default format. Enabling this variable causes SHOW CREATE TABLE to display ROW_FORMAT regardless of whether it is the default format.

  • show_old_temporals

    Property Value
    Command-Line Format --show-old-temporals[={OFF|ON}]
    Deprecated Yes
    System Variable show_old_temporals
    Scope Global, Session
    Dynamic Yes
    Type Boolean
    Default Value OFF

    Whether SHOW CREATE TABLE output includes comments to flag temporal columns found to be in pre-5.6.4 format (TIME, DATETIME, and TIMESTAMP columns without support for fractional seconds precision). This variable is disabled by default. If enabled, SHOW CREATE TABLE output looks like this:

    CREATE TABLE `mytbl` (
      `ts` timestamp /* 5.5 binary format */ NOT NULL DEFAULT CURRENT_TIMESTAMP,
      `dt` datetime /* 5.5 binary format */ DEFAULT NULL,
      `t` time /* 5.5 binary format */ DEFAULT NULL
    ) DEFAULT CHARSET=latin1

    Output for the COLUMN_TYPE column of the INFORMATION_SCHEMA.COLUMNS table is affected similarly.

    This variable is deprecated and will be removed in a future MySQL release.

  • skip_external_locking

    Property Value
    Command-Line Format --skip-external-locking[={OFF|ON}]
    System Variable skip_external_locking
    Scope Global
    Dynamic No
    Type Boolean
    Default Value ON

    This is OFF if mysqld uses external locking (system locking), ON if external locking is disabled. This affects only MyISAM table access.

    This variable is set by the --external-locking or --skip-external-locking option. External locking is disabled by default.

    External locking affects only MyISAM table access. For more information, including conditions under which it can and cannot be used, see Section 8.11.5, “External Locking”.

  • skip_name_resolve

    Property Value
    Command-Line Format --skip-name-resolve[={OFF|ON}]
    System Variable skip_name_resolve
    Scope Global
    Dynamic No
    Type Boolean
    Default Value OFF

    Whether to resolve host names when checking client connections. If this variable is OFF, mysqld resolves host names when checking client connections. If it is ON, mysqld uses only IP numbers; in this case, all Host column values in the grant tables must be IP addresses. See Section 8.12.5.2, “DNS Lookup Optimization and the Host Cache”.

    Depending on the network configuration of your system and the Host values for your accounts, clients may need to connect using an explicit --host option, such as --host=127.0.0.1 or --host=::1.

    An attempt to connect to the host 127.0.0.1 normally resolves to the localhost account. However, this fails if the server is run with skip_name_resolve enabled. If you plan to do that, make sure an account exists that can accept a connection. For example, to be able to connect as root using --host=127.0.0.1 or --host=::1, create these accounts:

    CREATE USER 'root'@'127.0.0.1' IDENTIFIED BY 'root-password';
    CREATE USER 'root'@'::1' IDENTIFIED BY 'root-password';
    
  • skip_networking

    Property Value
    Command-Line Format --skip-networking[={OFF|ON}]
    System Variable skip_networking
    Scope Global
    Dynamic No
    Type Boolean
    Default Value OFF

    This variable controls whether the server permits TCP/IP connections. By default, it is disabled (permit TCP connections). If enabled, the server permits only local (non-TCP/IP) connections and all interaction with mysqld must be made using named pipes or shared memory (on Windows) or Unix socket files (on Unix). This option is highly recommended for systems where only local clients are permitted. See Section 8.12.5.2, “DNS Lookup Optimization and the Host Cache”.

  • skip_show_database

    Property Value
    Command-Line Format --skip-show-database
    System Variable skip_show_database
    Scope Global
    Dynamic No

    This prevents people from using the SHOW DATABASES statement if they do not have the SHOW DATABASES privilege. This can improve security if you have concerns about users being able to see databases belonging to other users. Its effect depends on the SHOW DATABASES privilege: If the variable value is ON, the SHOW DATABASES statement is permitted only to users who have the SHOW DATABASES privilege, and the statement displays all database names. If the value is OFF, SHOW DATABASES is permitted to all users, but displays the names of only those databases for which the user has the SHOW DATABASES or other privilege.

    Caution

    Because a global privilege is considered a privilege for all databases, any global privilege enables a user to see all database names with SHOW DATABASES or by examining the INFORMATION_SCHEMA SCHEMATA table.

  • slow_launch_time

    Property Value
    Command-Line Format --slow-launch-time=#
    System Variable slow_launch_time
    Scope Global
    Dynamic Yes
    Type Integer
    Default Value 2

    If creating a thread takes longer than this many seconds, the server increments the Slow_launch_threads status variable.

  • slow_query_log

    Property Value
    Command-Line Format --slow-query-log[={OFF|ON}]
    System Variable slow_query_log
    Scope Global
    Dynamic Yes
    Type Boolean
    Default Value OFF

    Whether the slow query log is enabled. The value can be 0 (or OFF) to disable the log or 1 (or ON) to enable the log. The destination for log output is controlled by the log_output system variable; if that value is NONE, no log entries are written even if the log is enabled.

    Slow is determined by the value of the long_query_time variable. See Section 5.4.5, “The Slow Query Log”.

  • slow_query_log_file

    Property Value
    Command-Line Format --slow-query-log-file=file_name
    System Variable slow_query_log_file
    Scope Global
    Dynamic Yes
    Type File name
    Default Value host_name-slow.log

    The name of the slow query log file. The default value is host_name-slow.log, but the initial value can be changed with the --slow_query_log_file option.

  • socket

    Property Value
    Command-Line Format --socket={file_name|pipe_name}
    System Variable socket
    Scope Global
    Dynamic No
    Type String
    Default Value (Other) /tmp/mysql.sock
    Default Value (Windows) MySQL

    On Unix platforms, this variable is the name of the socket file that is used for local client connections. The default is /tmp/mysql.sock. (For some distribution formats, the directory might be different, such as /var/lib/mysql for RPMs.)

    On Windows, this variable is the name of the named pipe that is used for local client connections. The default value is MySQL (not case-sensitive).

  • sort_buffer_size

    Property Value
    Command-Line Format --sort-buffer-size=#
    System Variable sort_buffer_size
    Scope Global, Session
    Dynamic Yes
    Type Integer
    Default Value 262144
    Minimum Value 32768
    Maximum Value (Other, 64-bit platforms) 18446744073709551615
    Maximum Value (Other, 32-bit platforms) 4294967295
    Maximum Value (Windows) 4294967295

    Each session that must perform a sort allocates a buffer of this size. sort_buffer_size is not specific to any storage engine and applies in a general manner for optimization. At minimum the sort_buffer_size value must be large enough to accommodate fifteen tuples in the sort buffer. Also, increasing the value of max_sort_length may require increasing the value of sort_buffer_size. For more information, see Section 8.2.1.14, “ORDER BY Optimization”

    If you see many Sort_merge_passes per second in SHOW GLOBAL STATUS output, you can consider increasing the sort_buffer_size value to speed up ORDER BY or GROUP BY operations that cannot be improved with query optimization or improved indexing.

    The optimizer tries to work out how much space is needed but can allocate more, up to the limit. Setting it larger than required globally will slow down most queries that sort. It is best to increase it as a session setting, and only for the sessions that need a larger size. On Linux, there are thresholds of 256KB and 2MB where larger values may significantly slow down memory allocation, so you should consider staying below one of those values. Experiment to find the best value for your workload. See Section B.4.3.5, “Where MySQL Stores Temporary Files”.

    The maximum permissible setting for sort_buffer_size is 4GB−1. Larger values are permitted for 64-bit platforms (except 64-bit Windows, for which large values are truncated to 4GB−1 with a warning).

  • sql_auto_is_null

    Property Value
    System Variable sql_auto_is_null
    Scope Global, Session
    Dynamic Yes
    Type Boolean
    Default Value OFF

    If this variable is enabled, then after a statement that successfully inserts an automatically generated AUTO_INCREMENT value, you can find that value by issuing a statement of the following form:

    SELECT * FROM tbl_name WHERE auto_col IS NULL
    

    If the statement returns a row, the value returned is the same as if you invoked the LAST_INSERT_ID() function. For details, including the return value after a multiple-row insert, see Section 12.15, “Information Functions”. If no AUTO_INCREMENT value was successfully inserted, the SELECT statement returns no row.

    The behavior of retrieving an AUTO_INCREMENT value by using an IS NULL comparison is used by some ODBC programs, such as Access. See Obtaining Auto-Increment Values. This behavior can be disabled by setting sql_auto_is_null to OFF.

    The default value of sql_auto_is_null is OFF.

  • sql_big_selects

    Property Value
    System Variable sql_big_selects
    Scope Global, Session
    Dynamic Yes
    Type Boolean
    Default Value ON

    If set to OFF, MySQL aborts SELECT statements that are likely to take a very long time to execute (that is, statements for which the optimizer estimates that the number of examined rows exceeds the value of max_join_size). This is useful when an inadvisable WHERE statement has been issued. The default value for a new connection is ON, which permits all SELECT statements.

    If you set the max_join_size system variable to a value other than DEFAULT, sql_big_selects is set to OFF.

  • sql_buffer_result

    Property Value
    System Variable sql_buffer_result
    Scope Global, Session
    Dynamic Yes
    Type Boolean
    Default Value OFF

    If enabled, sql_buffer_result forces results from SELECT statements to be put into temporary tables. This helps MySQL free the table locks early and can be beneficial in cases where it takes a long time to send results to the client. The default value is OFF.

  • sql_log_off

    Property Value
    System Variable sql_log_off
    Scope Global, Session
    Dynamic Yes
    Type Boolean
    Default Value OFF
    Valid Values

    OFF (enable logging)

    ON (disable logging)

    This variable controls whether logging to the general query log is disabled for the current session (assuming that the general query log itself is enabled). The default value is OFF (that is, enable logging). To disable or enable general query logging for the current session, set the session sql_log_off variable to ON or OFF.

    Setting the session value of this system variable is a restricted operation. The session user must have privileges sufficient to set restricted session variables. See Section 5.1.8.1, “System Variable Privileges”.

  • sql_mode

    Property Value
    Command-Line Format --sql-mode=name
    System Variable sql_mode
    Scope Global, Session
    Dynamic Yes
    Type Set
    Default Value ONLY_FULL_GROUP_BY STRICT_TRANS_TABLES NO_ZERO_IN_DATE NO_ZERO_DATE ERROR_FOR_DIVISION_BY_ZERO NO_AUTO_CREATE_USER NO_ENGINE_SUBSTITUTION
    Valid Values

    ALLOW_INVALID_DATES

    ANSI_QUOTES

    ERROR_FOR_DIVISION_BY_ZERO

    HIGH_NOT_PRECEDENCE

    IGNORE_SPACE

    NO_AUTO_CREATE_USER

    NO_AUTO_VALUE_ON_ZERO

    NO_BACKSLASH_ESCAPES

    NO_DIR_IN_CREATE

    NO_ENGINE_SUBSTITUTION

    NO_FIELD_OPTIONS

    NO_KEY_OPTIONS

    NO_TABLE_OPTIONS

    NO_UNSIGNED_SUBTRACTION

    NO_ZERO_DATE

    NO_ZERO_IN_DATE

    ONLY_FULL_GROUP_BY

    PAD_CHAR_TO_FULL_LENGTH

    PIPES_AS_CONCAT

    REAL_AS_FLOAT

    STRICT_ALL_TABLES

    STRICT_TRANS_TABLES

    The current server SQL mode, which can be set dynamically. For details, see Section 5.1.10, “Server SQL Modes”.

    Note

    MySQL installation programs may configure the SQL mode during the installation process. If the SQL mode differs from the default or from what you expect, check for a setting in an option file that the server reads at startup.

  • sql_notes

    Property Value
    System Variable sql_notes
    Scope Global, Session
    Dynamic Yes
    Type Boolean
    Default Value ON

    If enabled (the default), diagnostics of Note level increment warning_count and the server records them. If disabled, Note diagnostics do not increment warning_count and the server does not record them. mysqldump includes output to disable this variable so that reloading the dump file does not produce warnings for events that do not affect the integrity of the reload operation.

  • sql_quote_show_create

    Property Value
    System Variable sql_quote_show_create
    Scope Global, Session
    Dynamic Yes
    Type Boolean
    Default Value ON

    If enabled (the default), the server quotes identifiers for SHOW CREATE TABLE and SHOW CREATE DATABASE statements. If disabled, quoting is disabled. This option is enabled by default so that replication works for identifiers that require quoting. See Section 13.7.5.10, “SHOW CREATE TABLE Statement”, and Section 13.7.5.6, “SHOW CREATE DATABASE Statement”.

  • sql_safe_updates

    Property Value
    System Variable sql_safe_updates
    Scope Global, Session
    Dynamic Yes
    Type Boolean
    Default Value OFF

    If this variable is enabled, UPDATE and DELETE statements that do not use a key in the WHERE clause or a LIMIT clause produce an error. This makes it possible to catch UPDATE and DELETE statements where keys are not used properly and that would probably change or delete a large number of rows. The default value is OFF.

    For the mysql client, sql_safe_updates can be enabled by using the --safe-updates option. For more information, see Using Safe-Updates Mode (--safe-updates).

  • sql_select_limit

    Property Value
    System Variable sql_select_limit
    Scope Global, Session
    Dynamic Yes
    Type Integer

    The maximum number of rows to return from SELECT statements. For more information, see Using Safe-Updates Mode (--safe-updates).

    The default value for a new connection is the maximum number of rows that the server permits per table. Typical default values are (232)−1 or (264)−1. If you have changed the limit, the default value can be restored by assigning a value of DEFAULT.

    If a SELECT has a LIMIT clause, the LIMIT takes precedence over the value of sql_select_limit.

  • sql_warnings

    Property Value
    System Variable sql_warnings
    Scope Global, Session
    Dynamic Yes
    Type Boolean
    Default Value OFF

    This variable controls whether single-row INSERT statements produce an information string if warnings occur. The default is OFF. Set the value to ON to produce an information string.

  • ssl_ca

    Property Value
    Command-Line Format --ssl-ca=file_name
    System Variable ssl_ca
    Scope Global
    Dynamic No
    Type File name

    The path to a file with a list of trusted SSL Certificate Authorities.

  • ssl_capath

    Property Value
    Command-Line Format --ssl-capath=dir_name
    System Variable ssl_capath
    Scope Global
    Dynamic No
    Type Directory name

    The path to a directory that contains trusted SSL CA certificates in PEM format. Support for this capability depends on the SSL library used to compile MySQL. See Section 6.3.4, “SSL Library-Dependent Capabilities”.

  • ssl_cert

    Property Value
    Command-Line Format --ssl-cert=file_name
    System Variable ssl_cert
    Scope Global
    Dynamic No
    Type File name

    The name of the SSL certificate file to use for establishing secure connections.

  • ssl_cipher

    Property Value
    Command-Line Format --ssl-cipher=name
    System Variable ssl_cipher
    Scope Global
    Dynamic No
    Type String

    The list of permissible ciphers for SSL encryption.

  • ssl_crl

    Property Value
    Command-Line Format --ssl-crl=file_name
    System Variable ssl_crl
    Scope Global
    Dynamic No
    Type File name

    The path to a file containing certificate revocation lists in PEM format. Support for revocation-list capability depends on the SSL library used to compile MySQL. See Section 6.3.4, “SSL Library-Dependent Capabilities”.

  • ssl_crlpath

    Property Value
    Command-Line Format --ssl-crlpath=dir_name
    System Variable ssl_crlpath
    Scope Global
    Dynamic No
    Type Directory name

    The path to a directory that contains files containing certificate revocation lists in PEM format. Support for revocation-list capability depends on the SSL library used to compile MySQL. See Section 6.3.4, “SSL Library-Dependent Capabilities”.

  • ssl_key

    Property Value
    Command-Line Format --ssl-key=file_name
    System Variable ssl_key
    Scope Global
    Dynamic No
    Type File name

    The name of the SSL key file to use for establishing secure connections.

  • stored_program_cache

    Property Value
    Command-Line Format --stored-program-cache=#
    System Variable stored_program_cache
    Scope Global
    Dynamic Yes
    Type Integer
    Default Value 256
    Minimum Value 16
    Maximum Value 524288

    Sets a soft upper limit for the number of cached stored routines per connection. The value of this variable is specified in terms of the number of stored routines held in each of the two caches maintained by the MySQL Server for, respectively, stored procedures and stored functions.

    Whenever a stored routine is executed this cache size is checked before the first or top-level statement in the routine is parsed; if the number of routines of the same type (stored procedures or stored functions according to which is being executed) exceeds the limit specified by this variable, the corresponding cache is flushed and memory previously allocated for cached objects is freed. This allows the cache to be flushed safely, even when there are dependencies between stored routines.

  • super_read_only

    Property Value
    Command-Line Format --super-read-only[={OFF|ON}]
    System Variable super_read_only
    Scope Global
    Dynamic Yes
    Type Boolean
    Default Value OFF

    If the read_only system variable is enabled, the server permits client updates only from users who have the SUPER privilege. If the super_read_only system variable is also enabled, the server prohibits client updates even from users who have SUPER. See the description of the read_only system variable for a description of read-only mode and information about how read_only and super_read_only interact.

    Client updates prevented when super_read_only is enabled include operations that do not necessarily appear to be updates, such as CREATE FUNCTION (to install a UDF) and INSTALL PLUGIN. These operations are prohibited because they involve changes to tables in the mysql system database.

    Changes to super_read_only on a master server are not replicated to slave servers. The value can be set on a slave server independent of the setting on the master.

  • sync_frm

    Property Value
    Command-Line Format --sync-frm[={OFF|ON}]
    Deprecated Yes
    System Variable sync_frm
    Scope Global
    Dynamic Yes
    Type Boolean
    Default Value ON

    If this variable is set to 1, when any nontemporary table is created its .frm file is synchronized to disk (using fdatasync()). This is slower but safer in case of a crash. The default is 1.

    This variable is deprecated in MySQL 5.7 and will be removed in a future version of MySQL (when .frm files become obsolete).

  • system_time_zone

    Property Value
    System Variable system_time_zone
    Scope Global
    Dynamic No
    Type String

    The server system time zone. When the server begins executing, it inherits a time zone setting from the machine defaults, possibly modified by the environment of the account used for running the server or the startup script. The value is used to set system_time_zone. Typically the time zone is specified by the TZ environment variable. It also can be specified using the --timezone option of the mysqld_safe script.

    The system_time_zone variable differs from time_zone. Although they might have the same value, the latter variable is used to initialize the time zone for each client that connects. See Section 5.1.12, “MySQL Server Time Zone Support”.

  • table_definition_cache

    Property Value
    Command-Line Format --table-definition-cache=#
    System Variable table_definition_cache
    Scope Global
    Dynamic Yes
    Type Integer
    Default Value -1 (signifies autosizing; do not assign this literal value)
    Minimum Value 400
    Maximum Value 524288

    The number of table definitions (from .frm files) that can be stored in the definition cache. If you use a large number of tables, you can create a large table definition cache to speed up opening of tables. The table definition cache takes less space and does not use file descriptors, unlike the normal table cache. The minimum value is 400. The default value is based on the following formula, capped to a limit of 2000:

    400 + (table_open_cache / 2)

    For InnoDB, table_definition_cache acts as a soft limit for the number of open table instances in the InnoDB data dictionary cache. If the number of open table instances exceeds the table_definition_cache setting, the LRU mechanism begins to mark table instances for eviction and eventually removes them from the data dictionary cache. The limit helps address situations in which significant amounts of memory would be used to cache rarely used table instances until the next server restart. The number of table instances with cached metadata could be higher than the limit defined by table_definition_cache, because InnoDB system table instances and parent and child table instances with foreign key relationships are not placed on the LRU list and are not subject to eviction from memory.

    Additionally, table_definition_cache defines a soft limit for the number of InnoDB file-per-table tablespaces that can be open at one time, which is also controlled by innodb_open_files. If both table_definition_cache and innodb_open_files are set, the highest setting is used. If neither variable is set, table_definition_cache, which has a higher default value, is used. If the number of open tablespace file handles exceeds the limit defined by table_definition_cache or innodb_open_files, the LRU mechanism searches the tablespace file LRU list for files that are fully flushed and are not currently being extended. This process is performed each time a new tablespace is opened. If there are no inactive tablespaces, no tablespace files are closed.

  • table_open_cache

    Property Value
    Command-Line Format --table-open-cache=#
    System Variable table_open_cache
    Scope Global
    Dynamic Yes
    Type Integer
    Default Value 2000
    Minimum Value 1
    Maximum Value 524288

    The number of open tables for all threads. Increasing this value increases the number of file descriptors that mysqld requires. You can check whether you need to increase the table cache by checking the Opened_tables status variable. See Section 5.1.9, “Server Status Variables”. If the value of Opened_tables is large and you do not use FLUSH TABLES often (which just forces all tables to be closed and reopened), then you should increase the value of the table_open_cache variable. For more information about the table cache, see Section 8.4.3.1, “How MySQL Opens and Closes Tables”.

  • table_open_cache_instances

    Property Value
    Command-Line Format --table-open-cache-instances=#
    System Variable table_open_cache_instances
    Scope Global
    Dynamic No
    Type Integer
    Default Value 16
    Minimum Value 1
    Maximum Value 64

    The number of open tables cache instances. To improve scalability by reducing contention among sessions, the open tables cache can be partitioned into several smaller cache instances of size table_open_cache / table_open_cache_instances . A session needs to lock only one instance to access it for DML statements. This segments cache access among instances, permitting higher performance for operations that use the cache when there are many sessions accessing tables. (DDL statements still require a lock on the entire cache, but such statements are much less frequent than DML statements.)

    A value of 8 or 16 is recommended on systems that routinely use 16 or more cores.

  • thread_cache_size

    Property Value
    Command-Line Format --thread-cache-size=#
    System Variable thread_cache_size
    Scope Global
    Dynamic Yes
    Type Integer
    Default Value -1 (signifies autosizing; do not assign this literal value)
    Minimum Value 0
    Maximum Value 16384

    How many threads the server should cache for reuse. When a client disconnects, the client's threads are put in the cache if there are fewer than thread_cache_size threads there. Requests for threads are satisfied by reusing threads taken from the cache if possible, and only when the cache is empty is a new thread created. This variable can be increased to improve performance if you have a lot of new connections. Normally, this does not provide a notable performance improvement if you have a good thread implementation. However, if your server sees hundreds of connections per second you should normally set thread_cache_size high enough so that most new connections use cached threads. By examining the difference between the Connections and Threads_created status variables, you can see how efficient the thread cache is. For details, see Section 5.1.9, “Server Status Variables”.

    The default value is based on the following formula, capped to a limit of 100:

    8 + (max_connections / 100)

    This variable has no effect for the embedded server (libmysqld) and as of MySQL 5.7.2 is no longer visible within the embedded server.

  • thread_handling

    Property Value
    Command-Line Format --thread-handling=name
    System Variable thread_handling
    Scope Global
    Dynamic No
    Type Enumeration
    Default Value one-thread-per-connection
    Valid Values

    no-threads

    one-thread-per-connection

    loaded-dynamically

    The thread-handling model used by the server for connection threads. The permissible values are no-threads (the server uses a single thread to handle one connection) and one-thread-per-connection (the server uses one thread to handle each client connection). no-threads is useful for debugging under Linux; see Section 28.5, “Debugging and Porting MySQL”.

    This variable has no effect for the embedded server (libmysqld) and as of MySQL 5.7.2 is no longer visible within the embedded server.

  • thread_pool_algorithm

    Property Value
    Command-Line Format --thread-pool-algorithm=#
    System Variable thread_pool_algorithm
    Scope Global
    Dynamic No
    Type Integer
    Default Value 0
    Minimum Value 0
    Maximum Value 1

    This variable controls which algorithm the thread pool plugin uses:

    • A value of 0 (the default) uses a conservative low-concurrency algorithm which is most well tested and is known to produce very good results.

    • A value of 1 increases the concurrency and uses a more aggressive algorithm which at times has been known to perform 5–10% better on optimal thread counts, but has degrading performance as the number of connections increases. Its use should be considered as experimental and not supported.

    This variable is available only if the thread pool plugin is enabled. See Section 5.5.3, “MySQL Enterprise Thread Pool”

  • thread_pool_high_priority_connection

    Property Value
    Command-Line Format --thread-pool-high-priority-connection=#
    System Variable thread_pool_high_priority_connection
    Scope Global, Session
    Dynamic Yes
    Type Integer
    Default Value 0
    Minimum Value 0
    Maximum Value 1

    This variable affects queuing of new statements prior to execution. If the value is 0 (false, the default), statement queuing uses both the low-priority and high-priority queues. If the value is 1 (true), queued statements always go to the high-priority queue.

    This variable is available only if the thread pool plugin is enabled. See Section 5.5.3, “MySQL Enterprise Thread Pool”

  • thread_pool_max_unused_threads

    Property Value
    Command-Line Format --thread-pool-max-unused-threads=#
    System Variable thread_pool_max_unused_threads
    Scope Global
    Dynamic Yes
    Type Integer
    Default Value 0
    Minimum Value 0
    Maximum Value 4096

    The maximum permitted number of unused threads in the thread pool. This variable makes it possible to limit the amount of memory used by sleeping threads.

    A value of 0 (the default) means no limit on the number of sleeping threads. A value of N where N is greater than 0 means 1 consumer thread and N−1 reserve threads. In this case, if a thread is ready to sleep but the number of sleeping threads is already at the maximum, the thread exits rather than going to sleep.

    A sleeping thread is either sleeping as a consumer thread or a reserve thread. The thread pool permits one thread to be the consumer thread when sleeping. If a thread goes to sleep and there is no existing consumer thread, it will sleep as a consumer thread. When a thread must be woken up, a consumer thread is selected if there is one. A reserve thread is selected only when there is no consumer thread to wake up.

    This variable is available only if the thread pool plugin is enabled. See Section 5.5.3, “MySQL Enterprise Thread Pool”

  • thread_pool_prio_kickup_timer

    Property Value
    Command-Line Format --thread-pool-prio-kickup-timer=#
    System Variable thread_pool_prio_kickup_timer
    Scope Global, Session
    Dynamic Yes
    Type Integer
    Default Value 1000
    Minimum Value 0
    Maximum Value 4294967294

    This variable affects statements waiting for execution in the low-priority queue. The value is the number of milliseconds before a waiting statement is moved to the high-priority queue. The default is 1000 (1 second). The range of values is 0 to 232 − 2.

    This variable is available only if the thread pool plugin is enabled. See Section 5.5.3, “MySQL Enterprise Thread Pool”

  • thread_pool_size

    Property Value
    Command-Line Format --thread-pool-size=#
    System Variable thread_pool_size
    Scope Global
    Dynamic No
    Type Integer
    Default Value 16
    Minimum Value 1
    Maximum Value 64

    The number of thread groups in the thread pool. This is the most important parameter controlling thread pool performance. It affects how many statements can execute simultaneously. The default value is 16, with a range from 1 to 64 of permissible values. If a value outside this range is specified, the thread pool plugin does not load and the server writes a message to the error log.

    This variable is available only if the thread pool plugin is enabled. See Section 5.5.3, “MySQL Enterprise Thread Pool”

  • thread_pool_stall_limit

    Property Value
    Command-Line Format --thread-pool-stall-limit=#
    System Variable thread_pool_stall_limit
    Scope Global
    Dynamic Yes
    Type Integer
    Default Value 6
    Minimum Value 4
    Maximum Value 600

    This variable affects executing statements. The value is the amount of time a statement has to finish after starting to execute before it becomes defined as stalled, at which point the thread pool permits the thread group to begin executing another statement. The value is measured in 10 millisecond units, so a value of 6 (the default) means 60ms. The range of values is 4 to 600 (40ms to 6s). Short wait values permit threads to start more quickly. Short values are also better for avoiding deadlock situations. Long wait values are useful for workloads that include long-running statements, to avoid starting too many new statements while the current ones execute.

    This variable is available only if the thread pool plugin is enabled. See Section 5.5.3, “MySQL Enterprise Thread Pool”

  • thread_stack

    Property Value
    Command-Line Format --thread-stack=#
    System Variable thread_stack
    Scope Global
    Dynamic No
    Type Integer
    Default Value (64-bit platforms) 262144
    Default Value (32-bit platforms) 196608
    Minimum Value 131072
    Maximum Value (64-bit platforms) 18446744073709551615
    Maximum Value (32-bit platforms) 4294967295
    Block Size 1024

    The stack size for each thread. The default is large enough for normal operation. If the thread stack size is too small, it limits the complexity of the SQL statements that the server can handle, the recursion depth of stored procedures, and other memory-consuming actions.

  • time_format

    This variable is unused. It is deprecated and is removed in MySQL 8.0.

  • time_zone

    Property Value
    System Variable time_zone
    Scope Global, Session
    Dynamic Yes
    Type String
    Default Value SYSTEM
    Minimum Value -12:59
    Maximum Value +13:00

    The current time zone. This variable is used to initialize the time zone for each client that connects. By default, the initial value of this is 'SYSTEM' (which means, use the value of system_time_zone). The value can be specified explicitly at server startup with the --default-time-zone option. See Section 5.1.12, “MySQL Server Time Zone Support”.

    Note

    If set to SYSTEM, every MySQL function call that requires a time zone calculation makes a system library call to determine the current system time zone. This call may be protected by a global mutex, resulting in contention.

  • timestamp

    Property Value
    System Variable timestamp
    Scope Session
    Dynamic Yes
    Type Numeric
    Default Value UNIX_TIMESTAMP()
    Minimum Value 1
    Maximum Value 2147483647

    Set the time for this client. This is used to get the original timestamp if you use the binary log to restore rows. timestamp_value should be a Unix epoch timestamp (a value like that returned by UNIX_TIMESTAMP(), not a value in 'YYYY-MM-DD hh:mm:ss' format) or DEFAULT.

    Setting timestamp to a constant value causes it to retain that value until it is changed again. Setting timestamp to DEFAULT causes its value to be the current date and time as of the time it is accessed. The maximum value corresponds to '2038-01-19 03:14:07' UTC, the same as for the TIMESTAMP data type.

    timestamp is a DOUBLE rather than BIGINT because its value includes a microseconds part.

    SET timestamp affects the value returned by NOW() but not by SYSDATE(). This means that timestamp settings in the binary log have no effect on invocations of SYSDATE(). The server can be started with the --sysdate-is-now option to cause SYSDATE() to be an alias for NOW(), in which case SET timestamp affects both functions.

  • tls_version

    Property Value
    Command-Line Format --tls-version=protocol_list
    Introduced 5.7.10
    System Variable tls_version
    Scope Global
    Dynamic No
    Type String
    Default Value (>= 5.7.28) TLSv1,TLSv1.1,TLSv1.2
    Default Value (<= 5.7.27)

    TLSv1,TLSv1.1,TLSv1.2 (OpenSSL)

    TLSv1,TLSv1.1 (yaSSL)

    Which protocols the server permits for encrypted connections. The value is a comma-separated list containing one or more protocol versions. The protocols that can be named for this variable depend on the SSL library used to compile MySQL. Permitted protocols should be chosen such as not to leave holes in the list. For details, see Section 6.3.2, “Encrypted Connection TLS Protocols and Ciphers”.

  • tmp_table_size

    Property Value
    Command-Line Format --tmp-table-size=#
    System Variable tmp_table_size
    Scope Global, Session
    Dynamic Yes
    Type Integer
    Default Value 16777216
    Minimum Value 1024
    Maximum Value 18446744073709551615

    The maximum size of internal in-memory temporary tables. This variable does not apply to user-created MEMORY tables.

    The actual limit is determined from whichever of the values of tmp_table_size and max_heap_table_size is smaller. If an in-memory temporary table exceeds the limit, MySQL automatically converts it to an on-disk temporary table. The internal_tmp_disk_storage_engine option defines the storage engine used for on-disk temporary tables.

    Increase the value of tmp_table_size (and max_heap_table_size if necessary) if you do many advanced GROUP BY queries and you have lots of memory.

    You can compare the number of internal on-disk temporary tables created to the total number of internal temporary tables created by comparing the values of the Created_tmp_disk_tables and Created_tmp_tables variables.

    See also Section 8.4.4, “Internal Temporary Table Use in MySQL”.

  • tmpdir

    Property Value
    Command-Line Format --tmpdir=dir_name
    System Variable tmpdir
    Scope Global
    Dynamic No
    Type Directory name

    The path of the directory to use for creating temporary files. It might be useful if your default /tmp directory resides on a partition that is too small to hold temporary tables. This variable can be set to a list of several paths that are used in round-robin fashion. Paths should be separated by colon characters (:) on Unix and semicolon characters (;) on Windows.

    tmpdir can be a non-permanent location, such as a directory on a memory-based file system or a directory that is cleared when the server host restarts. If the MySQL server is acting as a replication slave, and you are using a non-permanent location for tmpdir, consider setting a different temporary directory for the slave using the slave_load_tmpdir variable. For a replication slave, the temporary files used to replicate LOAD DATA statements are stored in this directory, so with a permanent location they can survive machine restarts, although replication can now continue after a restart if the temporary files have been removed.

    For more information about the storage location of temporary files, see Section B.4.3.5, “Where MySQL Stores Temporary Files”.

  • transaction_alloc_block_size

    Property Value
    Command-Line Format --transaction-alloc-block-size=#
    System Variable transaction_alloc_block_size
    Scope Global, Session
    Dynamic Yes
    Type Integer
    Default Value 8192
    Minimum Value 1024
    Maximum Value 131072
    Block Size 1024

    The amount in bytes by which to increase a per-transaction memory pool which needs memory. See the description of transaction_prealloc_size.

  • transaction_isolation

    Property Value
    Command-Line Format --transaction-isolation=name
    System Variable (>= 5.7.20) transaction_isolation
    Scope (>= 5.7.20) Global, Session
    Dynamic (>= 5.7.20) Yes
    Type Enumeration
    Default Value REPEATABLE-READ
    Valid Values

    READ-UNCOMMITTED

    READ-COMMITTED

    REPEATABLE-READ

    SERIALIZABLE

    The transaction isolation level. The default is REPEATABLE-READ.

    The transaction isolation level has three scopes: global, session, and next transaction. This three-scope implementation leads to some nonstandard isolation-level assignment semantics, as described later.

    To set the global transaction isolation level at startup, use the --transaction-isolation server option.

    At runtime, the isolation level can be set directly using the SET statement to assign a value to the transaction_isolation system variable, or indirectly using the SET TRANSACTION statement. If you set transaction_isolation directly to an isolation level name that contains a space, the name should be enclosed within quotation marks, with the space replaced by a dash. For example, use this SET statement to set the global value:

    SET GLOBAL transaction_isolation = 'READ-COMMITTED';

    Setting the global transaction_isolation value sets the isolation level for all subsequent sessions. Existing sessions are unaffected.

    To set the session or next-level transaction_isolation value, use the SET statement. For most session system variables, these statements are equivalent ways to set the value:

    SET @@SESSION.var_name = value;
    SET SESSION var_name = value;
    SET var_name = value;
    SET @@var_name = value;
    

    As mentioned previously, the transaction isolation level has a next-transaction scope, in addition to the global and session scopes. To enable the next-transaction scope to be set, SET syntax for assigning session system variable values has nonstandard semantics for transaction_isolation:

    • To set the session isolation level, use any of these syntaxes:

      SET @@SESSION.transaction_isolation = value;
      SET SESSION transaction_isolation = value;
      SET transaction_isolation = value;
      

      For each of those syntaxes, these semantics apply:

      • Sets the isolation level for all subsequent transactions performed within the session.

      • Permitted within transactions, but does not affect the current ongoing transaction.

      • If executed between transactions, overrides any preceding statement that sets the next-transaction isolation level.

      • Corresponds to SET SESSION TRANSACTION ISOLATION LEVEL (with the SESSION keyword).

    • To set the next-transaction isolation level, use this syntax:

      SET @@transaction_isolation = value;
      

      For that syntax, these semantics apply:

      • Sets the isolation level only for the next single transaction performed within the session.

      • Subsequent transactions revert to the session isolation level.

      • Not permitted within transactions.

      • Corresponds to SET TRANSACTION ISOLATION LEVEL (without the SESSION keyword).

    For more information about SET TRANSACTION and its relationship to the transaction_isolation system variable, see Section 13.3.6, “SET TRANSACTION Statement”.

    Note

    transaction_isolation was added in MySQL 5.7.20 as an alias for tx_isolation, which is now deprecated and is removed in MySQL 8.0. Applications should be adjusted to use transaction_isolation in preference to tx_isolation.

  • transaction_prealloc_size

    Property Value
    Command-Line Format --transaction-prealloc-size=#
    System Variable transaction_prealloc_size
    Scope Global, Session
    Dynamic Yes
    Type Integer
    Default Value 4096
    Minimum Value 1024
    Maximum Value 131072
    Block Size 1024

    There is a per-transaction memory pool from which various transaction-related allocations take memory. The initial size of the pool in bytes is transaction_prealloc_size. For every allocation that cannot be satisfied from the pool because it has insufficient memory available, the pool is increased by transaction_alloc_block_size bytes. When the transaction ends, the pool is truncated to transaction_prealloc_size bytes.

    By making transaction_prealloc_size sufficiently large to contain all statements within a single transaction, you can avoid many malloc() calls.

  • transaction_read_only

    Property Value
    Command-Line Format --transaction-read-only[={OFF|ON}]
    System Variable (>= 5.7.20) transaction_read_only
    Scope (>= 5.7.20) Global, Session
    Dynamic (>= 5.7.20) Yes
    Type Boolean
    Default Value OFF

    The transaction access mode. The value can be OFF (read/write; the default) or ON (read only).

    The transaction access mode has three scopes: global, session, and next transaction. This three-scope implementation leads to some nonstandard access-mode assignment semantics, as described later.

    To set the global transaction access mode at startup, use the --transaction-read-only server option.

    At runtime, the access mode can be set directly using the SET statement to assign a value to the transaction_read_only system variable, or indirectly using the SET TRANSACTION statement. For example, use this SET statement to set the global value:

    SET GLOBAL transaction_read_only = ON;

    Setting the global transaction_read_only value sets the access mode for all subsequent sessions. Existing sessions are unaffected.

    To set the session or next-level transaction_read_only value, use the SET statement. For most session system variables, these statements are equivalent ways to set the value:

    SET @@SESSION.var_name = value;
    SET SESSION var_name = value;
    SET var_name = value;
    SET @@var_name = value;
    

    As mentioned previously, the transaction access mode has a next-transaction scope, in addition to the global and session scopes. To enable the next-transaction scope to be set, SET syntax for assigning session system variable values has nonstandard semantics for transaction_read_only,

    • To set the session access mode, use any of these syntaxes:

      SET @@SESSION.transaction_read_only = value;
      SET SESSION transaction_read_only = value;
      SET transaction_read_only = value;
      

      For each of those syntaxes, these semantics apply:

      • Sets the access mode for all subsequent transactions performed within the session.

      • Permitted within transactions, but does not affect the current ongoing transaction.

      • If executed between transactions, overrides any preceding statement that sets the next-transaction access mode.

      • Corresponds to SET SESSION TRANSACTION {READ WRITE | READ ONLY} (with the SESSION keyword).

    • To set the next-transaction access mode, use this syntax:

      SET @@transaction_read_only = value;
      

      For that syntax, these semantics apply:

      • Sets the access mode only for the next single transaction performed within the session.

      • Subsequent transactions revert to the session access mode.

      • Not permitted within transactions.

      • Corresponds to SET TRANSACTION {READ WRITE | READ ONLY} (without the SESSION keyword).

    For more information about SET TRANSACTION and its relationship to the transaction_read_only system variable, see Section 13.3.6, “SET TRANSACTION Statement”.

    Note

    transaction_read_only was added in MySQL 5.7.20 as an alias for tx_read_only, which is now deprecated and is removed in MySQL 8.0. Applications should be adjusted to use transaction_read_only in preference to tx_read_only.

  • tx_isolation

    Property Value
    Deprecated 5.7.20
    System Variable tx_isolation
    Scope Global, Session
    Dynamic Yes
    Type Enumeration
    Default Value REPEATABLE-READ
    Valid Values

    READ-UNCOMMITTED

    READ-COMMITTED

    REPEATABLE-READ

    SERIALIZABLE

    The default transaction isolation level. Defaults to REPEATABLE-READ.

    Note

    transaction_isolation was added in MySQL 5.7.20 as an alias for tx_isolation, which is now deprecated and is removed in MySQL 8.0. Applications should be adjusted to use transaction_isolation in preference to tx_isolation. See the description of transaction_isolation for details.

  • tx_read_only

    Property Value
    Deprecated 5.7.20
    System Variable tx_read_only
    Scope Global, Session
    Dynamic Yes
    Type Boolean
    Default Value OFF

    The default transaction access mode. The value can be OFF (read/write, the default) or ON (read only).

    Note

    transaction_read_only was added in MySQL 5.7.20 as an alias for tx_read_only, which is now deprecated and is removed in MySQL 8.0. Applications should be adjusted to use transaction_read_only in preference to tx_read_only. See the description of transaction_read_only for details.

  • unique_checks

    Property Value
    System Variable unique_checks
    Scope Global, Session
    Dynamic Yes
    Type Boolean
    Default Value ON

    If set to 1 (the default), uniqueness checks for secondary indexes in InnoDB tables are performed. If set to 0, storage engines are permitted to assume that duplicate keys are not present in input data. If you know for certain that your data does not contain uniqueness violations, you can set this to 0 to speed up large table imports to InnoDB.

    Setting this variable to 0 does not require storage engines to ignore duplicate keys. An engine is still permitted to check for them and issue duplicate-key errors if it detects them.

  • updatable_views_with_limit

    Property Value
    Command-Line Format --updatable-views-with-limit[={OFF|ON}]
    System Variable updatable_views_with_limit
    Scope Global, Session
    Dynamic Yes
    Type Boolean
    Default Value 1

    This variable controls whether updates to a view can be made when the view does not contain all columns of the primary key defined in the underlying table, if the update statement contains a LIMIT clause. (Such updates often are generated by GUI tools.) An update is an UPDATE or DELETE statement. Primary key here means a PRIMARY KEY, or a UNIQUE index in which no column can contain NULL.

    The variable can have two values:

    • 1 or YES: Issue a warning only (not an error message). This is the default value.

    • 0 or NO: Prohibit the update.

  • validate_password_xxx

    The validate_password plugin implements a set of system variables having names of the form validate_password_xxx. These variables affect password testing by that plugin; see Section 6.4.3.2, “Password Validation Plugin Options and Variables”.

  • validate_user_plugins

    Property Value
    Command-Line Format --validate-user-plugins[={OFF|ON}]
    System Variable validate_user_plugins
    Scope Global
    Dynamic No
    Type Boolean
    Default Value ON

    If this variable is enabled (the default), the server checks each user account and produces a warning if conditions are found that would make the account unusable:

    • The account requires an authentication plugin that is not loaded.

    • The account requires the sha256_password authentication plugin but the server was started with neither SSL nor RSA enabled as required by this plugin.

    Enabling validate_user_plugins slows down server initialization and FLUSH PRIVILEGES. If you do not require the additional checking, you can disable this variable at startup to avoid the performance decrement.

  • version

    The version number for the server. The value might also include a suffix indicating server build or configuration information. -log indicates that one or more of the general log, slow query log, or binary log are enabled. -debug indicates that the server was built with debugging support enabled.

  • version_comment

    Property Value
    System Variable version_comment
    Scope Global
    Dynamic No
    Type String

    The CMake configuration program has a COMPILATION_COMMENT option that permits a comment to be specified when building MySQL. This variable contains the value of that comment. See Section 2.9.7, “MySQL Source-Configuration Options”.

  • version_compile_machine

    Property Value
    System Variable version_compile_machine
    Scope Global
    Dynamic No
    Type String

    The type of the server binary.

  • version_compile_os

    Property Value
    System Variable version_compile_os
    Scope Global
    Dynamic No
    Type String

    The type of operating system on which MySQL was built.

  • wait_timeout

    Property Value
    Command-Line Format --wait-timeout=#
    System Variable wait_timeout
    Scope Global, Session
    Dynamic Yes
    Type Integer
    Default Value 28800
    Minimum Value 1
    Maximum Value (Other) 31536000
    Maximum Value (Windows) 2147483

    The number of seconds the server waits for activity on a noninteractive connection before closing it.

    On thread startup, the session wait_timeout value is initialized from the global wait_timeout value or from the global interactive_timeout value, depending on the type of client (as defined by the CLIENT_INTERACTIVE connect option to mysql_real_connect()). See also interactive_timeout.

  • warning_count

    The number of errors, warnings, and notes that resulted from the last statement that generated messages. This variable is read only. See Section 13.7.5.40, “SHOW WARNINGS Statement”.

5.1.8 Using System Variables

The MySQL server maintains many system variables that configure its operation. Section 5.1.7, “Server System Variables”, describes the meaning of these variables. Each system variable has a default value. System variables can be set at server startup using options on the command line or in an option file. Most of them can be changed dynamically while the server is running by means of the SET statement, which enables you to modify operation of the server without having to stop and restart it. You can also use system variable values in expressions.

Many system variables are built in. System variables implemented by a server plugin are exposed when the plugin is installed and have names that begin with the plugin name. For example, the audit_log plugin implements a system variable named audit_log_policy.

There are two scopes in which system variables exist. Global variables affect the overall operation of the server. Session variables affect its operation for individual client connections. A given system variable can have both a global and a session value. Global and session system variables are related as follows:

  • When the server starts, it initializes each global variable to its default value. These defaults can be changed by options specified on the command line or in an option file. (See Section 4.2.2, “Specifying Program Options”.)

  • The server also maintains a set of session variables for each client that connects. The client's session variables are initialized at connect time using the current values of the corresponding global variables. For example, a client's SQL mode is controlled by the session sql_mode value, which is initialized when the client connects to the value of the global sql_mode value.

    For some system variables, the session value is not initialized from the corresponding global value; if so, that is indicated in the variable description.

System variable values can be set globally at server startup by using options on the command line or in an option file. At startup, the syntax for system variables is the same as for command options, so within variable names, dashes and underscores may be used interchangeably. For example, --general_log=ON and --general-log=ON are equivalent.

When you use a startup option to set a variable that takes a numeric value, the value can be given with a suffix of K, M, or G (either uppercase or lowercase) to indicate a multiplier of 1024, 10242 or 10243; that is, units of kilobytes, megabytes, or gigabytes, respectively. Thus, the following command starts the server with an InnoDB log file size of 16 megabytes and a maximum packet size of one gigabyte:

mysqld --innodb-log-file-size=16M --max-allowed-packet=1G

Within an option file, those variables are set like this:

[mysqld]
innodb_log_file_size=16M
max_allowed_packet=1G

The lettercase of suffix letters does not matter; 16M and 16m are equivalent, as are 1G and 1g.

To restrict the maximum value to which a system variable can be set at runtime with the SET statement, specify this maximum by using an option of the form --maximum-var_name=value at server startup. For example, to prevent the value of innodb_log_file_size from being increased to more than 32MB at runtime, use the option --maximum-innodb-log-file-size=32M.

Many system variables are dynamic and can be changed at runtime by using the SET statement. For a list, see Section 5.1.8.2, “Dynamic System Variables”. To change a system variable with SET, refer to it by name, optionally preceded by a modifier. At runtime, system variable names must be written using underscores, not dashes. The following examples briefly illustrate this syntax:

  • Set a global system variable:

    SET GLOBAL max_connections = 1000;
    SET @@GLOBAL.max_connections = 1000;
  • Set a session system variable:

    SET SESSION sql_mode = 'TRADITIONAL';
    SET @@SESSION.sql_mode = 'TRADITIONAL';
    SET @@sql_mode = 'TRADITIONAL';

For complete details about SET syntax, see Section 13.7.4.1, “SET Syntax for Variable Assignment”. For a description of the privilege requirements for setting system variables, see Section 5.1.8.1, “System Variable Privileges”

Suffixes for specifying a value multiplier can be used when setting a variable at server startup, but not to set the value with SET at runtime. On the other hand, with SET you can assign a variable's value using an expression, which is not true when you set a variable at server startup. For example, the first of the following lines is legal at server startup, but the second is not:

shell> mysql --max_allowed_packet=16M
shell> mysql --max_allowed_packet=16*1024*1024

Conversely, the second of the following lines is legal at runtime, but the first is not:

mysql> SET GLOBAL max_allowed_packet=16M;
mysql> SET GLOBAL max_allowed_packet=16*1024*1024;
Note

Some system variables can be enabled with the SET statement by setting them to ON or 1, or disabled by setting them to OFF or 0. However, to set such a variable on the command line or in an option file, you must set it to 1 or 0; setting it to ON or OFF will not work. For example, on the command line, --delay_key_write=1 works but --delay_key_write=ON does not.

To display system variable names and values, use the SHOW VARIABLES statement:

mysql> SHOW VARIABLES;
+---------------------------------+-----------------------------------+
| Variable_name                   | Value                             |
+---------------------------------+-----------------------------------+
| auto_increment_increment        | 1                                 |
| auto_increment_offset           | 1                                 |
| automatic_sp_privileges         | ON                                |
| back_log                        | 50                                |
| basedir                         | /home/mysql/                      |
| binlog_cache_size               | 32768                             |
| bulk_insert_buffer_size         | 8388608                           |
| character_set_client            | utf8                              |
| character_set_connection        | utf8                              |
| character_set_database          | latin1                            |
| character_set_filesystem        | binary                            |
| character_set_results           | utf8                              |
| character_set_server            | latin1                            |
| character_set_system            | utf8                              |
| character_sets_dir              | /home/mysql/share/mysql/charsets/ |
| collation_connection            | utf8_general_ci                   |
| collation_database              | latin1_swedish_ci                 |
| collation_server                | latin1_swedish_ci                 |
...
| innodb_autoextend_increment     | 8                                 |
| innodb_buffer_pool_size         | 8388608                           |
| innodb_checksums                | ON                                |
| innodb_commit_concurrency       | 0                                 |
| innodb_concurrency_tickets      | 500                               |
| innodb_data_file_path           | ibdata1:10M:autoextend            |
| innodb_data_home_dir            |                                   |
...
| version                         | 5.7.18-log                        |
| version_comment                 | Source distribution               |
| version_compile_machine         | i686                              |
| version_compile_os              | suse-linux                        |
| wait_timeout                    | 28800                             |
+---------------------------------+-----------------------------------+

With a LIKE clause, the statement displays only those variables that match the pattern. To obtain a specific variable name, use a LIKE clause as shown:

SHOW VARIABLES LIKE 'max_join_size';
SHOW SESSION VARIABLES LIKE 'max_join_size';

To get a list of variables whose name match a pattern, use the % wildcard character in a LIKE clause:

SHOW VARIABLES LIKE '%size%';
SHOW GLOBAL VARIABLES LIKE '%size%';

Wildcard characters can be used in any position within the pattern to be matched. Strictly speaking, because _ is a wildcard that matches any single character, you should escape it as \_ to match it literally. In practice, this is rarely necessary.

For SHOW VARIABLES, if you specify neither GLOBAL nor SESSION, MySQL returns SESSION values.

The reason for requiring the GLOBAL keyword when setting GLOBAL-only variables but not when retrieving them is to prevent problems in the future:

  • Were a SESSION variable to be removed that has the same name as a GLOBAL variable, a client with privileges sufficient to modify global variables might accidentally change the GLOBAL variable rather than just the SESSION variable for its own session.

  • Were a SESSION variable to be added with the same name as a GLOBAL variable, a client that intends to change the GLOBAL variable might find only its own SESSION variable changed.

5.1.8.1 System Variable Privileges

A system variable can have a global value that affects server operation as a whole, a session value that affects only the current session, or both. To modify system variable runtime values, use the SET statement. See Section 13.7.4.1, “SET Syntax for Variable Assignment”. This section describes the privileges required to assign values to system variables at runtime.

Setting a global system variable runtime value requires the SUPER privilege.

To set a session system variable runtime value, use the SET SESSION statement. In contrast to setting global runtime values, setting session runtime values normally requires no special privileges and can be done by any user to affect the current session. For some system variables, setting the session value may have effects outside the current session and thus is a restricted operation that can be done only by users who have the SUPER privilege. If a session system variable is restricted in this way, the variable description indicates that restriction. Examples include binlog_format and sql_log_bin. Setting the session value of these variables affects binary logging for the current session, but may also have wider implications for the integrity of server replication and backups.

5.1.8.2 Dynamic System Variables

Many server system variables are dynamic and can be set at runtime. See Section 13.7.4.1, “SET Syntax for Variable Assignment”. For a description of the privilege requirements for setting system variables, see Section 5.1.8.1, “System Variable Privileges”

The following table lists all dynamic system variables applicable within mysqld.

The table lists each variable's data type and scope. The last column indicates whether the scope for each variable is Global, Session, or both. Please see the corresponding item descriptions for details on setting and using the variables. Where appropriate, direct links to further information about the items are provided.

Variables that have a type of string take a string value. Variables that have a type of numeric take a numeric value. Variables that have a type of boolean can be set to 0, 1, ON or OFF. Variables that are marked as enumeration normally should be set to one of the available values for the variable, but can also be set to the number that corresponds to the desired enumeration value. For enumerated system variables, the first enumeration value corresponds to 0. This differs from the ENUM data type used for table columns, for which the first enumeration value corresponds to 1.

Table 5.4 Dynamic System Variable Summary

Variable Name Variable Type Variable Scope
audit_log_connection_policy Enumeration Global
audit_log_exclude_accounts String Global
audit_log_flush Boolean Global
audit_log_include_accounts String Global
audit_log_read_buffer_size Integer Varies
audit_log_rotate_on_size Integer Global
audit_log_statement_policy Enumeration Global
authentication_ldap_sasl_auth_method_name String Global
authentication_ldap_sasl_bind_base_dn String Global
authentication_ldap_sasl_bind_root_dn String Global
authentication_ldap_sasl_bind_root_pwd String Global
authentication_ldap_sasl_ca_path String Global
authentication_ldap_sasl_group_search_attr String Global
authentication_ldap_sasl_group_search_filter String Global
authentication_ldap_sasl_init_pool_size Integer Global
authentication_ldap_sasl_log_status Integer Global
authentication_ldap_sasl_max_pool_size Integer Global
authentication_ldap_sasl_server_host String Global
authentication_ldap_sasl_server_port Integer Global
authentication_ldap_sasl_tls Boolean Global
authentication_ldap_sasl_user_search_attr String Global
authentication_ldap_simple_auth_method_name String Global
authentication_ldap_simple_bind_base_dn String Global
authentication_ldap_simple_bind_root_dn String Global
authentication_ldap_simple_bind_root_pwd String Global
authentication_ldap_simple_ca_path String Global
authentication_ldap_simple_group_search_attr String Global
authentication_ldap_simple_group_search_filter String Global
authentication_ldap_simple_init_pool_size Integer Global
authentication_ldap_simple_log_status Integer Global
authentication_ldap_simple_max_pool_size Integer Global
authentication_ldap_simple_server_host String Global
authentication_ldap_simple_server_port Integer Global
authentication_ldap_simple_tls Boolean Global
authentication_ldap_simple_user_search_attr String Global
auto_increment_increment Integer Both
auto_increment_offset Integer Both
autocommit Boolean Both
automatic_sp_privileges Boolean Global
avoid_temporal_upgrade Boolean Global
big_tables Boolean Both
binlog_cache_size Integer Global
binlog_checksum String Global
binlog_direct_non_transactional_updates Boolean Both
binlog_error_action Enumeration Global
binlog_format Enumeration Both
binlog_group_commit_sync_delay Integer Global
binlog_group_commit_sync_no_delay_count Integer Global
binlog_max_flush_queue_time Integer Global
binlog_order_commits Boolean Global
binlog_row_image Enumeration Both
binlog_rows_query_log_events Boolean Both
binlog_stmt_cache_size Integer Global
binlog_transaction_dependency_history_size Integer Global
binlog_transaction_dependency_tracking Enumeration Global
block_encryption_mode String Both
bulk_insert_buffer_size Integer Both
character_set_client String Both
character_set_connection String Both
character_set_database String Both
character_set_filesystem String Both
character_set_results String Both
character_set_server String Both
check_proxy_users Boolean Global
collation_connection String Both
collation_database String Both
collation_server String Both
completion_type Enumeration Both
concurrent_insert Enumeration Global
connect_timeout Integer Global
connection_control_failed_connections_threshold Integer Global
connection_control_max_connection_delay Integer Global
connection_control_min_connection_delay Integer Global
debug String Both
debug_sync String Session
default_password_lifetime Integer Global
default_storage_engine Enumeration Both
default_tmp_storage_engine Enumeration Both
default_week_format Integer Both
delay_key_write Enumeration Global
delayed_insert_limit Integer Global
delayed_insert_timeout Integer Global
delayed_queue_size Integer Global
div_precision_increment Integer Both
end_markers_in_json Boolean Both
enforce_gtid_consistency Enumeration Global
eq_range_index_dive_limit Integer Both
event_scheduler Enumeration Global
expire_logs_days Integer Global
explicit_defaults_for_timestamp Boolean Both
flush Boolean Global
flush_time Integer Global
foreign_key_checks Boolean Both
ft_boolean_syntax String Global
general_log Boolean Global
general_log_file File name Global
group_concat_max_len Integer Both
group_replication_allow_local_disjoint_gtids_join Boolean Global
group_replication_allow_local_lower_version_join Boolean Global
group_replication_auto_increment_increment Integer Global
group_replication_bootstrap_group Boolean Global
group_replication_components_stop_timeout Integer Global
group_replication_compression_threshold Integer Global
group_replication_enforce_update_everywhere_checks Boolean Global
group_replication_exit_state_action Enumeration Global
group_replication_flow_control_applier_threshold Integer Global
group_replication_flow_control_certifier_threshold Integer Global
group_replication_flow_control_mode Enumeration Global
group_replication_force_members String Global
group_replication_group_name String Global
group_replication_group_seeds String Global
group_replication_gtid_assignment_block_size Integer Global
group_replication_ip_whitelist String Global
group_replication_local_address String Global
group_replication_member_weight Integer Global
group_replication_poll_spin_loops Integer Global
group_replication_recovery_complete_at Enumeration Global
group_replication_recovery_reconnect_interval Integer Global
group_replication_recovery_retry_count Integer Global
group_replication_recovery_ssl_ca String Global
group_replication_recovery_ssl_capath String Global
group_replication_recovery_ssl_cert String Global
group_replication_recovery_ssl_cipher String Global
group_replication_recovery_ssl_crl File name Global
group_replication_recovery_ssl_crlpath Directory name Global
group_replication_recovery_ssl_key String Global
group_replication_recovery_ssl_verify_server_cert Boolean Global
group_replication_recovery_use_ssl Boolean Global
group_replication_single_primary_mode Boolean Global
group_replication_ssl_mode Enumeration Global
group_replication_start_on_boot Boolean Global
group_replication_transaction_size_limit Integer Global
group_replication_unreachable_majority_timeout Integer Global
gtid_executed_compression_period Integer Global
gtid_mode Enumeration Global
gtid_next Enumeration Session
gtid_purged String Global
host_cache_size Integer Global
identity Integer Session
init_connect String Global
init_slave String Global
innodb_adaptive_flushing Boolean Global
innodb_adaptive_flushing_lwm Integer Global
innodb_adaptive_hash_index Boolean Global
innodb_adaptive_max_sleep_delay Integer Global
innodb_api_bk_commit_interval Integer Global
innodb_api_trx_level Integer Global
innodb_autoextend_increment Integer Global
innodb_background_drop_list_empty Boolean Global
innodb_buffer_pool_dump_at_shutdown Boolean Global
innodb_buffer_pool_dump_now Boolean Global
innodb_buffer_pool_dump_pct Integer Global
innodb_buffer_pool_filename File name Global
innodb_buffer_pool_load_abort Boolean Global
innodb_buffer_pool_load_now Boolean Global
innodb_buffer_pool_size Integer Global
innodb_change_buffer_max_size Integer Global
innodb_change_buffering Enumeration Global
innodb_change_buffering_debug Integer Global
innodb_checksum_algorithm Enumeration Global
innodb_cmp_per_index_enabled Boolean Global
innodb_commit_concurrency Integer Global
innodb_compress_debug Enumeration Global
innodb_compression_failure_threshold_pct Integer Global
innodb_compression_level Integer Global
innodb_compression_pad_pct_max Integer Global
innodb_concurrency_tickets Integer Global
innodb_deadlock_detect Boolean Global
innodb_default_row_format Enumeration Global
innodb_disable_resize_buffer_pool_debug Boolean Global
innodb_disable_sort_file_cache Boolean Global
innodb_fast_shutdown Integer Global
innodb_fil_make_page_dirty_debug Integer Global
innodb_file_format String Global
innodb_file_format_max String Global
innodb_file_per_table Boolean Global
innodb_fill_factor Integer Global
innodb_flush_log_at_timeout Integer Global
innodb_flush_log_at_trx_commit Enumeration Global
innodb_flush_neighbors Enumeration Global
innodb_flush_sync Boolean Global
innodb_flushing_avg_loops Integer Global
innodb_ft_aux_table String Global
innodb_ft_enable_diag_print Boolean Global
innodb_ft_enable_stopword Boolean Both
innodb_ft_num_word_optimize Integer Global
innodb_ft_result_cache_limit Integer Global
innodb_ft_server_stopword_table String Global
innodb_ft_user_stopword_table String Both
innodb_io_capacity Integer Global
innodb_io_capacity_max Integer Global
innodb_large_prefix Boolean Global
innodb_limit_optimistic_insert_debug Integer Global
innodb_lock_wait_timeout Integer Both
innodb_log_checkpoint_now Boolean Global
innodb_log_checksums Boolean Global
innodb_log_compressed_pages Boolean Global
innodb_log_write_ahead_size Integer Global
innodb_lru_scan_depth Integer Global
innodb_max_dirty_pages_pct Numeric Global
innodb_max_dirty_pages_pct_lwm Numeric Global
innodb_max_purge_lag Integer Global
innodb_max_purge_lag_delay Integer Global
innodb_max_undo_log_size Integer Global
innodb_merge_threshold_set_all_debug Integer Global
innodb_monitor_disable String Global
innodb_monitor_enable String Global
innodb_monitor_reset Enumeration Global
innodb_monitor_reset_all Enumeration Global
innodb_old_blocks_pct Integer Global
innodb_old_blocks_time Integer Global
innodb_online_alter_log_max_size Integer Global
innodb_optimize_fulltext_only Boolean Global
innodb_print_all_deadlocks Boolean Global
innodb_purge_batch_size Integer Global
innodb_purge_rseg_truncate_frequency Integer Global
innodb_random_read_ahead Boolean Global
innodb_read_ahead_threshold Integer Global
innodb_replication_delay Integer Global
innodb_rollback_segments Integer Global
innodb_saved_page_number_debug Integer Global
innodb_spin_wait_delay Integer Global
innodb_stats_auto_recalc Boolean Global
innodb_stats_include_delete_marked Boolean Global
innodb_stats_method Enumeration Global
innodb_stats_on_metadata Boolean Global
innodb_stats_persistent Boolean Global
innodb_stats_persistent_sample_pages Integer Global
innodb_stats_sample_pages Integer Global
innodb_stats_transient_sample_pages Integer Global
innodb_status_output Boolean Global
innodb_status_output_locks Boolean Global
innodb_strict_mode Boolean Both
innodb_support_xa Boolean Both
innodb_sync_spin_loops Integer Global
innodb_table_locks Boolean Both
innodb_thread_concurrency Integer Global
innodb_thread_sleep_delay Integer Global
innodb_tmpdir Directory name Both
innodb_trx_purge_view_update_only_debug Boolean Global
innodb_trx_rseg_n_slots_debug Integer Global
innodb_undo_log_truncate Boolean Global
innodb_undo_logs Integer Global
insert_id Integer Session
interactive_timeout Integer Both
internal_tmp_disk_storage_engine Enumeration Global
join_buffer_size Integer Both
keep_files_on_create Boolean Both
key_buffer_size Integer Global
key_cache_age_threshold Integer Global
key_cache_block_size Integer Global
key_cache_division_limit Integer Global
keyring_aws_cmk_id String Global
keyring_aws_region Enumeration Global
keyring_encrypted_file_data File name Global
keyring_encrypted_file_password String Global
keyring_file_data File name Global
keyring_okv_conf_dir Directory name Global
keyring_operations Boolean Global
last_insert_id Integer Session
lc_messages String Both
lc_time_names String Both
local_infile Boolean Global
lock_wait_timeout Integer Both
log_bin_trust_function_creators Boolean Global
log_builtin_as_identified_by_password Boolean Global
log_error_verbosity Integer Global
log_output Set Global
log_queries_not_using_indexes Boolean Global
log_slow_admin_statements Boolean Global
log_slow_slave_statements Boolean Global
log_statements_unsafe_for_binlog Boolean Global
log_syslog Boolean Global
log_syslog_facility String Global
log_syslog_include_pid Boolean Global
log_syslog_tag String Global
log_throttle_queries_not_using_indexes Integer Global
log_timestamps Enumeration Global
log_warnings Integer Global
long_query_time Numeric Both
low_priority_updates Boolean Both
master_info_repository String Global
master_verify_checksum Boolean Global
max_allowed_packet Integer Both
max_binlog_cache_size Integer Global
max_binlog_size Integer Global
max_binlog_stmt_cache_size Integer Global
max_connect_errors Integer Global
max_connections Integer Global
max_delayed_threads Integer Both
max_error_count Integer Both
max_execution_time Integer Both
max_heap_table_size Integer Both
max_insert_delayed_threads Integer Both
max_join_size Integer Both
max_length_for_sort_data Integer Both
max_points_in_geometry Integer Both
max_prepared_stmt_count Integer Global
max_relay_log_size Integer Global
max_seeks_for_key Integer Both
max_sort_length Integer Both
max_sp_recursion_depth Integer Both
max_tmp_tables Integer Both
max_user_connections Integer Both
max_write_lock_count Integer Global
min_examined_row_limit Integer Both
multi_range_count Integer Both
myisam_data_pointer_size Integer Global
myisam_max_sort_file_size Integer Global
myisam_repair_threads Integer Both
myisam_sort_buffer_size Integer Both
myisam_stats_method Enumeration Both
myisam_use_mmap Boolean Global
mysql_firewall_mode Boolean Global
mysql_firewall_trace Boolean Global
mysql_native_password_proxy_users Boolean Global
mysqlx_connect_timeout Integer Global
mysqlx_idle_worker_thread_timeout Integer Global
mysqlx_max_allowed_packet Integer Global
mysqlx_max_connections Integer Global
mysqlx_min_worker_threads Integer Global
ndb_allow_copying_alter_table Boolean Both
ndb_autoincrement_prefetch_sz Integer Both
ndb_blob_read_batch_bytes Integer Both
ndb_blob_write_batch_bytes Integer Both
ndb_cache_check_time Integer Global
ndb_clear_apply_status Boolean Global
ndb_data_node_neighbour Integer Global
ndb_default_column_format Enumeration Global
ndb_default_column_format Enumeration Global
ndb_deferred_constraints Integer Both
ndb_deferred_constraints Integer Both
ndb_distribution Enumeration Global
ndb_distribution Enumeration Global
ndb_eventbuffer_free_percent Integer Global
ndb_eventbuffer_max_alloc Integer Global
ndb_extra_logging Integer Global
ndb_force_send Boolean Both
ndb_fully_replicated Boolean Both
ndb_index_stat_enable Boolean Both
ndb_index_stat_option String Both
ndb_join_pushdown Boolean Both
ndb_log_bin Boolean Both
ndb_log_binlog_index Boolean Global
ndb_log_empty_epochs Boolean Global
ndb_log_empty_epochs Boolean Global
ndb_log_empty_update Boolean Global
ndb_log_empty_update Boolean Global
ndb_log_exclusive_reads Boolean Both
ndb_log_exclusive_reads Boolean Both
ndb_log_update_as_write Boolean Global
ndb_log_update_minimal Boolean Global
ndb_log_updated_only Boolean Global
ndb_optimization_delay Integer Global
ndb_read_backup Boolean Global
ndb_recv_thread_activation_threshold Integer Global
ndb_recv_thread_cpu_mask Bitmap Global
ndb_report_thresh_binlog_epoch_slip Integer Global
ndb_report_thresh_binlog_mem_usage Integer Global
ndb_row_checksum Integer Both
ndb_show_foreign_key_mock_tables Boolean Global
ndb_slave_conflict_role Enumeration Global
ndb_table_no_logging Boolean Session
ndb_table_temporary Boolean Session
ndb_use_exact_count Boolean Both
ndb_use_transactions Boolean Both
ndbinfo_max_bytes Integer Both
ndbinfo_max_rows Integer Both
ndbinfo_offline Boolean Global
ndbinfo_show_hidden Boolean Both
ndbinfo_table_prefix String Both
net_buffer_length Integer Both
net_read_timeout Integer Both
net_retry_count Integer Both
net_write_timeout Integer Both
new Boolean Both
offline_mode Boolean Global
old_alter_table Boolean Both
old_passwords Enumeration Both
optimizer_prune_level Integer Both
optimizer_search_depth Integer Both
optimizer_switch Set Both
optimizer_trace String Both
optimizer_trace_features String Both
optimizer_trace_limit Integer Both
optimizer_trace_max_mem_size Integer Both
optimizer_trace_offset Integer Both
parser_max_mem_size Integer Both
preload_buffer_size Integer Both
profiling Boolean Both
profiling_history_size Integer Both
pseudo_slave_mode Integer Session
pseudo_thread_id Integer Session
query_alloc_block_size Integer Both
query_cache_limit Integer Global
query_cache_min_res_unit Integer Global
query_cache_size Integer Global
query_cache_type Enumeration Both
query_cache_wlock_invalidate Boolean Both
query_prealloc_size Integer Both
rand_seed1 Integer Session
rand_seed2 Integer Session
range_alloc_block_size Integer Both
range_optimizer_max_mem_size Integer Both
rbr_exec_mode Enumeration Both
read_buffer_size Integer Both
read_only Boolean Global
read_rnd_buffer_size Integer Both
relay_log_info_repository String Global
relay_log_purge Boolean Global
require_secure_transport Boolean Global
rewriter_enabled Boolean Global
rewriter_verbose Integer Global
rpl_semi_sync_master_enabled Boolean Global
rpl_semi_sync_master_timeout Integer Global
rpl_semi_sync_master_trace_level Integer Global
rpl_semi_sync_master_wait_for_slave_count Integer Global
rpl_semi_sync_master_wait_no_slave Boolean Global
rpl_semi_sync_master_wait_point Enumeration Global
rpl_semi_sync_slave_enabled Boolean Global
rpl_semi_sync_slave_trace_level Integer Global
rpl_stop_slave_timeout Integer Global
secure_auth Boolean Global
server_id Integer Global
session_track_gtids Enumeration Both
session_track_schema Boolean Both
session_track_state_change Boolean Both
session_track_system_variables String Both
session_track_transaction_info Enumeration Both
sha256_password_proxy_users Boolean Global
show_compatibility_56 Boolean Global
show_create_table_verbosity Boolean Both
show_old_temporals Boolean Both
slave_allow_batching Boolean Global
slave_checkpoint_group Integer Global
slave_checkpoint_period Integer Global
slave_compressed_protocol Boolean Global
slave_exec_mode Enumeration Global
slave_max_allowed_packet Integer Global
slave_net_timeout Integer Global
slave_parallel_type Enumeration Global
slave_parallel_workers Integer Global
slave_pending_jobs_size_max Integer Global
slave_preserve_commit_order Boolean Global
slave_rows_search_algorithms Set Global
slave_sql_verify_checksum Boolean Global
slave_transaction_retries Integer Global
slow_launch_time Integer Global
slow_query_log Boolean Global
slow_query_log_file File name Global
sort_buffer_size Integer Both
sql_auto_is_null Boolean Both
sql_big_selects Boolean Both
sql_buffer_result Boolean Both
sql_log_bin Boolean Session
sql_log_off Boolean Both
sql_mode Set Both
sql_notes Boolean Both
sql_quote_show_create Boolean Both
sql_safe_updates Boolean Both
sql_select_limit Integer Both
sql_slave_skip_counter Integer Global
sql_warnings Boolean Both
stored_program_cache Integer Global
super_read_only Boolean Global
sync_binlog Integer Global
sync_frm Boolean Global
sync_master_info Integer Global
sync_relay_log Integer Global
sync_relay_log_info Integer Global
table_definition_cache Integer Global
table_open_cache Integer Global
thread_cache_size Integer Global
thread_pool_high_priority_connection Integer Both
thread_pool_max_unused_threads Integer Global
thread_pool_prio_kickup_timer Integer Both
thread_pool_stall_limit Integer Global
time_zone String Both
timestamp Numeric Session
tmp_table_size Integer Both
transaction_alloc_block_size Integer Both
transaction_allow_batching Boolean Session
transaction_isolation Enumeration Both
transaction_prealloc_size Integer Both
transaction_read_only Boolean Both
transaction_write_set_extraction Enumeration Both
tx_isolation Enumeration Both
tx_read_only Boolean Both
unique_checks Boolean Both
updatable_views_with_limit Boolean Both
validate_password_check_user_name Boolean Global
validate_password_dictionary_file File name Global
validate_password_length Integer Global
validate_password_mixed_case_count Integer Global
validate_password_number_count Integer Global
validate_password_policy Enumeration Global
validate_password_special_char_count Integer Global
version_tokens_session String Both
wait_timeout Integer Both

5.1.8.3 Structured System Variables

A structured variable differs from a regular system variable in two respects:

  • Its value is a structure with components that specify server parameters considered to be closely related.

  • There might be several instances of a given type of structured variable. Each one has a different name and refers to a different resource maintained by the server.

MySQL supports one structured variable type, which specifies parameters governing the operation of key caches. A key cache structured variable has these components:

This section describes the syntax for referring to structured variables. Key cache variables are used for syntax examples, but specific details about how key caches operate are found elsewhere, in Section 8.10.2, “The MyISAM Key Cache”.

To refer to a component of a structured variable instance, you can use a compound name in instance_name.component_name format. Examples:

hot_cache.key_buffer_size
hot_cache.key_cache_block_size
cold_cache.key_cache_block_size

For each structured system variable, an instance with the name of default is always predefined. If you refer to a component of a structured variable without any instance name, the default instance is used. Thus, default.key_buffer_size and key_buffer_size both refer to the same system variable.

Structured variable instances and components follow these naming rules:

  • For a given type of structured variable, each instance must have a name that is unique within variables of that type. However, instance names need not be unique across structured variable types. For example, each structured variable has an instance named default, so default is not unique across variable types.

  • The names of the components of each structured variable type must be unique across all system variable names. If this were not true (that is, if two different types of structured variables could share component member names), it would not be clear which default structured variable to use for references to member names that are not qualified by an instance name.

  • If a structured variable instance name is not legal as an unquoted identifier, refer to it as a quoted identifier using backticks. For example, hot-cache is not legal, but `hot-cache` is.

  • global, session, and local are not legal instance names. This avoids a conflict with notation such as @@GLOBAL.var_name for referring to nonstructured system variables.

Currently, the first two rules have no possibility of being violated because the only structured variable type is the one for key caches. These rules will assume greater significance if some other type of structured variable is created in the future.

With one exception, you can refer to structured variable components using compound names in any context where simple variable names can occur. For example, you can assign a value to a structured variable using a command-line option:

shell> mysqld --hot_cache.key_buffer_size=64K

In an option file, use this syntax:

[mysqld]
hot_cache.key_buffer_size=64K

If you start the server with this option, it creates a key cache named hot_cache with a size of 64KB in addition to the default key cache that has a default size of 8MB.

Suppose that you start the server as follows:

shell> mysqld --key_buffer_size=256K \
         --extra_cache.key_buffer_size=128K \
         --extra_cache.key_cache_block_size=2048

In this case, the server sets the size of the default key cache to 256KB. (You could also have written --default.key_buffer_size=256K.) In addition, the server creates a second key cache named extra_cache that has a size of 128KB, with the size of block buffers for caching table index blocks set to 2048 bytes.

The following example starts the server with three different key caches having sizes in a 3:1:1 ratio:

shell> mysqld --key_buffer_size=6M \
         --hot_cache.key_buffer_size=2M \
         --cold_cache.key_buffer_size=2M

Structured variable values may be set and retrieved at runtime as well. For example, to set a key cache named hot_cache to a size of 10MB, use either of these statements:

mysql> SET GLOBAL hot_cache.key_buffer_size = 10*1024*1024;
mysql> SET @@GLOBAL.hot_cache.key_buffer_size = 10*1024*1024;

To retrieve the cache size, do this:

mysql> SELECT @@GLOBAL.hot_cache.key_buffer_size;

However, the following statement does not work. The variable is not interpreted as a compound name, but as a simple string for a LIKE pattern-matching operation:

mysql> SHOW GLOBAL VARIABLES LIKE 'hot_cache.key_buffer_size';

This is the exception to being able to use structured variable names anywhere a simple variable name may occur.

5.1.9 Server Status Variables

The MySQL server maintains many status variables that provide information about its operation. You can view these variables and their values by using the SHOW [GLOBAL | SESSION] STATUS statement (see Section 13.7.5.35, “SHOW STATUS Statement”). The optional GLOBAL keyword aggregates the values over all connections, and SESSION shows the values for the current connection.

mysql> SHOW GLOBAL STATUS;
+-----------------------------------+------------+
| Variable_name                     | Value      |
+-----------------------------------+------------+
| Aborted_clients                   | 0          |
| Aborted_connects                  | 0          |
| Bytes_received                    | 155372598  |
| Bytes_sent                        | 1176560426 |
...
| Connections                       | 30023      |
| Created_tmp_disk_tables           | 0          |
| Created_tmp_files                 | 3          |
| Created_tmp_tables                | 2          |
...
| Threads_created                   | 217        |
| Threads_running                   | 88         |
| Uptime                            | 1389872    |
+-----------------------------------+------------+

Many status variables are reset to 0 by the FLUSH STATUS statement.

This section provides a description of each status variable. For a status variable summary, see Section 5.1.5, “Server Status Variable Reference”.

The status variables have the meanings shown in the following list. For meanings of status variables specific to NDB Cluster, see Section 21.3.3.9.3, “NDB Cluster Status Variables”.

5.1.10 Server SQL Modes

The MySQL server can operate in different SQL modes, and can apply these modes differently for different clients, depending on the value of the sql_mode system variable. DBAs can set the global SQL mode to match site server operating requirements, and each application can set its session SQL mode to its own requirements.

Modes affect the SQL syntax MySQL supports and the data validation checks it performs. This makes it easier to use MySQL in different environments and to use MySQL together with other database servers.

For answers to questions often asked about server SQL modes in MySQL, see Section A.3, “MySQL 5.7 FAQ: Server SQL Mode”.

When working with InnoDB tables, consider also the innodb_strict_mode system variable. It enables additional error checks for InnoDB tables.

Setting the SQL Mode

The default SQL mode in MySQL 5.7 includes these modes: ONLY_FULL_GROUP_BY, STRICT_TRANS_TABLES, NO_ZERO_IN_DATE, NO_ZERO_DATE, ERROR_FOR_DIVISION_BY_ZERO, NO_AUTO_CREATE_USER, and NO_ENGINE_SUBSTITUTION.

These modes were added to the default SQL mode in MySQL 5.7: The ONLY_FULL_GROUP_BY and STRICT_TRANS_TABLES modes were added in MySQL 5.7.5. The NO_AUTO_CREATE_USER mode was added in MySQL 5.7.7. The ERROR_FOR_DIVISION_BY_ZERO, NO_ZERO_DATE, and NO_ZERO_IN_DATE modes were added in MySQL 5.7.8. For additional discussion regarding these changes to the default SQL mode value, see SQL Mode Changes in MySQL 5.7.

To set the SQL mode at server startup, use the --sql-mode="modes" option on the command line, or sql-mode="modes" in an option file such as my.cnf (Unix operating systems) or my.ini (Windows). modes is a list of different modes separated by commas. To clear the SQL mode explicitly, set it to an empty string using --sql-mode="" on the command line, or sql-mode="" in an option file.

Note

MySQL installation programs may configure the SQL mode during the installation process. If the SQL mode differs from the default or from what you expect, check for a setting in an option file that the server reads at startup.

To change the SQL mode at runtime, set the global or session sql_mode system variable using a SET statement:

SET GLOBAL sql_mode = 'modes';
SET SESSION sql_mode = 'modes';

Setting the GLOBAL variable requires the SUPER privilege and affects the operation of all clients that connect from that time on. Setting the SESSION variable affects only the current client. Each client can change its session sql_mode value at any time.

To determine the current global or session sql_mode setting, select its value:

SELECT @@GLOBAL.sql_mode;
SELECT @@SESSION.sql_mode;
Important

SQL mode and user-defined partitioning.  Changing the server SQL mode after creating and inserting data into partitioned tables can cause major changes in the behavior of such tables, and could lead to loss or corruption of data. It is strongly recommended that you never change the SQL mode once you have created tables employing user-defined partitioning.

When replicating partitioned tables, differing SQL modes on the master and slave can also lead to problems. For best results, you should always use the same server SQL mode on the master and slave.

For more information, see Section 22.6, “Restrictions and Limitations on Partitioning”.

The Most Important SQL Modes

The most important sql_mode values are probably these:

  • ANSI

    This mode changes syntax and behavior to conform more closely to standard SQL. It is one of the special combination modes listed at the end of this section.

  • STRICT_TRANS_TABLES

    If a value could not be inserted as given into a transactional table, abort the statement. For a nontransactional table, abort the statement if the value occurs in a single-row statement or the first row of a multiple-row statement. More details are given later in this section.

    As of MySQL 5.7.5, the default SQL mode includes STRICT_TRANS_TABLES.

  • TRADITIONAL

    Make MySQL behave like a traditional SQL database system. A simple description of this mode is give an error instead of a warning when inserting an incorrect value into a column. It is one of the special combination modes listed at the end of this section.

    Note

    With TRADITIONAL mode enabled, an INSERT or UPDATE aborts as soon as an error occurs. If you are using a nontransactional storage engine, this may not be what you want because data changes made prior to the error may not be rolled back, resulting in a partially done update.

When this manual refers to strict mode, it means a mode with either or both STRICT_TRANS_TABLES or STRICT_ALL_TABLES enabled.

Full List of SQL Modes

The following list describes all supported SQL modes:

  • ALLOW_INVALID_DATES

    Do not perform full checking of dates. Check only that the month is in the range from 1 to 12 and the day is in the range from 1 to 31. This may be useful for Web applications that obtain year, month, and day in three different fields and store exactly what the user inserted, without date validation. This mode applies to DATE and DATETIME columns. It does not apply TIMESTAMP columns, which always require a valid date.

    With ALLOW_INVALID_DATES disabled, the server requires that month and day values be legal, and not merely in the range 1 to 12 and 1 to 31, respectively. With strict mode disabled, invalid dates such as '2004-04-31' are converted to '0000-00-00' and a warning is generated. With strict mode enabled, invalid dates generate an error. To permit such dates, enable ALLOW_INVALID_DATES.

  • ANSI_QUOTES

    Treat " as an identifier quote character (like the ` quote character) and not as a string quote character. You can still use ` to quote identifiers with this mode enabled. With ANSI_QUOTES enabled, you cannot use double quotation marks to quote literal strings because they are interpreted as identifiers.

  • ERROR_FOR_DIVISION_BY_ZERO

    The ERROR_FOR_DIVISION_BY_ZERO mode affects handling of division by zero, which includes MOD(N,0). For data-change operations (INSERT, UPDATE), its effect also depends on whether strict SQL mode is enabled.

    • If this mode is not enabled, division by zero inserts NULL and produces no warning.

    • If this mode is enabled, division by zero inserts NULL and produces a warning.

    • If this mode and strict mode are enabled, division by zero produces an error, unless IGNORE is given as well. For INSERT IGNORE and UPDATE IGNORE, division by zero inserts NULL and produces a warning.

    For SELECT, division by zero returns NULL. Enabling ERROR_FOR_DIVISION_BY_ZERO causes a warning to be produced as well, regardless of whether strict mode is enabled.

    ERROR_FOR_DIVISION_BY_ZERO is deprecated. ERROR_FOR_DIVISION_BY_ZERO is not part of strict mode, but should be used in conjunction with strict mode and is enabled by default. A warning occurs if ERROR_FOR_DIVISION_BY_ZERO is enabled without also enabling strict mode or vice versa. For additional discussion, see SQL Mode Changes in MySQL 5.7.

    Because ERROR_FOR_DIVISION_BY_ZERO is deprecated, it will be removed in a future MySQL release as a separate mode name and its effect included in the effects of strict SQL mode.

  • HIGH_NOT_PRECEDENCE

    The precedence of the NOT operator is such that expressions such as NOT a BETWEEN b AND c are parsed as NOT (a BETWEEN b AND c). In some older versions of MySQL, the expression was parsed as (NOT a) BETWEEN b AND c. The old higher-precedence behavior can be obtained by enabling the HIGH_NOT_PRECEDENCE SQL mode.

    mysql> SET sql_mode = '';
    mysql> SELECT NOT 1 BETWEEN -5 AND 5;
            -> 0
    mysql> SET sql_mode = 'HIGH_NOT_PRECEDENCE';
    mysql> SELECT NOT 1 BETWEEN -5 AND 5;
            -> 1
    
  • IGNORE_SPACE

    Permit spaces between a function name and the ( character. This causes built-in function names to be treated as reserved words. As a result, identifiers that are the same as function names must be quoted as described in Section 9.2, “Schema Object Names”. For example, because there is a COUNT() function, the use of count as a table name in the following statement causes an error:

    mysql> CREATE TABLE count (i INT);
    ERROR 1064 (42000): You have an error in your SQL syntax
    

    The table name should be quoted:

    mysql> CREATE TABLE `count` (i INT);
    Query OK, 0 rows affected (0.00 sec)
    

    The IGNORE_SPACE SQL mode applies to built-in functions, not to user-defined functions or stored functions. It is always permissible to have spaces after a UDF or stored function name, regardless of whether IGNORE_SPACE is enabled.

    For further discussion of IGNORE_SPACE, see Section 9.2.5, “Function Name Parsing and Resolution”.

  • NO_AUTO_CREATE_USER

    Prevent the GRANT statement from automatically creating new user accounts if it would otherwise do so, unless authentication information is specified. The statement must specify a nonempty password using IDENTIFIED BY or an authentication plugin using IDENTIFIED WITH.

    It is preferable to create MySQL accounts with CREATE USER rather than GRANT. NO_AUTO_CREATE_USER is deprecated and the default SQL mode includes NO_AUTO_CREATE_USER. Assignments to sql_mode that change the NO_AUTO_CREATE_USER mode state produce a warning, except assignments that set sql_mode to DEFAULT. NO_AUTO_CREATE_USER will be removed in a future MySQL release, at which point its effect will be enabled at all times (GRANT will not create accounts).

    Previously, before NO_AUTO_CREATE_USER was deprecated, one reason not to enable it was that it was not replication safe. Now it can be enabled and replication-safe user management performed with CREATE USER IF NOT EXISTS, DROP USER IF EXISTS, and ALTER USER IF EXISTS rather than GRANT. These statements enable safe replication when slaves may have different grants than those on the master. See Section 13.7.1.2, “CREATE USER Statement”, Section 13.7.1.3, “DROP USER Statement”, and Section 13.7.1.1, “ALTER USER Statement”.

  • NO_AUTO_VALUE_ON_ZERO

    NO_AUTO_VALUE_ON_ZERO affects handling of AUTO_INCREMENT columns. Normally, you generate the next sequence number for the column by inserting either NULL or 0 into it. NO_AUTO_VALUE_ON_ZERO suppresses this behavior for 0 so that only NULL generates the next sequence number.

    This mode can be useful if 0 has been stored in a table's AUTO_INCREMENT column. (Storing 0 is not a recommended practice, by the way.) For example, if you dump the table with mysqldump and then reload it, MySQL normally generates new sequence numbers when it encounters the 0 values, resulting in a table with contents different from the one that was dumped. Enabling NO_AUTO_VALUE_ON_ZERO before reloading the dump file solves this problem. For this reason, mysqldump automatically includes in its output a statement that enables NO_AUTO_VALUE_ON_ZERO.

  • NO_BACKSLASH_ESCAPES

    Disable the use of the backslash character (\) as an escape character within strings and identifiers. With this mode enabled, backslash becomes an ordinary character like any other.

  • NO_DIR_IN_CREATE

    When creating a table, ignore all INDEX DIRECTORY and DATA DIRECTORY directives. This option is useful on slave replication servers.

  • NO_ENGINE_SUBSTITUTION

    Control automatic substitution of the default storage engine when a statement such as CREATE TABLE or ALTER TABLE specifies a storage engine that is disabled or not compiled in.

    By default, NO_ENGINE_SUBSTITUTION is enabled.

    Because storage engines can be pluggable at runtime, unavailable engines are treated the same way:

    With NO_ENGINE_SUBSTITUTION disabled, for CREATE TABLE the default engine is used and a warning occurs if the desired engine is unavailable. For ALTER TABLE, a warning occurs and the table is not altered.

    With NO_ENGINE_SUBSTITUTION enabled, an error occurs and the table is not created or altered if the desired engine is unavailable.

  • NO_FIELD_OPTIONS

    Do not print MySQL-specific column options in the output of SHOW CREATE TABLE. This mode is used by mysqldump in portability mode.

    Note

    As of MySQL 5.7.22, NO_FIELD_OPTIONS is deprecated. It will be removed in a future version of MySQL.

  • NO_KEY_OPTIONS

    Do not print MySQL-specific index options in the output of SHOW CREATE TABLE. This mode is used by mysqldump in portability mode.

    Note

    As of MySQL 5.7.22, NO_KEY_OPTIONS is deprecated. It will be removed in a future version of MySQL.

  • NO_TABLE_OPTIONS

    Do not print MySQL-specific table options (such as ENGINE) in the output of SHOW CREATE TABLE. This mode is used by mysqldump in portability mode.

    Note

    As of MySQL 5.7.22, NO_TABLE_OPTIONS is deprecated. It will be removed in a future version of MySQL.

  • NO_UNSIGNED_SUBTRACTION

    Subtraction between integer values, where one is of type UNSIGNED, produces an unsigned result by default. If the result would otherwise have been negative, an error results:

    mysql> SET sql_mode = '';
    Query OK, 0 rows affected (0.00 sec)
    
    mysql> SELECT CAST(0 AS UNSIGNED) - 1;
    ERROR 1690 (22003): BIGINT UNSIGNED value is out of range in '(cast(0 as unsigned) - 1)'
    

    If the NO_UNSIGNED_SUBTRACTION SQL mode is enabled, the result is negative:

    mysql> SET sql_mode = 'NO_UNSIGNED_SUBTRACTION';
    mysql> SELECT CAST(0 AS UNSIGNED) - 1;
    +-------------------------+
    | CAST(0 AS UNSIGNED) - 1 |
    +-------------------------+
    |                      -1 |
    +-------------------------+
    

    If the result of such an operation is used to update an UNSIGNED integer column, the result is clipped to the maximum value for the column type, or clipped to 0 if NO_UNSIGNED_SUBTRACTION is enabled. With strict SQL mode enabled, an error occurs and the column remains unchanged.

    When NO_UNSIGNED_SUBTRACTION is enabled, the subtraction result is signed, even if any operand is unsigned. For example, compare the type of column c2 in table t1 with that of column c2 in table t2:

    mysql> SET sql_mode='';
    mysql> CREATE TABLE test (c1 BIGINT UNSIGNED NOT NULL);
    mysql> CREATE TABLE t1 SELECT c1 - 1 AS c2 FROM test;
    mysql> DESCRIBE t1;
    +-------+---------------------+------+-----+---------+-------+
    | Field | Type                | Null | Key | Default | Extra |
    +-------+---------------------+------+-----+---------+-------+
    | c2    | bigint(21) unsigned | NO   |     | 0       |       |
    +-------+---------------------+------+-----+---------+-------+
    
    mysql> SET sql_mode='NO_UNSIGNED_SUBTRACTION';
    mysql> CREATE TABLE t2 SELECT c1 - 1 AS c2 FROM test;
    mysql> DESCRIBE t2;
    +-------+------------+------+-----+---------+-------+
    | Field | Type       | Null | Key | Default | Extra |
    +-------+------------+------+-----+---------+-------+
    | c2    | bigint(21) | NO   |     | 0       |       |
    +-------+------------+------+-----+---------+-------+
    

    This means that BIGINT UNSIGNED is not 100% usable in all contexts. See Section 12.10, “Cast Functions and Operators”.

  • NO_ZERO_DATE

    The NO_ZERO_DATE mode affects whether the server permits '0000-00-00' as a valid date. Its effect also depends on whether strict SQL mode is enabled.

    • If this mode is not enabled, '0000-00-00' is permitted and inserts produce no warning.

    • If this mode is enabled, '0000-00-00' is permitted and inserts produce a warning.

    • If this mode and strict mode are enabled, '0000-00-00' is not permitted and inserts produce an error, unless IGNORE is given as well. For INSERT IGNORE and UPDATE IGNORE, '0000-00-00' is permitted and inserts produce a warning.

    NO_ZERO_DATE is deprecated. NO_ZERO_DATE is not part of strict mode, but should be used in conjunction with strict mode and is enabled by default. A warning occurs if NO_ZERO_DATE is enabled without also enabling strict mode or vice versa. For additional discussion, see SQL Mode Changes in MySQL 5.7.

    Because NO_ZERO_DATE is deprecated, it will be removed in a future MySQL release as a separate mode name and its effect included in the effects of strict SQL mode.

  • NO_ZERO_IN_DATE

    The NO_ZERO_IN_DATE mode affects whether the server permits dates in which the year part is nonzero but the month or day part is 0. (This mode affects dates such as '2010-00-01' or '2010-01-00', but not '0000-00-00'. To control whether the server permits '0000-00-00', use the NO_ZERO_DATE mode.) The effect of NO_ZERO_IN_DATE also depends on whether strict SQL mode is enabled.

    • If this mode is not enabled, dates with zero parts are permitted and inserts produce no warning.

    • If this mode is enabled, dates with zero parts are inserted as '0000-00-00' and produce a warning.

    • If this mode and strict mode are enabled, dates with zero parts are not permitted and inserts produce an error, unless IGNORE is given as well. For INSERT IGNORE and UPDATE IGNORE, dates with zero parts are inserted as '0000-00-00' and produce a warning.

    NO_ZERO_IN_DATE is deprecated. NO_ZERO_IN_DATE is not part of strict mode, but should be used in conjunction with strict mode and is enabled by default. A warning occurs if NO_ZERO_IN_DATE is enabled without also enabling strict mode or vice versa. For additional discussion, see SQL Mode Changes in MySQL 5.7.

    Because NO_ZERO_IN_DATE is deprecated, it will be removed in a future MySQL release as a separate mode name and its effect included in the effects of strict SQL mode.

  • ONLY_FULL_GROUP_BY

    Reject queries for which the select list, HAVING condition, or ORDER BY list refer to nonaggregated columns that are neither named in the GROUP BY clause nor are functionally dependent on (uniquely determined by) GROUP BY columns.

    As of MySQL 5.7.5, the default SQL mode includes ONLY_FULL_GROUP_BY. (Before 5.7.5, MySQL does not detect functional dependency and ONLY_FULL_GROUP_BY is not enabled by default. For a description of pre-5.7.5 behavior, see the MySQL 5.6 Reference Manual.)

    A MySQL extension to standard SQL permits references in the HAVING clause to aliased expressions in the select list. Before MySQL 5.7.5, enabling ONLY_FULL_GROUP_BY disables this extension, thus requiring the HAVING clause to be written using unaliased expressions. As of MySQL 5.7.5, this restriction is lifted so that the HAVING clause can refer to aliases regardless of whether ONLY_FULL_GROUP_BY is enabled.

    For additional discussion and examples, see Section 12.20.3, “MySQL Handling of GROUP BY”.

  • PAD_CHAR_TO_FULL_LENGTH

    By default, trailing spaces are trimmed from CHAR column values on retrieval. If PAD_CHAR_TO_FULL_LENGTH is enabled, trimming does not occur and retrieved CHAR values are padded to their full length. This mode does not apply to VARCHAR columns, for which trailing spaces are retained on retrieval.

    mysql> CREATE TABLE t1 (c1 CHAR(10));
    Query OK, 0 rows affected (0.37 sec)
    
    mysql> INSERT INTO t1 (c1) VALUES('xy');
    Query OK, 1 row affected (0.01 sec)
    
    mysql> SET sql_mode = '';
    Query OK, 0 rows affected (0.00 sec)
    
    mysql> SELECT c1, CHAR_LENGTH(c1) FROM t1;
    +------+-----------------+
    | c1   | CHAR_LENGTH(c1) |
    +------+-----------------+
    | xy   |               2 |
    +------+-----------------+
    1 row in set (0.00 sec)
    
    mysql> SET sql_mode = 'PAD_CHAR_TO_FULL_LENGTH';
    Query OK, 0 rows affected (0.00 sec)
    
    mysql> SELECT c1, CHAR_LENGTH(c1) FROM t1;
    +------------+-----------------+
    | c1         | CHAR_LENGTH(c1) |
    +------------+-----------------+
    | xy         |              10 |
    +------------+-----------------+
    1 row in set (0.00 sec)
    
  • PIPES_AS_CONCAT

    Treat || as a string concatenation operator (same as CONCAT()) rather than as a synonym for OR.

  • REAL_AS_FLOAT

    Treat REAL as a synonym for FLOAT. By default, MySQL treats REAL as a synonym for DOUBLE.

  • STRICT_ALL_TABLES

    Enable strict SQL mode for all storage engines. Invalid data values are rejected. For details, see Strict SQL Mode.

    From MySQL 5.7.4 through 5.7.7, STRICT_ALL_TABLES includes the effect of the ERROR_FOR_DIVISION_BY_ZERO, NO_ZERO_DATE, and NO_ZERO_IN_DATE modes. For additional discussion, see SQL Mode Changes in MySQL 5.7.

  • STRICT_TRANS_TABLES

    Enable strict SQL mode for transactional storage engines, and when possible for nontransactional storage engines. For details, see Strict SQL Mode.

    From MySQL 5.7.4 through 5.7.7, STRICT_TRANS_TABLES includes the effect of the ERROR_FOR_DIVISION_BY_ZERO, NO_ZERO_DATE, and NO_ZERO_IN_DATE modes. For additional discussion, see SQL Mode Changes in MySQL 5.7.

Combination SQL Modes

The following special modes are provided as shorthand for combinations of mode values from the preceding list.

Strict SQL Mode

Strict mode controls how MySQL handles invalid or missing values in data-change statements such as INSERT or UPDATE. A value can be invalid for several reasons. For example, it might have the wrong data type for the column, or it might be out of range. A value is missing when a new row to be inserted does not contain a value for a non-NULL column that has no explicit DEFAULT clause in its definition. (For a NULL column, NULL is inserted if the value is missing.) Strict mode also affects DDL statements such as CREATE TABLE.

If strict mode is not in effect, MySQL inserts adjusted values for invalid or missing values and produces warnings (see Section 13.7.5.40, “SHOW WARNINGS Statement”). In strict mode, you can produce this behavior by using INSERT IGNORE or UPDATE IGNORE.

For statements such as SELECT that do not change data, invalid values generate a warning in strict mode, not an error.

Strict mode produces an error for attempts to create a key that exceeds the maximum key length. When strict mode is not enabled, this results in a warning and truncation of the key to the maximum key length.

Strict mode does not affect whether foreign key constraints are checked. foreign_key_checks can be used for that. (See Section 5.1.7, “Server System Variables”.)

Strict SQL mode is in effect if either STRICT_ALL_TABLES or STRICT_TRANS_TABLES is enabled, although the effects of these modes differ somewhat:

  • For transactional tables, an error occurs for invalid or missing values in a data-change statement when either STRICT_ALL_TABLES or STRICT_TRANS_TABLES is enabled. The statement is aborted and rolled back.

  • For nontransactional tables, the behavior is the same for either mode if the bad value occurs in the first row to be inserted or updated: The statement is aborted and the table remains unchanged. If the statement inserts or modifies multiple rows and the bad value occurs in the second or later row, the result depends on which strict mode is enabled:

    • For STRICT_ALL_TABLES, MySQL returns an error and ignores the rest of the rows. However, because the earlier rows have been inserted or updated, the result is a partial update. To avoid this, use single-row statements, which can be aborted without changing the table.

    • For STRICT_TRANS_TABLES, MySQL converts an invalid value to the closest valid value for the column and inserts the adjusted value. If a value is missing, MySQL inserts the implicit default value for the column data type. In either case, MySQL generates a warning rather than an error and continues processing the statement. Implicit defaults are described in Section 11.6, “Data Type Default Values”.

Strict mode affects handling of division by zero, zero dates, and zeros in dates as follows:

  • Strict mode affects handling of division by zero, which includes MOD(N,0):

    For data-change operations (INSERT, UPDATE):

    • If strict mode is not enabled, division by zero inserts NULL and produces no warning.

    • If strict mode is enabled, division by zero produces an error, unless IGNORE is given as well. For INSERT IGNORE and UPDATE IGNORE, division by zero inserts NULL and produces a warning.

    For SELECT, division by zero returns NULL. Enabling strict mode causes a warning to be produced as well.

  • Strict mode affects whether the server permits '0000-00-00' as a valid date:

    • If strict mode is not enabled, '0000-00-00' is permitted and inserts produce no warning.

    • If strict mode is enabled, '0000-00-00' is not permitted and inserts produce an error, unless IGNORE is given as well. For INSERT IGNORE and UPDATE IGNORE, '0000-00-00' is permitted and inserts produce a warning.

  • Strict mode affects whether the server permits dates in which the year part is nonzero but the month or day part is 0 (dates such as '2010-00-01' or '2010-01-00'):

    • If strict mode is not enabled, dates with zero parts are permitted and inserts produce no warning.

    • If strict mode is enabled, dates with zero parts are not permitted and inserts produce an error, unless IGNORE is given as well. For INSERT IGNORE and UPDATE IGNORE, dates with zero parts are inserted as '0000-00-00' (which is considered valid with IGNORE) and produce a warning.

For more information about strict mode with respect to IGNORE, see Comparison of the IGNORE Keyword and Strict SQL Mode.

Before MySQL 5.7.4, and in MySQL 5.7.8 and later, strict mode affects handling of division by zero, zero dates, and zeros in dates in conjunction with the ERROR_FOR_DIVISION_BY_ZERO, NO_ZERO_DATE, and NO_ZERO_IN_DATE modes. From MySQL 5.7.4 though 5.7.7, the ERROR_FOR_DIVISION_BY_ZERO, NO_ZERO_DATE, and NO_ZERO_IN_DATE modes do nothing when named explicitly and their effects are included in the effects of strict mode. For additional discussion, see SQL Mode Changes in MySQL 5.7.

Comparison of the IGNORE Keyword and Strict SQL Mode

This section compares the effect on statement execution of the IGNORE keyword (which downgrades errors to warnings) and strict SQL mode (which upgrades warnings to errors). It describes which statements they affect, and which errors they apply to.

The following table presents a summary comparison of statement behavior when the default is to produce an error versus a warning. An example of when the default is to produce an error is inserting a NULL into a NOT NULL column. An example of when the default is to produce a warning is inserting a value of the wrong data type into a column (such as inserting the string 'abc' into an integer column).

Operational Mode When Statement Default is Error When Statement Default is Warning
Without IGNORE or strict SQL mode Error Warning
With IGNORE Warning Warning (same as without IGNORE or strict SQL mode)
With strict SQL mode Error (same as without IGNORE or strict SQL mode) Error
With IGNORE and strict SQL mode Warning Warning

One conclusion to draw from the table is that when the IGNORE keyword and strict SQL mode are both in effect, IGNORE takes precedence. This means that, although IGNORE and strict SQL mode can be considered to have opposite effects on error handling, they do not cancel when used together.

The Effect of IGNORE on Statement Execution

Several statements in MySQL support an optional IGNORE keyword. This keyword causes the server to downgrade certain types of errors and generate warnings instead. For a multiple-row statement, IGNORE causes the statement to skip to the next row instead of aborting.

For example, if the table t has a primary key column i, attempting to insert the same value of i into multiple rows normally produces a duplicate-key error:

mysql> INSERT INTO t (i) VALUES(1),(1);
ERROR 1062 (23000): Duplicate entry '1' for key 'PRIMARY'

With IGNORE, the row containing the duplicate key still is not inserted, but a warning occurs instead of an error:

mysql> INSERT IGNORE INTO t (i) VALUES(1),(1);
Query OK, 1 row affected, 1 warning (0.01 sec)
Records: 2  Duplicates: 1  Warnings: 1

mysql> SHOW WARNINGS;
+---------+------+---------------------------------------+
| Level   | Code | Message                               |
+---------+------+---------------------------------------+
| Warning | 1062 | Duplicate entry '1' for key 'PRIMARY' |
+---------+------+---------------------------------------+
1 row in set (0.00 sec)

These statements support the IGNORE keyword:

  • CREATE TABLE ... SELECT: IGNORE does not apply to the CREATE TABLE or SELECT parts of the statement but to inserts into the table of rows produced by the SELECT. Rows that duplicate an existing row on a unique key value are discarded.

  • DELETE: IGNORE causes MySQL to ignore errors during the process of deleting rows.

  • INSERT: With IGNORE, rows that duplicate an existing row on a unique key value are discarded. Rows set to values that would cause data conversion errors are set to the closest valid values instead.

    For partitioned tables where no partition matching a given value is found, IGNORE causes the insert operation to fail silently for rows containing the unmatched value.

  • LOAD DATA, LOAD XML: With IGNORE, rows that duplicate an existing row on a unique key value are discarded.

  • UPDATE: With IGNORE, rows for which duplicate-key conflicts occur on a unique key value are not updated. Rows updated to values that would cause data conversion errors are updated to the closest valid values instead.

The IGNORE keyword applies to the following errors:

ER_BAD_NULL_ERROR
ER_DUP_ENTRY
ER_DUP_ENTRY_WITH_KEY_NAME
ER_DUP_KEY
ER_NO_PARTITION_FOR_GIVEN_VALUE
ER_NO_PARTITION_FOR_GIVEN_VALUE_SILENT
ER_NO_REFERENCED_ROW_2
ER_ROW_DOES_NOT_MATCH_GIVEN_PARTITION_SET
ER_ROW_IS_REFERENCED_2
ER_SUBQUERY_NO_1_ROW
ER_VIEW_CHECK_FAILED
The Effect of Strict SQL Mode on Statement Execution

The MySQL server can operate in different SQL modes, and can apply these modes differently for different clients, depending on the value of the sql_mode system variable. In strict SQL mode, the server upgrades certain warnings to errors.

For example, in non-strict SQL mode, inserting the string 'abc' into an integer column results in conversion of the value to 0 and a warning:

mysql> SET sql_mode = '';
Query OK, 0 rows affected (0.00 sec)

mysql> INSERT INTO t (i) VALUES('abc');
Query OK, 1 row affected, 1 warning (0.01 sec)

mysql> SHOW WARNINGS;
+---------+------+--------------------------------------------------------+
| Level   | Code | Message                                                |
+---------+------+--------------------------------------------------------+
| Warning | 1366 | Incorrect integer value: 'abc' for column 'i' at row 1 |
+---------+------+--------------------------------------------------------+
1 row in set (0.00 sec)

In strict SQL mode, the invalid value is rejected with an error:

mysql> SET sql_mode = 'STRICT_ALL_TABLES';
Query OK, 0 rows affected (0.00 sec)

mysql> INSERT INTO t (i) VALUES('abc');
ERROR 1366 (HY000): Incorrect integer value: 'abc' for column 'i' at row 1

For more information about possible settings of the sql_mode system variable, see Section 5.1.10, “Server SQL Modes”.

Strict SQL mode applies to the following statements under conditions for which some value might be out of range or an invalid row is inserted into or deleted from a table:

Within stored programs, individual statements of the types just listed execute in strict SQL mode if the program was defined while strict mode was in effect.

Strict SQL mode applies to the following errors, represent a class of errors in which an input value is either invalid or missing. A value is invalid if it has the wrong data type for the column or might be out of range. A value is missing if a new row to be inserted does not contain a value for a NOT NULL column that has no explicit DEFAULT clause in its definition.

ER_BAD_NULL_ERROR
ER_CUT_VALUE_GROUP_CONCAT
ER_DATA_TOO_LONG
ER_DATETIME_FUNCTION_OVERFLOW
ER_DIVISION_BY_ZERO
ER_INVALID_ARGUMENT_FOR_LOGARITHM
ER_NO_DEFAULT_FOR_FIELD
ER_NO_DEFAULT_FOR_VIEW_FIELD
ER_TOO_LONG_KEY
ER_TRUNCATED_WRONG_VALUE
ER_TRUNCATED_WRONG_VALUE_FOR_FIELD
ER_WARN_DATA_OUT_OF_RANGE
ER_WARN_NULL_TO_NOTNULL
ER_WARN_TOO_FEW_RECORDS
ER_WRONG_ARGUMENTS
ER_WRONG_VALUE_FOR_TYPE
WARN_DATA_TRUNCATED

SQL Mode Changes in MySQL 5.7

In MySQL 5.7.22, these SQL modes are deprecated and will be removed in a future version of MySQL: DB2, MAXDB, MSSQL, MYSQL323, MYSQL40, ORACLE, POSTGRESQL, NO_FIELD_OPTIONS, NO_KEY_OPTIONS, NO_TABLE_OPTIONS.

In MySQL 5.7, the ONLY_FULL_GROUP_BY SQL mode is enabled by default because GROUP BY processing has become more sophisticated to include detection of functional dependencies. However, if you find that having ONLY_FULL_GROUP_BY enabled causes queries for existing applications to be rejected, either of these actions should restore operation:

  • If it is possible to modify an offending query, do so, either so that nonaggregated columns are functionally dependent on GROUP BY columns, or by referring to nonaggregated columns using ANY_VALUE().

  • If it is not possible to modify an offending query (for example, if it is generated by a third-party application), set the sql_mode system variable at server startup to not enable ONLY_FULL_GROUP_BY.

In MySQL 5.7, the ERROR_FOR_DIVISION_BY_ZERO, NO_ZERO_DATE, and NO_ZERO_IN_DATE SQL modes are deprecated. The long term plan is to have the three modes be included in strict SQL mode and to remove them as explicit modes in a future MySQL release. For compatibility in MySQL 5.7 with MySQL 5.6 strict mode and to provide additional time for affected applications to be modified, the following behaviors apply:

With the preceding changes, stricter data checking is still enabled by default, but the individual modes can be disabled in environments where it is currently desirable or necessary to do so.

5.1.11 IPv6 Support

Support for IPv6 in MySQL includes these capabilities:

The following sections describe how to set up MySQL so that clients can connect to the server over IPv6.

5.1.11.1 Verifying System Support for IPv6

Before MySQL Server can accept IPv6 connections, the operating system on your server host must support IPv6. As a simple test to determine whether that is true, try this command:

shell> ping6 ::1
16 bytes from ::1, icmp_seq=0 hlim=64 time=0.171 ms
16 bytes from ::1, icmp_seq=1 hlim=64 time=0.077 ms
...

To produce a description of your system's network interfaces, invoke ifconfig -a and look for IPv6 addresses in the output.

If your host does not support IPv6, consult your system documentation for instructions on enabling it. It might be that you need only reconfigure an existing network interface to add an IPv6 address. Or a more extensive change might be needed, such as rebuilding the kernel with IPv6 options enabled.

These links may be helpful in setting up IPv6 on various platforms:

5.1.11.2 Configuring the MySQL Server to Permit IPv6 Connections

The MySQL server listens on a single network socket for TCP/IP connections. This socket is bound to a single address, but it is possible for an address to map onto multiple network interfaces. To specify an address, set bind_address=addr at server startup, where addr is an IPv4 or IPv6 address or a host name. For details, see the bind_address description in Section 5.1.7, “Server System Variables”.

5.1.11.3 Connecting Using the IPv6 Local Host Address

The following procedure shows how to configure MySQL to permit IPv6 connections by clients that connect to the local server using the ::1 local host address. The instructions given here assume that your system supports IPv6.

  1. Start the MySQL server with an appropriate bind_address setting to permit it to accept IPv6 connections. For example, put the following lines in the server option file and restart the server:

    [mysqld]
    bind_address = *

    Alternatively, you can bind the server to ::1, but that makes the server more restrictive for TCP/IP connections. It accepts only IPv6 connections for that single address and rejects IPv4 connections. For more information, see the bind_address description in Section 5.1.7, “Server System Variables”.

  2. As an administrator, connect to the server and create an account for a local user who will connect from the ::1 local IPv6 host address:

    mysql> CREATE USER 'ipv6user'@'::1' IDENTIFIED BY 'ipv6pass';
    

    For the permitted syntax of IPv6 addresses in account names, see Section 6.2.4, “Specifying Account Names”. In addition to the CREATE USER statement, you can issue GRANT statements that give specific privileges to the account, although that is not necessary for the remaining steps in this procedure.

  3. Invoke the mysql client to connect to the server using the new account:

    shell> mysql -h ::1 -u ipv6user -pipv6pass
    
  4. Try some simple statements that show connection information:

    mysql> STATUS
    ...
    Connection:   ::1 via TCP/IP
    ...
    
    mysql> SELECT CURRENT_USER(), @@bind_address;
    +----------------+----------------+
    | CURRENT_USER() | @@bind_address |
    +----------------+----------------+
    | ipv6user@::1   | ::             |
    +----------------+----------------+
    

5.1.11.4 Connecting Using IPv6 Nonlocal Host Addresses

The following procedure shows how to configure MySQL to permit IPv6 connections by remote clients. It is similar to the preceding procedure for local clients, but the server and client hosts are distinct and each has its own nonlocal IPv6 address. The example uses these addresses:

Server host: 2001:db8:0:f101::1
Client host: 2001:db8:0:f101::2

These addresses are chosen from the nonroutable address range recommended by IANA for documentation purposes and suffice for testing on your local network. To accept IPv6 connections from clients outside the local network, the server host must have a public address. If your network provider assigns you an IPv6 address, you can use that. Otherwise, another way to obtain an address is to use an IPv6 broker; see Section 5.1.11.5, “Obtaining an IPv6 Address from a Broker”.

  1. Start the MySQL server with an appropriate bind_address setting to permit it to accept IPv6 connections. For example, put the following lines in the server option file and restart the server:

    [mysqld]
    bind_address = *

    Alternatively, you can bind the server to 2001:db8:0:f101::1, but that makes the server more restrictive for TCP/IP connections. It accepts only IPv6 connections for that single address and rejects IPv4 connections. For more information, see the bind_address description in Section 5.1.7, “Server System Variables”.

  2. On the server host (2001:db8:0:f101::1), create an account for a user who will connect from the client host (2001:db8:0:f101::2):

    mysql> CREATE USER 'remoteipv6user'@'2001:db8:0:f101::2' IDENTIFIED BY 'remoteipv6pass';
    
  3. On the client host (2001:db8:0:f101::2), invoke the mysql client to connect to the server using the new account:

    shell> mysql -h 2001:db8:0:f101::1 -u remoteipv6user -premoteipv6pass
    
  4. Try some simple statements that show connection information:

    mysql> STATUS
    ...
    Connection:   2001:db8:0:f101::1 via TCP/IP
    ...
    
    mysql> SELECT CURRENT_USER(), @@bind_address;
    +-----------------------------------+----------------+
    | CURRENT_USER()                    | @@bind_address |
    +-----------------------------------+----------------+
    | remoteipv6user@2001:db8:0:f101::2 | ::             |
    +-----------------------------------+----------------+
    

5.1.11.5 Obtaining an IPv6 Address from a Broker

If you do not have a public IPv6 address that enables your system to communicate over IPv6 outside your local network, you can obtain one from an IPv6 broker. The Wikipedia IPv6 Tunnel Broker page lists several brokers and their features, such as whether they provide static addresses and the supported routing protocols.

After configuring your server host to use a broker-supplied IPv6 address, start the MySQL server with an appropriate bind_address setting to permit the server to accept IPv6 connections. For example, put the following lines in the server option file and restart the server:

[mysqld]
bind_address = *

Alternatively, you can bind the server to the specific IPv6 address provided by the broker, but that makes the server more restrictive for TCP/IP connections. It accepts only IPv6 connections for that single address and rejects IPv4 connections. For more information, see the bind_address description in Section 5.1.7, “Server System Variables”. In addition, if the broker allocates dynamic addresses, the address provided for your system might change the next time you connect to the broker. If so, any accounts you create that name the original address become invalid. To bind to a specific address but avoid this change-of-address problem, you may be able to arrange with the broker for a static IPv6 address.

The following example shows how to use Freenet6 as the broker and the gogoc IPv6 client package on Gentoo Linux.

  1. Create an account at Freenet6 by visiting this URL and signing up:

    http://gogonet.gogo6.com
    
  2. After creating the account, go to this URL, sign in, and create a user ID and password for the IPv6 broker:

    http://gogonet.gogo6.com/page/freenet6-registration
    

  3. As root, install gogoc:

    shell> emerge gogoc
    
  4. Edit /etc/gogoc/gogoc.conf to set the userid and password values. For example:

    userid=gogouser
    passwd=gogopass
  5. Start gogoc:

    shell> /etc/init.d/gogoc start
    

    To start gogoc each time your system boots, execute this command:

    shell> rc-update add gogoc default
    
  6. Use ping6 to try to ping a host:

    shell> ping6 ipv6.google.com
    
  7. To see your IPv6 address:

    shell> ifconfig tun
    

5.1.12 MySQL Server Time Zone Support

This section describes the time zone settings maintained by MySQL, how to load the system tables required for named time support, how to stay current with time zone changes, and how to enable leap-second support.

For information about time zone settings in replication setups, see Section 16.4.1.15, “Replication and System Functions” and Section 16.4.1.31, “Replication and Time Zones”.

Time Zone Variables

MySQL Server maintains several time zone settings:

  • The system time zone. When the server starts, it attempts to determine the time zone of the host machine automatically and uses it to set the system_time_zone system variable. The value does not change thereafter.

    To explicitly specify the system time zone for MySQL Server at startup, set the TZ environment variable before you start mysqld. If you start the server using mysqld_safe, its --timezone option provides another way to set the system time zone. The permissible values for TZ and --timezone are system dependent. Consult your operating system documentation to see what values are acceptable.

  • The server current time zone. The global time_zone system variable indicates the time zone the server currently is operating in. The initial time_zone value is 'SYSTEM', which indicates that the server time zone is the same as the system time zone.

    Note

    If set to SYSTEM, every MySQL function call that requires a time zone calculation makes a system library call to determine the current system time zone. This call may be protected by a global mutex, resulting in contention.

    The initial global server time zone value can be specified explicitly at startup with the --default-time-zone option on the command line, or you can use the following line in an option file:

    default-time-zone='timezone'
    

    If you have the SUPER privilege, you can set the global server time zone value at runtime with this statement:

    SET GLOBAL time_zone = timezone;
    
  • Per-session time zones. Each client that connects has its own session time zone setting, given by the session time_zone variable. Initially, the session variable takes its value from the global time_zone variable, but the client can change its own time zone with this statement:

    SET time_zone = timezone;
    

The session time zone setting affects display and storage of time values that are zone-sensitive. This includes the values displayed by functions such as NOW() or CURTIME(), and values stored in and retrieved from TIMESTAMP columns. Values for TIMESTAMP columns are converted from the session time zone to UTC for storage, and from UTC to the session time zone for retrieval.

The session time zone setting does not affect values displayed by functions such as UTC_TIMESTAMP() or values in DATE, TIME, or DATETIME columns. Nor are values in those data types stored in UTC; the time zone applies for them only when converting from TIMESTAMP values. If you want locale-specific arithmetic for DATE, TIME, or DATETIME values, convert them to UTC, perform the arithmetic, and then convert back.

The current global and session time zone values can be retrieved like this:

SELECT @@GLOBAL.time_zone, @@SESSION.time_zone;

timezone values can be given in several formats, none of which are case-sensitive:

  • As the value 'SYSTEM', indicating that the server time zone is the same as the system time zone.

  • As a string indicating an offset from UTC of the form [H]H:MM, prefixed with a + or -, such as '+10:00', '-6:00', or '+05:30'. A leading zero can optionally be used for hours values less than 10; MySQL prepends a leading zero when storing and retriving the value in such cases. MySQL converts '-00:00' or '-0:00' to '+00:00'.

    A time zone offset must be in the range '-12:59' to '+13:00', inclusive.

  • As a named time zone, such as 'Europe/Helsinki', 'US/Eastern', or 'MET'. Named time zones can be used only if the time zone information tables in the mysql database have been created and populated.

Populating the Time Zone Tables

Several tables in the mysql system database exist to store time zone information (see Section 5.3, “The mysql System Database”). The MySQL installation procedure creates the time zone tables, but does not load them. To do so manually, use the following instructions.

Note

Loading the time zone information is not necessarily a one-time operation because the information changes occasionally. When such changes occur, applications that use the old rules become out of date and you may find it necessary to reload the time zone tables to keep the information used by your MySQL server current. See Staying Current with Time Zone Changes.

If your system has its own zoneinfo database (the set of files describing time zones), use the mysql_tzinfo_to_sql program to load the time zone tables. Examples of such systems are Linux, macOS, FreeBSD, and Solaris. One likely location for these files is the /usr/share/zoneinfo directory. If your system has no zoneinfo database, you can use a downloadable package, as described later in this section.

To load the time zone tables from the command line, pass the zoneinfo directory path name to mysql_tzinfo_to_sql and send the output into the mysql program. For example:

mysql_tzinfo_to_sql /usr/share/zoneinfo | mysql -u root -p mysql

The mysql command shown here assumes that you connect to the server using an account such as root that has privileges for modifying tables in the mysql system database. Adjust the connection parameters as required.

mysql_tzinfo_to_sql reads your system's time zone files and generates SQL statements from them. mysql processes those statements to load the time zone tables.

mysql_tzinfo_to_sql also can be used to load a single time zone file or generate leap second information:

  • To load a single time zone file tz_file that corresponds to a time zone name tz_name, invoke mysql_tzinfo_to_sql like this:

    mysql_tzinfo_to_sql tz_file tz_name | mysql -u root -p mysql
    

    With this approach, you must execute a separate command to load the time zone file for each named zone that the server needs to know about.

  • If your time zone must account for leap seconds, initialize leap second information like this, where tz_file is the name of your time zone file:

    mysql_tzinfo_to_sql --leap tz_file | mysql -u root -p mysql
    

After running mysql_tzinfo_to_sql, restart the server so that it does not continue to use any previously cached time zone data.

If your system has no zoneinfo database (for example, Windows), you can use a package containing SQL statements that is available for download at the MySQL Developer Zone:

https://dev.mysql.com/downloads/timezones.html
Warning

Do not use a downloadable time zone package if your system has a zoneinfo database. Use the mysql_tzinfo_to_sql utility instead. Otherwise, you may cause a difference in datetime handling between MySQL and other applications on your system.

To use an SQL-statement time zone package that you have downloaded, unpack it, then load the unpacked file contents into the time zone tables:

mysql -u root -p mysql < file_name

Then restart the server.

Warning

Do not use a downloadable time zone package that contains MyISAM tables. That is intended for older MySQL versions. MySQL 5.7 and higher uses InnoDB for the time zone tables. Trying to replace them with MyISAM tables will cause problems.

Staying Current with Time Zone Changes

When time zone rules change, applications that use the old rules become out of date. To stay current, it is necessary to make sure that your system uses current time zone information is used. For MySQL, there are multiple factors to consider in staying current:

  • The operating system time affects the value that the MySQL server uses for times if its time zone is set to SYSTEM. Make sure that your operating system is using the latest time zone information. For most operating systems, the latest update or service pack prepares your system for the time changes. Check the website for your operating system vendor for an update that addresses the time changes.

  • If you replace the system's /etc/localtime time zone file with a version that uses rules differing from those in effect at mysqld startup, restart mysqld so that it uses the updated rules. Otherwise, mysqld might not notice when the system changes its time.

  • If you use named time zones with MySQL, make sure that the time zone tables in the mysql database are up to date:

    • If your system has its own zoneinfo database, reload the MySQL time zone tables whenever the zoneinfo database is updated.

    • For systems that do not have their own zoneinfo database, check the MySQL Developer Zone for updates. When a new update is available, download it and use it to replace the content of your current time zone tables.

    For instructions for both methods, see Populating the Time Zone Tables. mysqld caches time zone information that it looks up, so after updating the time zone tables, restart mysqld to make sure that it does not continue to serve outdated time zone data.

If you are uncertain whether named time zones are available, for use either as the server's time zone setting or by clients that set their own time zone, check whether your time zone tables are empty. The following query determines whether the table that contains time zone names has any rows:

mysql> SELECT COUNT(*) FROM mysql.time_zone_name;
+----------+
| COUNT(*) |
+----------+
|        0 |
+----------+

A count of zero indicates that the table is empty. In this case, no applications currently are using named time zones, and you need not update the tables (unless you want to enable named time zone support). A count greater than zero indicates that the table is not empty and that its contents are available to be used for named time zone support. In this case, be sure to reload your time zone tables so that applications that use named time zones will obtain correct query results.

To check whether your MySQL installation is updated properly for a change in Daylight Saving Time rules, use a test like the one following. The example uses values that are appropriate for the 2007 DST 1-hour change that occurs in the United States on March 11 at 2 a.m.

The test uses this query:

SELECT
  CONVERT_TZ('2007-03-11 2:00:00','US/Eastern','US/Central') AS time1,
  CONVERT_TZ('2007-03-11 3:00:00','US/Eastern','US/Central') AS time2;

The two time values indicate the times at which the DST change occurs, and the use of named time zones requires that the time zone tables be used. The desired result is that both queries return the same result (the input time, converted to the equivalent value in the 'US/Central' time zone).

Before updating the time zone tables, you see an incorrect result like this:

+---------------------+---------------------+
| time1               | time2               |
+---------------------+---------------------+
| 2007-03-11 01:00:00 | 2007-03-11 02:00:00 |
+---------------------+---------------------+

After updating the tables, you should see the correct result:

+---------------------+---------------------+
| time1               | time2               |
+---------------------+---------------------+
| 2007-03-11 01:00:00 | 2007-03-11 01:00:00 |
+---------------------+---------------------+

Time Zone Leap Second Support

Leap second values are returned with a time part that ends with :59:59. This means that a function such as NOW() can return the same value for two or three consecutive seconds during the leap second. It remains true that literal temporal values having a time part that ends with :59:60 or :59:61 are considered invalid.

If it is necessary to search for TIMESTAMP values one second before the leap second, anomalous results may be obtained if you use a comparison with 'YYYY-MM-DD hh:mm:ss' values. The following example demonstrates this. It changes the session time zone to UTC so there is no difference between internal TIMESTAMP values (which are in UTC) and displayed values (which have time zone correction applied).

mysql> CREATE TABLE t1 (
         a INT,
         ts TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
         PRIMARY KEY (ts)
       );
Query OK, 0 rows affected (0.01 sec)

mysql> -- change to UTC
mysql> SET time_zone = '+00:00';
Query OK, 0 rows affected (0.00 sec)

mysql> -- Simulate NOW() = '2008-12-31 23:59:59'
mysql> SET timestamp = 1230767999;
Query OK, 0 rows affected (0.00 sec)

mysql> INSERT INTO t1 (a) VALUES (1);
Query OK, 1 row affected (0.00 sec)

mysql> -- Simulate NOW() = '2008-12-31 23:59:60'
mysql> SET timestamp = 1230768000;
Query OK, 0 rows affected (0.00 sec)

mysql> INSERT INTO t1 (a) VALUES (2);
Query OK, 1 row affected (0.00 sec)

mysql> -- values differ internally but display the same
mysql> SELECT a, ts, UNIX_TIMESTAMP(ts) FROM t1;
+------+---------------------+--------------------+
| a    | ts                  | UNIX_TIMESTAMP(ts) |
+------+---------------------+--------------------+
|    1 | 2008-12-31 23:59:59 |         1230767999 |
|    2 | 2008-12-31 23:59:59 |         1230768000 |
+------+---------------------+--------------------+
2 rows in set (0.00 sec)

mysql> -- only the non-leap value matches
mysql> SELECT * FROM t1 WHERE ts = '2008-12-31 23:59:59';
+------+---------------------+
| a    | ts                  |
+------+---------------------+
|    1 | 2008-12-31 23:59:59 |
+------+---------------------+
1 row in set (0.00 sec)

mysql> -- the leap value with seconds=60 is invalid
mysql> SELECT * FROM t1 WHERE ts = '2008-12-31 23:59:60';
Empty set, 2 warnings (0.00 sec)

To work around this, you can use a comparison based on the UTC value actually stored in the column, which has the leap second correction applied:

mysql> -- selecting using UNIX_TIMESTAMP value return leap value
mysql> SELECT * FROM t1 WHERE UNIX_TIMESTAMP(ts) = 1230768000;
+------+---------------------+
| a    | ts                  |
+------+---------------------+
|    2 | 2008-12-31 23:59:59 |
+------+---------------------+
1 row in set (0.00 sec)

5.1.13 Server-Side Help Support

MySQL Server supports a HELP statement that returns information from the MySQL Reference Manual (see Section 13.8.3, “HELP Statement”). This information is stored in several tables in the mysql database (see Section 5.3, “The mysql System Database”). Proper operation of the HELP statement requires that these help tables be initialized.

For a new installation of MySQL using a binary or source distribution on Unix, help-table content initialization occurs when you initialize the data directory (see Section 2.10.1, “Initializing the Data Directory”). For an RPM distribution on Linux or binary distribution on Windows, content initialization occurs as part of the MySQL installation process.

For a MySQL upgrade using a binary distribution, help-table content is not upgraded automatically, but you can upgrade it manually. Locate the fill_help_tables.sql file in the share or share/mysql directory. Change location into that directory and process the file with the mysql client as follows:

mysql -u root -p mysql < fill_help_tables.sql

The command shown here assumes that you connect to the server using an account such as root that has privileges for modifying tables in the mysql database. Adjust the connection parameters as required.

If you are working with Git and a MySQL development source tree, the source tree contains only a stub version of fill_help_tables.sql. To obtain a non-stub copy, use one from a source or binary distribution.

Note

Each MySQL series has its own series-specific reference manual, so help-table content is series specific as well. This has implications for replication because help-table content should match the MySQL series. If you load MySQL 5.7 help content into a MySQL 5.7 master server, it does not make sense to replicate that content to a slave server from a different MySQL series and for which that content is not appropriate. For this reason, as you upgrade individual servers in a replication scenario, you should upgrade each server's help tables, using the instructions given earlier.

5.1.14 Server Tracking of Client Session State Changes

The MySQL server implements several session state trackers. A client can enable these trackers to receive notification of changes to its session state.

One use for the tracker mechanism is to provide a means for MySQL connectors and client applications to determine whether any session context is available to permit session migration from one server to another. (To change sessions in a load-balanced environment, it is necessary to detect whether there is session state to take into consideration when deciding whether a switch can be made.)

Another use for the tracker mechanism is to permit applications to know when transactions can be moved from one session to another. Transaction state tracking enables this, which is useful for applications that may wish to move transactions from a busy server to one that is less loaded. For example, a load-balancing connector managing a client connection pool could move transactions between available sessions in the pool.

However, session switching cannot be done at arbitrary times. If a session is in the middle of a transaction for which reads or writes have been done, switching to a different session implies a transaction rollback on the original session. A session switch must be done only when a transaction does not yet have any reads or writes performed within it.

Examples of when transactions might reasonably be switched:

In addition to knowing transaction state, it is useful to know transaction characteristics, so as to use the same characteristics if the transaction is moved to a different session. The following characteristics are relevant for this purpose:

READ ONLY 
READ WRITE
ISOLATION LEVEL
WITH CONSISTENT SNAPSHOT

To support the preceding session-switching activities, notification is available for these types of client session state information:

  • Changes to these attributes of client session state:

    • The default schema (database).

    • Session-specific values for system variables.

    • User-defined variables.

    • Temporary tables.

    • Prepared statements.

    The session_track_state_change system variable controls this tracker.

  • Changes to the default schema name. The session_track_schema system variable controls this tracker.

  • Changes to the session values of system variables. The session_track_system_variables system variable controls this tracker.

  • Available GTIDs. The session_track_gtids system variable controls this tracker.

  • Information about transaction state and characteristics. The session_track_transaction_info system variable controls this tracker.

For descriptions of the tracker-related system variables, see Section 5.1.7, “Server System Variables”. Those system variables permit control over which change notifications occur, but do not provide a way to access notification information. Notification occurs in the MySQL client/server protocol, which includes tracker information in OK packets so that session state changes can be detected. To enable client applications to extract state-change information from OK packets returned by the server, the MySQL C API provides a pair of functions:

The mysqltest program has disable_session_track_info and enable_session_track_info commands that control whether session tracker notifications occur. You can use these commands to see from the command line what notifications SQL statements produce. Suppose that a file testscript contains the following mysqltest script:

DROP TABLE IF EXISTS test.t1;
CREATE TABLE test.t1 (i INT, f FLOAT);
--enable_session_track_info
SET @@SESSION.session_track_schema=ON;
SET @@SESSION.session_track_system_variables='*';
SET @@SESSION.session_track_state_change=ON;
USE information_schema;
SET NAMES 'utf8mb4';
SET @@SESSION.session_track_transaction_info='CHARACTERISTICS';
SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;
SET TRANSACTION READ WRITE;
START TRANSACTION;
SELECT 1;
INSERT INTO test.t1 () VALUES();
INSERT INTO test.t1 () VALUES(1, RAND());
COMMIT;

Run the script as follows to see the information provided by the enabled trackers. For a description of the Tracker: information displayed by mysqltest for the various trackers, see Section 27.7.6.65, “mysql_session_track_get_first()”.

shell> mysqltest < testscript
DROP TABLE IF EXISTS test.t1;
CREATE TABLE test.t1 (i INT, f FLOAT);
SET @@SESSION.session_track_schema=ON;
SET @@SESSION.session_track_system_variables='*';
-- Tracker : SESSION_TRACK_SYSTEM_VARIABLES
-- session_track_system_variables
-- *

SET @@SESSION.session_track_state_change=ON;
-- Tracker : SESSION_TRACK_SYSTEM_VARIABLES
-- session_track_state_change
-- ON

USE information_schema;
-- Tracker : SESSION_TRACK_SCHEMA
-- information_schema

-- Tracker : SESSION_TRACK_STATE_CHANGE
-- 1

SET NAMES 'utf8mb4';
-- Tracker : SESSION_TRACK_SYSTEM_VARIABLES
-- character_set_client
-- utf8mb4
-- character_set_connection
-- utf8mb4
-- character_set_results
-- utf8mb4

-- Tracker : SESSION_TRACK_STATE_CHANGE
-- 1

SET @@SESSION.session_track_transaction_info='CHARACTERISTICS';
-- Tracker : SESSION_TRACK_SYSTEM_VARIABLES
-- session_track_transaction_info
-- CHARACTERISTICS

-- Tracker : SESSION_TRACK_STATE_CHANGE
-- 1

-- Tracker : SESSION_TRACK_TRANSACTION_CHARACTERISTICS
-- 

-- Tracker : SESSION_TRACK_TRANSACTION_STATE
-- ________

SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;
-- Tracker : SESSION_TRACK_TRANSACTION_CHARACTERISTICS
-- SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;

SET TRANSACTION READ WRITE;
-- Tracker : SESSION_TRACK_TRANSACTION_CHARACTERISTICS
-- SET TRANSACTION ISOLATION LEVEL SERIALIZABLE; SET TRANSACTION READ WRITE;

START TRANSACTION;
-- Tracker : SESSION_TRACK_TRANSACTION_CHARACTERISTICS
-- SET TRANSACTION ISOLATION LEVEL SERIALIZABLE; START TRANSACTION READ WRITE;

-- Tracker : SESSION_TRACK_TRANSACTION_STATE
-- T_______

SELECT 1;
1
1
-- Tracker : SESSION_TRACK_TRANSACTION_STATE
-- T_____S_

INSERT INTO test.t1 () VALUES();
-- Tracker : SESSION_TRACK_TRANSACTION_STATE
-- T___W_S_

INSERT INTO test.t1 () VALUES(1, RAND());
-- Tracker : SESSION_TRACK_TRANSACTION_STATE
-- T___WsS_

COMMIT;
-- Tracker : SESSION_TRACK_TRANSACTION_CHARACTERISTICS
-- 

-- Tracker : SESSION_TRACK_TRANSACTION_STATE
-- ________

ok

Preceding the START TRANSACTION statement, two SET TRANSACTION statements execute that set the isolation level and access mode characteristics for the next transaction. The SESSION_TRACK_TRANSACTION_CHARACTERISTICS value indicates those next-transaction values that have been set.

Following the COMMIT statement that ends the transaction, the SESSION_TRACK_TRANSACTION_CHARACTERISTICS value is reported as empty. This indicates that the next-transaction characteristics that were set preceding the start of the transaction have been reset, and that the session defaults apply. To track changes to those session defaults, track the session values of the transaction_isolation and transaction_read_only system variables.

5.1.15 The Server Shutdown Process

The server shutdown process takes place as follows:

  1. The shutdown process is initiated.

    This can occur initiated several ways. For example, a user with the SHUTDOWN privilege can execute a mysqladmin shutdown command. mysqladmin can be used on any platform supported by MySQL. Other operating system-specific shutdown initiation methods are possible as well: The server shuts down on Unix when it receives a SIGTERM signal. A server running as a service on Windows shuts down when the services manager tells it to.

  2. The server creates a shutdown thread if necessary.

    Depending on how shutdown was initiated, the server might create a thread to handle the shutdown process. If shutdown was requested by a client, a shutdown thread is created. If shutdown is the result of receiving a SIGTERM signal, the signal thread might handle shutdown itself, or it might create a separate thread to do so. If the server tries to create a shutdown thread and cannot (for example, if memory is exhausted), it issues a diagnostic message that appears in the error log:

    Error: Can't create thread to kill server
  3. The server stops accepting new connections.

    To prevent new activity from being initiated during shutdown, the server stops accepting new client connections by closing the handlers for the network interfaces to which it normally listens for connections: the TCP/IP port, the Unix socket file, the Windows named pipe, and shared memory on Windows.

  4. The server terminates current activity.

    For each thread associated with a client connection, the server breaks the connection to the client and marks the thread as killed. Threads die when they notice that they are so marked. Threads for idle connections die quickly. Threads that currently are processing statements check their state periodically and take longer to die. For additional information about thread termination, see Section 13.7.6.4, “KILL Statement”, in particular for the instructions about killed REPAIR TABLE or OPTIMIZE TABLE operations on MyISAM tables.

    For threads that have an open transaction, the transaction is rolled back. If a thread is updating a nontransactional table, an operation such as a multiple-row UPDATE or INSERT may leave the table partially updated because the operation can terminate before completion.

    If the server is a master replication server, it treats threads associated with currently connected slaves like other client threads. That is, each one is marked as killed and exits when it next checks its state.

    If the server is a slave replication server, it stops the I/O and SQL threads, if they are active, before marking client threads as killed. The SQL thread is permitted to finish its current statement (to avoid causing replication problems), and then stops. If the SQL thread is in the middle of a transaction at this point, the server waits until the current replication event group (if any) has finished executing, or until the user issues a KILL QUERY or KILL CONNECTION statement. See also Section 13.4.2.7, “STOP SLAVE Statement”. Since nontransactional statements cannot be rolled back, in order to guarantee crash-safe replication, only transactional tables should be used.

    Note

    To guarantee crash safety on the slave, you must run the slave with --relay-log-recovery enabled.

    See also Section 16.2.4, “Replication Relay and Status Logs”).

  5. The server shuts down or closes storage engines.

    At this stage, the server flushes the table cache and closes all open tables.

    Each storage engine performs any actions necessary for tables that it manages. InnoDB flushes its buffer pool to disk (unless innodb_fast_shutdown is 2), writes the current LSN to the tablespace, and terminates its own internal threads. MyISAM flushes any pending index writes for a table.

  6. The server exits.

To provide information to management processes, the server returns one of the exit codes described in the following list. The phrase in parentheses indicates the action taken by systemd in response to the code, for platforms on which systemd is used to manage the server.

  • 0 = successful termination (no restart done)

  • 1 = unsuccessful termination (no restart done)

  • 2 = unsuccessful termination (restart done)

5.2 The MySQL Data Directory

Information managed by the MySQL server is stored under a directory known as the data directory. The following list briefly describes the items typically found in the data directory, with cross references for additional information:

Some items in the preceding list can be relocated elsewhere by reconfiguring the server. In addition, the --datadir option enables the location of the data directory itself to be changed. For a given MySQL installation, check the server configuration to determine whether items have been moved.

5.3 The mysql System Database

The mysql database is the system database. It contains tables that store information required by the MySQL server as it runs.

Tables in the mysql database fall into these categories:

The remainder of this section enumerates the tables in each category, with cross references for additional information. System tables use the MyISAM storage engine unless otherwise indicated.

Warning

Do not convert MySQL system tables in the mysql database from MyISAM to InnoDB tables. This is an unsupported operation. If you do this, MySQL does not restart until you restore the old system tables from a backup or regenerate them by reinitializing the data directory (see Section 2.10.1, “Initializing the Data Directory”).

Grant System Tables

These system tables contain grant information about user accounts and the privileges held by them:

  • user: User accounts, global privileges, and other nonprivilege columns.

  • db: Database-level privileges.

  • tables_priv: Table-level privileges.

  • columns_priv: Column-level privileges.

  • procs_priv: Stored procedure and function privileges.

  • proxies_priv: Proxy-user privileges.

For more information about the structure, contents, and purpose of the grant tables, see Section 6.2.3, “Grant Tables”.

Object Information System Tables

These system tables contain information about stored programs, user-defined functions, and server-side plugins:

Log System Tables

The server uses these system tables for logging:

  • general_log: The general query log table.

  • slow_log: The slow query log table.

Log tables use the CSV storage engine.

For more information, see Section 5.4, “MySQL Server Logs”.

Server-Side Help System Tables

These system tables contain server-side help information:

  • help_category: Information about help categories.

  • help_keyword: Keywords associated with help topics.

  • help_relation: Mappings between help keywords and topics.

  • help_topic: Help topic contents.

These tables use the InnoDB storage engine as of MySQL 5.7.5, MyISAM before that.

For more information, see Section 5.1.13, “Server-Side Help Support”.

Time Zone System Tables

These system tables contain time zone information:

  • time_zone: Time zone IDs and whether they use leap seconds.

  • time_zone_leap_second: When leap seconds occur.

  • time_zone_name: Mappings between time zone IDs and names.

  • time_zone_transition, time_zone_transition_type: Time zone descriptions.

These tables use the InnoDB storage engine as of MySQL 5.7.5, MyISAM before that.

For more information, see Section 5.1.12, “MySQL Server Time Zone Support”.

Replication System Tables

The server uses these system tables to support replication:

Optimizer System Tables

These system tables are for use by the optimizer:

These tables use the InnoDB storage engine.

Miscellaneous System Tables

Other system tables do not fall into the preceding categories:

5.4 MySQL Server Logs

MySQL Server has several logs that can help you find out what activity is taking place.

Log Type Information Written to Log
Error log Problems encountered starting, running, or stopping mysqld
General query log Established client connections and statements received from clients
Binary log Statements that change data (also used for replication)
Relay log Data changes received from a replication master server
Slow query log Queries that took more than long_query_time seconds to execute
DDL log (metadata log) Metadata operations performed by DDL statements

By default, no logs are enabled, except the error log on Windows. (The DDL log is always created when required, and has no user-configurable options; see Section 5.4.6, “The DDL Log”.) The following log-specific sections provide information about the server options that enable logging.

By default, the server writes files for all enabled logs in the data directory. You can force the server to close and reopen the log files (or in some cases switch to a new log file) by flushing the logs. Log flushing occurs when you issue a FLUSH LOGS statement; execute mysqladmin with a flush-logs or refresh argument; or execute mysqldump with a --flush-logs or --master-data option. See Section 13.7.6.3, “FLUSH Statement”, Section 4.5.2, “mysqladmin — Client for Administering a MySQL Server”, and Section 4.5.4, “mysqldump — A Database Backup Program”. In addition, the binary log is flushed when its size reaches the value of the max_binlog_size system variable.

You can control the general query and slow query logs during runtime. You can enable or disable logging, or change the log file name. You can tell the server to write general query and slow query entries to log tables, log files, or both. For details, see Section 5.4.1, “Selecting General Query Log and Slow Query Log Output Destinations”, Section 5.4.3, “The General Query Log”, and Section 5.4.5, “The Slow Query Log”.

The relay log is used only on slave replication servers, to hold data changes from the master server that must also be made on the slave. For discussion of relay log contents and configuration, see Section 16.2.4.1, “The Slave Relay Log”.

For information about log maintenance operations such as expiration of old log files, see Section 5.4.7, “Server Log Maintenance”.

For information about keeping logs secure, see Section 6.1.2.3, “Passwords and Logging”.

5.4.1 Selecting General Query Log and Slow Query Log Output Destinations

MySQL Server provides flexible control over the destination of output written to the general query log and the slow query log, if those logs are enabled. Possible destinations for log entries are log files or the general_log and slow_log tables in the mysql system database. File output, table output, or both can be selected.

Log Control at Server Startup

The log_output system variable specifies the destination for log output. Setting this variable does not in itself enable the logs; they must be enabled separately.

  • If log_output is not specified at startup, the default logging destination is FILE.

  • If log_output is specified at startup, its value is a list one or more comma-separated words chosen from TABLE (log to tables), FILE (log to files), or NONE (do not log to tables or files). NONE, if present, takes precedence over any other specifiers.

The general_log system variable controls logging to the general query log for the selected log destinations. If specified at server startup, general_log takes an optional argument of 1 or 0 to enable or disable the log. To specify a file name other than the default for file logging, set the general_log_file variable. Similarly, the slow_query_log variable controls logging to the slow query log for the selected destinations and setting slow_query_log_file specifies a file name for file logging. If either log is enabled, the server opens the corresponding log file and writes startup messages to it. However, further logging of queries to the file does not occur unless the FILE log destination is selected.

Examples:

  • To write general query log entries to the log table and the log file, use --log_output=TABLE,FILE to select both log destinations and --general_log to enable the general query log.

  • To write general and slow query log entries only to the log tables, use --log_output=TABLE to select tables as the log destination and --general_log and --slow_query_log to enable both logs.

  • To write slow query log entries only to the log file, use --log_output=FILE to select files as the log destination and --slow_query_log to enable the slow query log. In this case, because the default log destination is FILE, you could omit the log_output setting.

Log Control at Runtime

The system variables associated with log tables and files enable runtime control over logging:

  • The log_output variable indicates the current logging destination. It can be modified at runtime to change the destination.

  • The general_log and slow_query_log variables indicate whether the general query log and slow query log are enabled (ON) or disabled (OFF). You can set these variables at runtime to control whether the logs are enabled.

  • The general_log_file and slow_query_log_file variables indicate the names of the general query log and slow query log files. You can set these variables at server startup or at runtime to change the names of the log files.

  • To disable or enable general query logging for the current session, set the session sql_log_off variable to ON or OFF. (This assumes that the general query log itself is enabled.)

Log Table Benefits and Characteristics

The use of tables for log output offers the following benefits:

  • Log entries have a standard format. To display the current structure of the log tables, use these statements:

    SHOW CREATE TABLE mysql.general_log;
    SHOW CREATE TABLE mysql.slow_log;
  • Log contents are accessible through SQL statements. This enables the use of queries that select only those log entries that satisfy specific criteria. For example, to select log contents associated with a particular client (which can be useful for identifying problematic queries from that client), it is easier to do this using a log table than a log file.

  • Logs are accessible remotely through any client that can connect to the server and issue queries (if the client has the appropriate log table privileges). It is not necessary to log in to the server host and directly access the file system.

The log table implementation has the following characteristics:

  • In general, the primary purpose of log tables is to provide an interface for users to observe the runtime execution of the server, not to interfere with its runtime execution.

  • CREATE TABLE, ALTER TABLE, and DROP TABLE are valid operations on a log table. For ALTER TABLE and DROP TABLE, the log table cannot be in use and must be disabled, as described later.

  • By default, the log tables use the CSV storage engine that writes data in comma-separated values format. For users who have access to the .CSV files that contain log table data, the files are easy to import into other programs such as spreadsheets that can process CSV input.

    The log tables can be altered to use the MyISAM storage engine. You cannot use ALTER TABLE to alter a log table that is in use. The log must be disabled first. No engines other than CSV or MyISAM are legal for the log tables.

    Log Tables and Too many open files Errors.  If you select TABLE as a log destination and the log tables use the CSV storage engine, you may find that disabling and enabling the general query log or slow query log repeatedly at runtime results in a number of open file descriptors for the .CSV file, possibly resulting in a Too many open files error. To work around this issue, execute FLUSH TABLES or ensure that the value of open_files_limit is greater than the value of table_open_cache_instances.

  • To disable logging so that you can alter (or drop) a log table, you can use the following strategy. The example uses the general query log; the procedure for the slow query log is similar but uses the slow_log table and slow_query_log system variable.

    SET @old_log_state = @@GLOBAL.general_log;
    SET GLOBAL general_log = 'OFF';
    ALTER TABLE mysql.general_log ENGINE = MyISAM;
    SET GLOBAL general_log = @old_log_state;
  • TRUNCATE TABLE is a valid operation on a log table. It can be used to expire log entries.

  • RENAME TABLE is a valid operation on a log table. You can atomically rename a log table (to perform log rotation, for example) using the following strategy:

    USE mysql;
    DROP TABLE IF EXISTS general_log2;
    CREATE TABLE general_log2 LIKE general_log;
    RENAME TABLE general_log TO general_log_backup, general_log2 TO general_log;
  • CHECK TABLE is a valid operation on a log table.

  • LOCK TABLES cannot be used on a log table.

  • INSERT, DELETE, and UPDATE cannot be used on a log table. These operations are permitted only internally to the server itself.

  • FLUSH TABLES WITH READ LOCK and the state of the read_only system variable have no effect on log tables. The server can always write to the log tables.

  • Entries written to the log tables are not written to the binary log and thus are not replicated to slave servers.

  • To flush the log tables or log files, use FLUSH TABLES or FLUSH LOGS, respectively.

  • Partitioning of log tables is not permitted.

  • A mysqldump dump includes statements to recreate those tables so that they are not missing after reloading the dump file. Log table contents are not dumped.

5.4.2 The Error Log

This section discusses how to configure the MySQL server for logging of diagnostic messages to the error log. For information about selecting the error message character set or language, see Section 10.6, “Error Message Character Set”, or Section 10.12, “Setting the Error Message Language”.

The error log contains a record of mysqld startup and shutdown times. It also contains diagnostic messages such as errors, warnings, and notes that occur during server startup and shutdown, and while the server is running. For example, if mysqld notices that a table needs to be automatically checked or repaired, it writes a message to the error log.

On some operating systems, the error log contains a stack trace if mysqld exits abnormally. The trace can be used to determine where mysqld exited. See Section 28.5, “Debugging and Porting MySQL”.

If used to start mysqld, mysqld_safe may write messages to the error log. For example, when mysqld_safe notices abnormal mysqld exits, it restarts mysqld and writes a mysqld restarted message to the error log.

The following sections discuss aspects of configuring error logging. In the discussion, console means stderr, the standard error output. This is your terminal or console window unless the standard error output has been redirected to a different destination.

The server interprets options that determine where to write error messages somewhat differently for Windows and Unix systems. Be sure to configure error logging using the information appropriate to your platform.

5.4.2.1 Error Logging on Windows

On Windows, mysqld uses the --log-error, --pid-file, and --console options to determine whether mysqld writes the error log to the console or a file, and, if to a file, the file name:

  • If --console is given, mysqld writes the error log to the console. (--console takes precedence over --log-error if both are given, and the following items regarding --log-error do not apply. Prior to MySQL 5.7, this is reversed: --log-error takes precedence over --console.)

  • If --log-error is not given, or is given without naming a file, mysqld writes the error log to a file named host_name.err in the data directory, unless the --pid-file option is specified. In that case, the file name is the PID file base name with a suffix of .err in the data directory.

  • If --log-error is given to name a file, mysqld writes the error log to that file (with an .err suffix added if the name has no suffix), located under the data directory unless an absolute path name is given to specify a different location.

If the server writes the error log to the console, it sets the log_error system variable to stderr. Otherwise, the server writes the error log to a file and sets log_error to the file name.

In addition, the server by default writes events and error messages to the Windows Event Log within the Application log:

  • Entries marked as Error, Warning, and Note are written to the Event Log, but not messages such as information statements from individual storage engines.

  • Event Log entries have a source of MySQL.

  • Information written to the Event Log is controlled using the log_syslog system variable, which on Windows is enabled by default. See Section 5.4.2.3, “Error Logging to the System Log”.

5.4.2.2 Error Logging on Unix and Unix-Like Systems

On Unix and Unix-like systems, mysqld uses the --log-error option to determine whether mysqld writes the error log to the console or a file, and, if to a file, the file name:

  • If --log-error is not given, mysqld writes the error log to the console.

  • If --log-error is given without naming a file, mysqld writes the error log to a file named host_name.err in the data directory.

  • If --log-error is given to name a file, mysqld writes the error log to that file (with an .err suffix added if the name has no suffix), located under the data directory unless an absolute path name is given to specify a different location.

  • If --log-error is given in an option file in a [mysqld], [server], or [mysqld_safe] section, mysqld_safe finds and uses the option, and passes it to mysqld.

Note

It is common for Yum or APT package installations to configure an error log file location under /var/log with an option like log-error=/var/log/mysqld.log in a server configuration file. Removing the file name from the option causes the host_name.err file in the data directory to be used.

If the server writes the error log to the console, it sets the log_error system variable to stderr. Otherwise, the server writes the error log to a file and sets log_error to the file name.

5.4.2.3 Error Logging to the System Log

It is possible to have mysqld write the error log to the system log (the Event Log on Windows, and syslog on Unix and Unix-like systems). To do so, use these system variables:

  • log_syslog: Enable this variable to send the error log to the system log. (On Windows, log_syslog is enabled by default.)

    If log_syslog is enabled, the following system variables can also be used for finer control.

  • log_syslog_facility: The default facility for syslog messages is daemon. Set this variable to specify a different facility.

  • log_syslog_include_pid: Whether to include the server process ID in each line of syslog output.

  • log_syslog_tag: This variable defines a tag to add to the server identifier (mysqld) in syslog messages. If defined, the tag is appended to the identifier with a leading hyphen.

Note

Error logging to the system log may require additional system configuration. Consult the system log documentation for your platform.

On Unix and Unix-like systems, control of output to syslog is also available using mysqld_safe, which can capture server error output and pass it to syslog.

Note

Using mysqld_safe for syslog error logging is deprecated; you should use the server system variables instead.

mysqld_safe has three error-logging options, --syslog, --skip-syslog, and --log-error. The default with no logging options or with --skip-syslog is to use the default log file. To explicitly specify use of an error log file, specify --log-error=file_name to mysqld_safe, which then arranges for mysqld to write messages to a log file. To use syslog, specify the --syslog option. For syslog output, a tag can be specified with --syslog-tag=tag_val; this is appended to the mysqld server identifier with a leading hyphen.

5.4.2.4 Error Log Filtering

The log_error_verbosity system variable controls server verbosity for writing error, warning, and note messages to the error log. Permitted values are 1 (errors only), 2 (errors and warnings), 3 (errors, warnings, and notes), with a default of 3. If the value is greater than 2, the server logs aborted connections and access-denied errors for new connection attempts. See Section B.4.2.10, “Communication Errors and Aborted Connections”.

5.4.2.5 Error Log Message Format

The ID included in error log messages is that of the thread within mysqld responsible for writing the message. This indicates which part of the server produced the message, and is consistent with general query log and slow query log messages, which include the connection thread ID.

The log_timestamps system variable controls the time zone of timestamps in messages written to the error log (as well as to general query log and slow query log files). Permitted values are UTC (the default) and SYSTEM (local system time zone).

5.4.2.6 Error Log File Flushing and Renaming

If you flush the error log using FLUSH ERROR LOGS, FLUSH LOGS, or mysqladmin flush-logs, the server closes and reopens any error log file to which it is writing. To rename an error log file, do so manually before flushing. Flushing the logs then opens a new file with the original file name. For example, assuming a log file name of host_name.err, to rename the file and create a new one, use the following commands:

mv host_name.err host_name.err-old
mysqladmin flush-logs
mv host_name.err-old backup-directory

On Windows, use rename rather than mv.

If the location of the error log file is not writable by the server, the log-flushing operation fails to create a new log file. For example, on Linux, the server might write the error log to the /var/log/mysqld.log file, where the /var/log directory is owned by root and is not writable by mysqld. For information about handling this case, see Section 5.4.7, “Server Log Maintenance”.

If the server is not writing to a named error log file, no error log file renaming occurs when the error log is flushed.

5.4.3 The General Query Log

The general query log is a general record of what mysqld is doing. The server writes information to this log when clients connect or disconnect, and it logs each SQL statement received from clients. The general query log can be very useful when you suspect an error in a client and want to know exactly what the client sent to mysqld.

Each line that shows when a client connects also includes using connection_type to indicate the protocol used to establish the connection. connection_type is one of TCP/IP (TCP/IP connection established without SSL), SSL/TLS (TCP/IP connection established with SSL), Socket (Unix socket file connection), Named Pipe (Windows named pipe connection), or Shared Memory (Windows shared memory connection).

mysqld writes statements to the query log in the order that it receives them, which might differ from the order in which they are executed. This logging order is in contrast with that of the binary log, for which statements are written after they are executed but before any locks are released. In addition, the query log may contain statements that only select data while such statements are never written to the binary log.

When using statement-based binary logging on a replication master server, statements received by its slaves are written to the query log of each slave. Statements are written to the query log of the master server if a client reads events with the mysqlbinlog utility and passes them to the server.

However, when using row-based binary logging, updates are sent as row changes rather than SQL statements, and thus these statements are never written to the query log when binlog_format is ROW. A given update also might not be written to the query log when this variable is set to MIXED, depending on the statement used. See Section 16.2.1.1, “Advantages and Disadvantages of Statement-Based and Row-Based Replication”, for more information.

By default, the general query log is disabled. To specify the initial general query log state explicitly, use --general_log[={0|1}]. With no argument or an argument of 1, --general_log enables the log. With an argument of 0, this option disables the log. To specify a log file name, use --general_log_file=file_name. To specify the log destination, use the log_output system variable (as described in Section 5.4.1, “Selecting General Query Log and Slow Query Log Output Destinations”).

Note

If you specify the TABLE log destination, see Log Tables and Too many open files Errors.

If you specify no name for the general query log file, the default name is host_name.log. The server creates the file in the data directory unless an absolute path name is given to specify a different directory.

To disable or enable the general query log or change the log file name at runtime, use the global general_log and general_log_file system variables. Set general_log to 0 (or OFF) to disable the log or to 1 (or ON) to enable it. Set general_log_file to specify the name of the log file. If a log file already is open, it is closed and the new file is opened.

When the general query log is enabled, the server writes output to any destinations specified by the log_output system variable. If you enable the log, the server opens the log file and writes startup messages to it. However, further logging of queries to the file does not occur unless the FILE log destination is selected. If the destination is NONE, the server writes no queries even if the general log is enabled. Setting the log file name has no effect on logging if the log destination value does not contain FILE.

Server restarts and log flushing do not cause a new general query log file to be generated (although flushing closes and reopens it). To rename the file and create a new one, use the following commands:

shell> mv host_name.log host_name-old.log
shell> mysqladmin flush-logs
shell> mv host_name-old.log backup-directory

On Windows, use rename rather than mv.

You can also rename the general query log file at runtime by disabling the log:

SET GLOBAL general_log = 'OFF';

With the log disabled, rename the log file externally (for example, from the command line). Then enable the log again:

SET GLOBAL general_log = 'ON';

This method works on any platform and does not require a server restart.

To disable or enable general query logging for the current session, set the session sql_log_off variable to ON or OFF. (This assumes that the general query log itself is enabled.)

Passwords in statements written to the general query log are rewritten by the server not to occur literally in plain text. Password rewriting can be suppressed for the general query log by starting the server with the --log-raw option. This option may be useful for diagnostic purposes, to see the exact text of statements as received by the server, but for security reasons is not recommended for production use. See also Section 6.1.2.3, “Passwords and Logging”.

An implication of password rewriting is that statements that cannot be parsed (due, for example, to syntax errors) are not written to the general query log because they cannot be known to be password free. Use cases that require logging of all statements including those with errors should use the --log-raw option, bearing in mind that this also bypasses password rewriting.

Password rewriting occurs only when plain text passwords are expected. For statements with syntax that expect a password hash value, no rewriting occurs. If a plain text password is supplied erroneously for such syntax, the password is logged as given, without rewriting. For example, the following statement is logged as shown because a password hash value is expected:

CREATE USER 'user1'@'localhost' IDENTIFIED BY PASSWORD 'not-so-secret';

The log_timestamps system variable controls the time zone of timestamps in messages written to the general query log file (as well as to the slow query log file and the error log). It does not affect the time zone of general query log and slow query log messages written to log tables, but rows retrieved from those tables can be converted from the local system time zone to any desired time zone with CONVERT_TZ() or by setting the session time_zone system variable.

5.4.4 The Binary Log

The binary log contains events that describe database changes such as table creation operations or changes to table data. It also contains events for statements that potentially could have made changes (for example, a DELETE which matched no rows), unless row-based logging is used. The binary log also contains information about how long each statement took that updated data. The binary log has two important purposes:

  • For replication, the binary log on a master replication server provides a record of the data changes to be sent to slave servers. The master server sends the events contained in its binary log to its slaves, which execute those events to make the same data changes that were made on the master. See Section 16.2, “Replication Implementation”.

  • Certain data recovery operations require use of the binary log. After a backup has been restored, the events in the binary log that were recorded after the backup was made are re-executed. These events bring databases up to date from the point of the backup. See Section 7.5, “Point-in-Time (Incremental) Recovery Using the Binary Log”.

The binary log is not used for statements such as SELECT or SHOW that do not modify data. To log all statements (for example, to identify a problem query), use the general query log. See Section 5.4.3, “The General Query Log”.

Running a server with binary logging enabled makes performance slightly slower. However, the benefits of the binary log in enabling you to set up replication and for restore operations generally outweigh this minor performance decrement.

The binary log is generally resilient to unexpected halts because only complete transactions are logged or read back. See Section 16.3.2, “Handling an Unexpected Halt of a Replication Slave” for more information.

Passwords in statements written to the binary log are rewritten by the server not to occur literally in plain text. See also Section 6.1.2.3, “Passwords and Logging”.

The following discussion describes some of the server options and variables that affect the operation of binary logging. For a complete list, see Section 16.1.6.4, “Binary Logging Options and Variables”.

To enable the binary log, start the server with the --log-bin[=base_name] option. If no base_name value is given, the default name is the value of the --pid-file option (which by default is the name of host machine) followed by -bin. If the base name is given, the server writes the file in the data directory unless the base name is given with a leading absolute path name to specify a different directory. It is recommended that you specify a base name explicitly rather than using the default of the host name; see Section B.4.7, “Known Issues in MySQL”, for the reason.

If you supply an extension in the log name (for example, --log-bin=base_name.extension), the extension is silently removed and ignored.

mysqld appends a numeric extension to the binary log base name to generate binary log file names. The number increases each time the server creates a new log file, thus creating an ordered series of files. The server creates a new file in the series each time it starts or flushes the logs. The server also creates a new binary log file automatically after the current log's size reaches max_binlog_size. A binary log file may become larger than max_binlog_size if you are using large transactions because a transaction is written to the file in one piece, never split between files.

To keep track of which binary log files have been used, mysqld also creates a binary log index file that contains the names of the binary log files. By default, this has the same base name as the binary log file, with the extension '.index'. You can change the name of the binary log index file with the --log-bin-index[=file_name] option. You should not manually edit this file while mysqld is running; doing so would confuse mysqld.

The term binary log file generally denotes an individual numbered file containing database events. The term binary log collectively denotes the set of numbered binary log files plus the index file.

A client that has privileges sufficient to set restricted session system variables (see Section 5.1.8.1, “System Variable Privileges”) can disable binary logging of its own statements by using a SET sql_log_bin=OFF statement.

By default, the server logs the length of the event as well as the event itself and uses this to verify that the event was written correctly. You can also cause the server to write checksums for the events by setting the binlog_checksum system variable. When reading back from the binary log, the master uses the event length by default, but can be made to use checksums if available by enabling the master_verify_checksum system variable. The slave I/O thread also verifies events received from the master. You can cause the slave SQL thread to use checksums if available when reading from the relay log by enabling the slave_sql_verify_checksum system variable.

The format of the events recorded in the binary log is dependent on the binary logging format. Three format types are supported, row-based logging, statement-based logging and mixed-base logging. The binary logging format used depends on the MySQL version. For general descriptions of the logging formats, see Section 5.4.4.1, “Binary Logging Formats”. For detailed information about the format of the binary log, see MySQL Internals: The Binary Log.

The server evaluates the --binlog-do-db and --binlog-ignore-db options in the same way as it does the --replicate-do-db and --replicate-ignore-db options. For information about how this is done, see Section 16.2.5.1, “Evaluation of Database-Level Replication and Binary Logging Options”.

A replication slave server by default does not write to its own binary log any data modifications that are received from the replication master. To log these modifications, start the slave with the --log-slave-updates option in addition to the --log-bin option (see Section 16.1.6.3, “Replication Slave Options and Variables”). This is done when a slave is also to act as a master to other slaves in chained replication.

You can delete all binary log files with the RESET MASTER statement, or a subset of them with PURGE BINARY LOGS. See Section 13.7.6.6, “RESET Statement”, and Section 13.4.1.1, “PURGE BINARY LOGS Statement”.

If you are using replication, you should not delete old binary log files on the master until you are sure that no slave still needs to use them. For example, if your slaves never run more than three days behind, once a day you can execute mysqladmin flush-logs on the master and then remove any logs that are more than three days old. You can remove the files manually, but it is preferable to use PURGE BINARY LOGS, which also safely updates the binary log index file for you (and which can take a date argument). See Section 13.4.1.1, “PURGE BINARY LOGS Statement”.

You can display the contents of binary log files with the mysqlbinlog utility. This can be useful when you want to reprocess statements in the log for a recovery operation. For example, you can update a MySQL server from the binary log as follows:

shell> mysqlbinlog log_file | mysql -h server_name

mysqlbinlog also can be used to display replication slave relay log file contents because they are written using the same format as binary log files. For more information on the mysqlbinlog utility and how to use it, see Section 4.6.7, “mysqlbinlog — Utility for Processing Binary Log Files”. For more information about the binary log and recovery operations, see Section 7.5, “Point-in-Time (Incremental) Recovery Using the Binary Log”.

Binary logging is done immediately after a statement or transaction completes but before any locks are released or any commit is done. This ensures that the log is logged in commit order.

Updates to nontransactional tables are stored in the binary log immediately after execution.

Within an uncommitted transaction, all updates (UPDATE, DELETE, or INSERT) that change transactional tables such as InnoDB tables are cached until a COMMIT statement is received by the server. At that point, mysqld writes the entire transaction to the binary log before the COMMIT is executed.

Modifications to nontransactional tables cannot be rolled back. If a transaction that is rolled back includes modifications to nontransactional tables, the entire transaction is logged with a ROLLBACK statement at the end to ensure that the modifications to those tables are replicated.

When a thread that handles the transaction starts, it allocates a buffer of binlog_cache_size to buffer statements. If a statement is bigger than this, the thread opens a temporary file to store the transaction. The temporary file is deleted when the thread ends.

The Binlog_cache_use status variable shows the number of transactions that used this buffer (and possibly a temporary file) for storing statements. The Binlog_cache_disk_use status variable shows how many of those transactions actually had to use a temporary file. These two variables can be used for tuning binlog_cache_size to a large enough value that avoids the use of temporary files.

The max_binlog_cache_size system variable (default 4GB, which is also the maximum) can be used to restrict the total size used to cache a multiple-statement transaction. If a transaction is larger than this many bytes, it fails and rolls back. The minimum value is 4096.

If you are using the binary log and row based logging, concurrent inserts are converted to normal inserts for CREATE ... SELECT or INSERT ... SELECT statements. This is done to ensure that you can re-create an exact copy of your tables by applying the log during a backup operation. If you are using statement-based logging, the original statement is written to the log.

The binary log format has some known limitations that can affect recovery from backups. See Section 16.4.1, “Replication Features and Issues”.

Binary logging for stored programs is done as described in Section 23.7, “Stored Program Binary Logging”.

Note that the binary log format differs in MySQL 5.7 from previous versions of MySQL, due to enhancements in replication. See Section 16.4.2, “Replication Compatibility Between MySQL Versions”.

If the server is unable to write to the binary log, flush binary log files, or synchronize the binary log to disk, the binary log on the replication master can become inconsistent and replication slaves can lose synchronization with the master. The binlog_error_action system variable controls the action taken if an error of this type is encountered with the binary log.

  • The default setting, ABORT_SERVER, makes the server halt binary logging and shut down. At this point, you can identify and correct the cause of the error. On restart, recovery proceeds as in the case of an unexpected server halt (see Section 16.3.2, “Handling an Unexpected Halt of a Replication Slave”).

  • The setting IGNORE_ERROR provides backward compatibility with older versions of MySQL. With this setting, the server continues the ongoing transaction and logs the error, then halts binary logging, but continues to perform updates. At this point, you can identify and correct the cause of the error. To resume binary logging, log_bin must be enabled again, which requires a server restart. Only use this option if you require backward compatibility, and the binary log is non-essential on this MySQL server instance. For example, you might use the binary log only for intermittent auditing or debugging of the server, and not use it for replication from the server or rely on it for point-in-time restore operations.

By default, the binary log is synchronized to disk at each write (sync_binlog=1). If sync_binlog was not enabled, and the operating system or machine (not only the MySQL server) crashed, there is a chance that the last statements of the binary log could be lost. To prevent this, enable the sync_binlog system variable to synchronize the binary log to disk after every N commit groups. See Section 5.1.7, “Server System Variables”. The safest value for sync_binlog is 1 (the default), but this is also the slowest.

For example, if you are using InnoDB tables and the MySQL server processes a COMMIT statement, it writes many prepared transactions to the binary log in sequence, synchronizes the binary log, and then commits this transaction into InnoDB. If the server crashes between those two operations, the transaction is rolled back by InnoDB at restart but still exists in the binary log. Such an issue is resolved assuming --innodb_support_xa is set to 1, the default. Although this option is related to the support of XA transactions in InnoDB, it also ensures that the binary log and InnoDB data files are synchronized. For this option to provide a greater degree of safety, the MySQL server should also be configured to synchronize the binary log and the InnoDB logs to disk before committing the transaction. The InnoDB logs are synchronized by default, and sync_binlog=1 can be used to synchronize the binary log. The effect of this option is that at restart after a crash, after doing a rollback of transactions, the MySQL server scans the latest binary log file to collect transaction xid values and calculate the last valid position in the binary log file. The MySQL server then tells InnoDB to complete any prepared transactions that were successfully written to the to the binary log, and truncates the binary log to the last valid position. This ensures that the binary log reflects the exact data of InnoDB tables, and therefore the slave remains in synchrony with the master because it does not receive a statement which has been rolled back.

Note

innodb_support_xa is deprecated and will be removed in a future release. InnoDB support for two-phase commit in XA transactions is always enabled as of MySQL 5.7.10.

If the MySQL server discovers at crash recovery that the binary log is shorter than it should have been, it lacks at least one successfully committed InnoDB transaction. This should not happen if sync_binlog=1 and the disk/file system do an actual sync when they are requested to (some do not), so the server prints an error message The binary log file_name is shorter than its expected size. In this case, this binary log is not correct and replication should be restarted from a fresh snapshot of the master's data.

The session values of the following system variables are written to the binary log and honored by the replication slave when parsing the binary log:

5.4.4.1 Binary Logging Formats

The server uses several logging formats to record information in the binary log. The exact format employed depends on the version of MySQL being used. There are three logging formats:

  • Replication capabilities in MySQL originally were based on propagation of SQL statements from master to slave. This is called statement-based logging. You can cause this format to be used by starting the server with --binlog-format=STATEMENT.

  • In row-based logging, the master writes events to the binary log that indicate how individual table rows are affected. It is important therefore that tables always use a primary key to ensure rows can be efficiently identified. You can cause the server to use row-based logging by starting it with --binlog-format=ROW.

  • A third option is also available: mixed logging. With mixed logging, statement-based logging is used by default, but the logging mode switches automatically to row-based in certain cases as described below. You can cause MySQL to use mixed logging explicitly by starting mysqld with the option --binlog-format=MIXED.

The logging format can also be set or limited by the storage engine being used. This helps to eliminate issues when replicating certain statements between a master and slave which are using different storage engines.

With statement-based replication, there may be issues with replicating nondeterministic statements. In deciding whether or not a given statement is safe for statement-based replication, MySQL determines whether it can guarantee that the statement can be replicated using statement-based logging. If MySQL cannot make this guarantee, it marks the statement as potentially unreliable and issues the warning, Statement may not be safe to log in statement format.

You can avoid these issues by using MySQL's row-based replication instead.

5.4.4.2 Setting The Binary Log Format

You can select the binary logging format explicitly by starting the MySQL server with --binlog-format=type. The supported values for type are:

  • STATEMENT causes logging to be statement based.

  • ROW causes logging to be row based.

  • MIXED causes logging to use mixed format.

The logging format also can be switched at runtime, although note that there are a number of situations in which you cannot do this, as discussed later in this section. Set the global value of the binlog_format system variable to specify the format for clients that connect subsequent to the change:

mysql> SET GLOBAL binlog_format = 'STATEMENT';
mysql> SET GLOBAL binlog_format = 'ROW';
mysql> SET GLOBAL binlog_format = 'MIXED';

An individual client can control the logging format for its own statements by setting the session value of binlog_format:

mysql> SET SESSION binlog_format = 'STATEMENT';
mysql> SET SESSION binlog_format = 'ROW';
mysql> SET SESSION binlog_format = 'MIXED';

Changing the global binlog_format value requires privileges sufficient to set global system variables. Changing the session binlog_format value requires privileges sufficient to set restricted session system variables. See Section 5.1.8.1, “System Variable Privileges”.

There are several reasons why a client might want to set binary logging on a per-session basis:

  • A session that makes many small changes to the database might want to use row-based logging.

  • A session that performs updates that match many rows in the WHERE clause might want to use statement-based logging because it will be more efficient to log a few statements than many rows.

  • Some statements require a lot of execution time on the master, but result in just a few rows being modified. It might therefore be beneficial to replicate them using row-based logging.

There are exceptions when you cannot switch the replication format at runtime:

  • From within a stored function or a trigger.

  • If the NDB storage engine is enabled.

  • If the session is currently in row-based replication mode and has open temporary tables.

Trying to switch the format in any of these cases results in an error.

Switching the replication format at runtime is not recommended when any temporary tables exist, because temporary tables are logged only when using statement-based replication, whereas with row-based replication they are not logged. With mixed replication, temporary tables are usually logged; exceptions happen with user-defined functions (UDFs) and with the UUID() function.

Switching the replication format while replication is ongoing can also cause issues. Each MySQL Server can set its own and only its own binary logging format (true whether binlog_format is set with global or session scope). This means that changing the logging format on a replication master does not cause a slave to change its logging format to match. When using STATEMENT mode, the binlog_format system variable is not replicated. When using MIXED or ROW logging mode, it is replicated but is ignored by the slave.

A replication slave is not able to convert binary log entries received in ROW logging format to STATEMENT format for use in its own binary log. The slave must therefore use ROW or MIXED format if the master does. Changing the binary logging format on the master from STATEMENT to ROW or MIXED while replication is ongoing to a slave with STATEMENT format can cause replication to fail with errors such as Error executing row event: 'Cannot execute statement: impossible to write to binary log since statement is in row format and BINLOG_FORMAT = STATEMENT.' Changing the binary logging format on the slave to STATEMENT format when the master is still using MIXED or ROW format also causes the same type of replication failure. To change the format safely, you must stop replication and ensure that the same change is made on both the master and the slave.

If you are using InnoDB tables and the transaction isolation level is READ COMMITTED or READ UNCOMMITTED, only row-based logging can be used. It is possible to change the logging format to STATEMENT, but doing so at runtime leads very rapidly to errors because InnoDB can no longer perform inserts.

With the binary log format set to ROW, many changes are written to the binary log using the row-based format. Some changes, however, still use the statement-based format. Examples include all DDL (data definition language) statements such as CREATE TABLE, ALTER TABLE, or DROP TABLE.

The --binlog-row-event-max-size option is available for servers that are capable of row-based replication. Rows are stored into the binary log in chunks having a size in bytes not exceeding the value of this option. The value must be a multiple of 256. The default value is 8192.

Warning

When using statement-based logging for replication, it is possible for the data on the master and slave to become different if a statement is designed in such a way that the data modification is nondeterministic; that is, it is left to the will of the query optimizer. In general, this is not a good practice even outside of replication. For a detailed explanation of this issue, see Section B.4.7, “Known Issues in MySQL”.

For information about logs kept by replication slaves, see Section 16.2.4, “Replication Relay and Status Logs”.

5.4.4.3 Mixed Binary Logging Format

When running in MIXED logging format, the server automatically switches from statement-based to row-based logging under the following conditions:

Note

A warning is generated if you try to execute a statement using statement-based logging that should be written using row-based logging. The warning is shown both in the client (in the output of SHOW WARNINGS) and through the mysqld error log. A warning is added to the SHOW WARNINGS table each time such a statement is executed. However, only the first statement that generated the warning for each client session is written to the error log to prevent flooding the log.

In addition to the decisions above, individual engines can also determine the logging format used when information in a table is updated. The logging capabilities of an individual engine can be defined as follows:

  • If an engine supports row-based logging, the engine is said to be row-logging capable.

  • If an engine supports statement-based logging, the engine is said to be statement-logging capable.

A given storage engine can support either or both logging formats. The following table lists the formats supported by each engine.

Storage Engine Row Logging Supported Statement Logging Supported
ARCHIVE Yes Yes
BLACKHOLE Yes Yes
CSV Yes Yes
EXAMPLE Yes No
FEDERATED Yes Yes
HEAP Yes Yes
InnoDB Yes Yes when the transaction isolation level is REPEATABLE READ or SERIALIZABLE; No otherwise.
MyISAM Yes Yes
MERGE Yes Yes
NDB Yes No

Whether a statement is to be logged and the logging mode to be used is determined according to the type of statement (safe, unsafe, or binary injected), the binary logging format (STATEMENT, ROW, or MIXED), and the logging capabilities of the storage engine (statement capable, row capable, both, or neither). (Binary injection refers to logging a change that must be logged using ROW format.)

Statements may be logged with or without a warning; failed statements are not logged, but generate errors in the log. This is shown in the following decision table. Type, binlog_format, SLC, and RLC columns outline the conditions, and Error / Warning and Logged as columns represent the corresponding actions. SLC stands for statement-logging capable, and RLC stands for row-logging capable.

Type binlog_format SLC RLC Error / Warning Logged as
* * No No Error: Cannot execute statement: Binary logging is impossible since at least one engine is involved that is both row-incapable and statement-incapable. -
Safe STATEMENT Yes No - STATEMENT
Safe MIXED Yes No - STATEMENT
Safe ROW Yes No Error: Cannot execute statement: Binary logging is impossible since BINLOG_FORMAT = ROW and at least one table uses a storage engine that is not capable of row-based logging. -
Unsafe STATEMENT Yes No Warning: Unsafe statement binlogged in statement format, since BINLOG_FORMAT = STATEMENT STATEMENT
Unsafe MIXED Yes No Error: Cannot execute statement: Binary logging of an unsafe statement is impossible when the storage engine is limited to statement-based logging, even if BINLOG_FORMAT = MIXED. -
Unsafe ROW Yes No Error: Cannot execute statement: Binary logging is impossible since BINLOG_FORMAT = ROW and at least one table uses a storage engine that is not capable of row-based logging. -
Row Injection STATEMENT Yes No Error: Cannot execute row injection: Binary logging is not possible since at least one table uses a storage engine that is not capable of row-based logging. -
Row Injection MIXED Yes No Error: Cannot execute row injection: Binary logging is not possible since at least one table uses a storage engine that is not capable of row-based logging. -
Row Injection ROW Yes No Error: Cannot execute row injection: Binary logging is not possible since at least one table uses a storage engine that is not capable of row-based logging. -
Safe STATEMENT No Yes Error: Cannot execute statement: Binary logging is impossible since BINLOG_FORMAT = STATEMENT and at least one table uses a storage engine that is not capable of statement-based logging. -
Safe MIXED No Yes - ROW
Safe ROW No Yes - ROW
Unsafe STATEMENT No Yes Error: Cannot execute statement: Binary logging is impossible since BINLOG_FORMAT = STATEMENT and at least one table uses a storage engine that is not capable of statement-based logging. -
Unsafe MIXED No Yes - ROW
Unsafe ROW No Yes - ROW
Row Injection STATEMENT No Yes Error: Cannot execute row injection: Binary logging is not possible since BINLOG_FORMAT = STATEMENT. -
Row Injection MIXED No Yes - ROW
Row Injection ROW No Yes - ROW
Safe STATEMENT Yes Yes - STATEMENT
Safe MIXED Yes Yes - STATEMENT
Safe ROW Yes Yes - ROW
Unsafe STATEMENT Yes Yes Warning: Unsafe statement binlogged in statement format since BINLOG_FORMAT = STATEMENT. STATEMENT
Unsafe MIXED Yes Yes - ROW
Unsafe ROW Yes Yes - ROW
Row Injection STATEMENT Yes Yes Error: Cannot execute row injection: Binary logging is not possible because BINLOG_FORMAT = STATEMENT. -
Row Injection MIXED Yes Yes - ROW
Row Injection ROW Yes Yes - ROW

When a warning is produced by the determination, a standard MySQL warning is produced (and is available using SHOW WARNINGS). The information is also written to the mysqld error log. Only one error for each error instance per client connection is logged to prevent flooding the log. The log message includes the SQL statement that was attempted.

If log_error_verbosity is 2 or greater on a slave server, the slave prints messages to the error log to provide information about its status, such as the binary log and relay log coordinates where it starts its job, when it is switching to another relay log, when it reconnects after a disconnect, statements that are unsafe for statement-based logging, and so forth.

5.4.4.4 Logging Format for Changes to mysql Database Tables

The contents of the grant tables in the mysql database can be modified directly (for example, with INSERT or DELETE) or indirectly (for example, with GRANT or CREATE USER). Statements that affect mysql database tables are written to the binary log using the following rules:

CREATE TABLE ... SELECT is a combination of data definition and data manipulation. The CREATE TABLE part is logged using statement format and the SELECT part is logged according to the value of binlog_format.

5.4.5 The Slow Query Log

The slow query log consists of SQL statements that take more than long_query_time seconds to execute and require at least min_examined_row_limit rows to be examined. The slow query log can be used to find queries that take a long time to execute and are therefore candidates for optimization. However, examining a long slow query log can be a time-consuming task. To make this easier, you can use the mysqldumpslow command to process a slow query log file and summarize its contents. See Section 4.6.8, “mysqldumpslow — Summarize Slow Query Log Files”.

The time to acquire the initial locks is not counted as execution time. mysqld writes a statement to the slow query log after it has been executed and after all locks have been released, so log order might differ from execution order.

Slow Query Log Parameters

The minimum and default values of long_query_time are 0 and 10, respectively. The value can be specified to a resolution of microseconds.

By default, administrative statements are not logged, nor are queries that do not use indexes for lookups. This behavior can be changed using log_slow_admin_statements and log_queries_not_using_indexes, as described later.

By default, the slow query log is disabled. To specify the initial slow query log state explicitly, use --slow_query_log[={0|1}]. With no argument or an argument of 1, --slow_query_log enables the log. With an argument of 0, this option disables the log. To specify a log file name, use --slow_query_log_file=file_name. To specify the log destination, use the log_output system variable (as described in Section 5.4.1, “Selecting General Query Log and Slow Query Log Output Destinations”).

Note

If you specify the TABLE log destination, see Log Tables and Too many open files Errors.

If you specify no name for the slow query log file, the default name is host_name-slow.log. The server creates the file in the data directory unless an absolute path name is given to specify a different directory.

To disable or enable the slow query log or change the log file name at runtime, use the global slow_query_log and slow_query_log_file system variables. Set slow_query_log to 0 to disable the log or to 1 to enable it. Set slow_query_log_file to specify the name of the log file. If a log file already is open, it is closed and the new file is opened.

The server writes less information to the slow query log if you use the --log-short-format option.

To include slow administrative statements in the slow query log, enable the log_slow_admin_statements system variable. Administrative statements include ALTER TABLE, ANALYZE TABLE, CHECK TABLE, CREATE INDEX, DROP INDEX, OPTIMIZE TABLE, and REPAIR TABLE.

To include queries that do not use indexes for row lookups in the statements written to the slow query log, enable the log_queries_not_using_indexes system variable. (Even with that variable enabled, the server does not log queries that would not benefit from the presence of an index due to the table having fewer than two rows.)

When queries that do not use an index are logged, the slow query log may grow quickly. It is possible to put a rate limit on these queries by setting the log_throttle_queries_not_using_indexes system variable. By default, this variable is 0, which means there is no limit. Positive values impose a per-minute limit on logging of queries that do not use indexes. The first such query opens a 60-second window within which the server logs queries up to the given limit, then suppresses additional queries. If there are suppressed queries when the window ends, the server logs a summary that indicates how many there were and the aggregate time spent in them. The next 60-second window begins when the server logs the next query that does not use indexes.

The server uses the controlling parameters in the following order to determine whether to write a query to the slow query log:

  1. The query must either not be an administrative statement, or log_slow_admin_statements must be enabled.

  2. The query must have taken at least long_query_time seconds, or log_queries_not_using_indexes must be enabled and the query used no indexes for row lookups.

  3. The query must have examined at least min_examined_row_limit rows.

  4. The query must not be suppressed according to the log_throttle_queries_not_using_indexes setting.

The log_timestamps system variable controls the time zone of timestamps in messages written to the slow query log file (as well as to the general query log file and the error log). It does not affect the time zone of general query log and slow query log messages written to log tables, but rows retrieved from those tables can be converted from the local system time zone to any desired time zone with CONVERT_TZ() or by setting the session time_zone system variable.

The server does not log queries handled by the query cache.

By default, a replication slave does not write replicated queries to the slow query log. To change this, enable the log_slow_slave_statements system variable. Note that if row-based replication is in use (binlog_format=ROW), log_slow_slave_statements has no effect. Queries are only added to the slave's slow query log when they are logged in statement format in the binary log, that is, when binlog_format=STATEMENT is set, or when binlog_format=MIXED is set and the statement is logged in statement format. Slow queries that are logged in row format when binlog_format=MIXED is set, or that are logged when binlog_format=ROW is set, are not added to the slave's slow query log, even if log_slow_slave_statements is enabled.

Slow Query Log Contents

When the slow query log is enabled, the server writes output to any destinations specified by the log_output system variable. If you enable the log, the server opens the log file and writes startup messages to it. However, further logging of queries to the file does not occur unless the FILE log destination is selected. If the destination is NONE, the server writes no queries even if the slow query log is enabled. Setting the log file name has no effect on logging if FILE is not selected as an output destination.

If the slow query log is enabled and FILE is selected as an output destination, each statement written to the log is preceded by a line that begins with a # character and has these fields (with all fields on a single line):

  • Query_time: duration

    The statement execution time in seconds.

  • Lock_time: duration

    The time to acquire locks in seconds.

  • Rows_sent: N

    The number of rows sent to the client.

  • Rows_examined:

    The number of rows examined by the server layer (not counting any processing internal to storage engines).

Each statement written to the slow query log file is preceded by a SET statement that includes a timestamp indicating when the slow statement was logged (which occurs after the statement finishes executing).

Passwords in statements written to the slow query log are rewritten by the server not to occur literally in plain text. See Section 6.1.2.3, “Passwords and Logging”.

5.4.6 The DDL Log

The DDL log, or metadata log, records metadata operations generated by data definition statements such as DROP TABLE and ALTER TABLE. MySQL uses this log to recover from crashes occurring in the middle of a metadata operation. When executing the statement DROP TABLE t1, t2, we need to ensure that both t1 and t2 are dropped, and that each table drop is complete. Another example of this type of SQL statement is ALTER TABLE t3 DROP PARTITION p2, where we must make certain that the partition is completely dropped and that its definition is removed from the list of partitions for table t3.

A record of metadata operations such as those just described are written to the file ddl_log.log, in the MySQL data directory. This is a binary file; it is not intended to be human-readable, and you should not attempt to modify its contents in any way.

ddl_log.log is not created until it is actually needed for recording metadata statements, and is removed following a successful start of mysqld. Thus, it is possible for this file not to be present on a MySQL server that is functioning in a completely normal manner.

Currently, ddl_log.log can hold up to 1048573 entries, equivalent to 4 GB in size. Once this limit is exceeded, you must rename or remove the file before it is possible to execute any additional DDL statements. This is a known issue which we are working to resolve (Bug #83708).

There are no user-configurable server options or variables associated with this file.

5.4.7 Server Log Maintenance

As described in Section 5.4, “MySQL Server Logs”, MySQL Server can create several different log files to help you see what activity is taking place. However, you must clean up these files regularly to ensure that the logs do not take up too much disk space.

When using MySQL with logging enabled, you may want to back up and remove old log files from time to time and tell MySQL to start logging to new files. See Section 7.2, “Database Backup Methods”.

On a Linux (Red Hat) installation, you can use the mysql-log-rotate script for log maintenance. If you installed MySQL from an RPM distribution, this script should have been installed automatically. Be careful with this script if you are using the binary log for replication. You should not remove binary logs until you are certain that their contents have been processed by all slaves.

On other systems, you must install a short script yourself that you start from cron (or its equivalent) for handling log files.

For the binary log, you can set the expire_logs_days system variable to expire binary log files automatically after a given number of days (see Section 5.1.7, “Server System Variables”). If you are using replication, you should set the variable no lower than the maximum number of days your slaves might lag behind the master. To remove binary logs on demand, use the PURGE BINARY LOGS statement (see Section 13.4.1.1, “PURGE BINARY LOGS Statement”).

To force MySQL to start using new log files, flush the logs. Log flushing occurs when you execute a FLUSH LOGS statement or a mysqladmin flush-logs, mysqladmin refresh, mysqldump --flush-logs, or mysqldump --master-data command. See Section 13.7.6.3, “FLUSH Statement”, Section 4.5.2, “mysqladmin — Client for Administering a MySQL Server”, and Section 4.5.4, “mysqldump — A Database Backup Program”. In addition, the server flushes the binary log automatically when current binary log file size reaches the value of the max_binlog_size system variable.

FLUSH LOGS supports optional modifiers to enable selective flushing of individual logs (for example, FLUSH BINARY LOGS). See Section 13.7.6.3, “FLUSH Statement”.

A log-flushing operation has the following effects:

  • If binary logging is enabled, the server closes the current binary log file and opens a new log file with the next sequence number.

  • If general query logging or slow query logging to a log file is enabled, the server closes and reopens the log file.

  • If the server was started with the --log-error option to cause the error log to be written to a file, the server closes and reopens the log file.

Execution of log-flushing statements or commands requires connecting to the server using an account that has the RELOAD privilege. On Unix and Unix-like systems, another way to flush the logs is to send a SIGHUP signal to the server, which can be done by root or the account that owns the server process. Signals enable log flushing to be performed without having to connect to the server. However, SIGHUP has additional effects other than log flushing that might be undesirable. For details, see Section 4.10, “Unix Signal Handling in MySQL”.

As mentioned previously, flushing the binary log creates a new binary log file, whereas flushing the general query log, slow query log, or error log just closes and reopens the log file. For the latter logs, to cause a new log file to be created on Unix, rename the current log file first before flushing it. At flush time, the server opens the new log file with the original name. For example, if the general query log, slow query log, and error log files are named mysql.log, mysql-slow.log, and err.log, you can use a series of commands like this from the command line:

cd mysql-data-directory
mv mysql.log mysql.log.old
mv mysql-slow.log mysql-slow.log.old
mv err.log err.log.old
mysqladmin flush-logs

On Windows, use rename rather than mv.

At this point, you can make a backup of mysql.log.old, mysql-slow.log.old, and err.log.old, then remove them from disk.

To rename the general query log or slow query log at runtime, first connect to the server and disable the log:

SET GLOBAL general_log = 'OFF';
SET GLOBAL slow_query_log = 'OFF';

With the logs disabled, rename the log files externally (for example, from the command line). Then enable the logs again:

SET GLOBAL general_log = 'ON';
SET GLOBAL slow_query_log = 'ON';

This method works on any platform and does not require a server restart.

Note

For the server to recreate a given log file after you have renamed the file externally, the file location must be writable by the server. This may not always be the case. For example, on Linux, the server might write the error log as /var/log/mysqld.log, where /var/log is owned by root and not writable by mysqld. In this case, log-flushing operations fail to create a new log file.

To handle this situation, you must manually create the new log file with the proper ownership after renaming the original log file. For example, execute these commands as root:

mv /var/log/mysqld.log /var/log/mysqld.log.old
install -omysql -gmysql -m0644 /dev/null /var/log/mysqld.log

5.5 MySQL Server Plugins

MySQL supports a plugin API that enables creation of server components. Plugins can be loaded at server startup, or loaded and unloaded at runtime without restarting the server. The components supported by this interface include, but are not limited to, storage engines, INFORMATION_SCHEMA tables, full-text parser plugins, partitioning support, and server extensions.

MySQL distributions include several plugins that implement server extensions:

  • Plugins for authenticating attempts by clients to connect to MySQL Server. Plugins are available for several authentication protocols. See Section 6.2.13, “Pluggable Authentication”.

  • A connection-control plugin that enables administrators to introduce an increasing delay after a certain number of consecutive failed client connection attempts. See Section 6.4.2, “The Connection-Control Plugins”.

  • A password-validation plugin implements password strength policies and assesses the strength of potential passwords. See Section 6.4.3, “The Password Validation Plugin”.

  • Semisynchronous replication plugins implement an interface to replication capabilities that permit the master to proceed as long as at least one slave has responded to each transaction. See Section 16.3.9, “Semisynchronous Replication”.

  • Group Replication enables you to create a highly available distributed MySQL service across a group of MySQL server instances, with data consistency, conflict detection and resolution, and group membership services all built-in. See Chapter 17, Group Replication.

  • MySQL Enterprise Edition includes a thread pool plugin that manages connection threads to increase server performance by efficiently managing statement execution threads for large numbers of client connections. See Section 5.5.3, “MySQL Enterprise Thread Pool”.

  • MySQL Enterprise Edition includes an audit plugin for monitoring and logging of connection and query activity. See Section 6.4.5, “MySQL Enterprise Audit”.

  • MySQL Enterprise Edition includes a firewall plugin that implements an application-level firewall to enable database administrators to permit or deny SQL statement execution based on matching against whitelists of accepted statement patterns. See Section 6.4.6, “MySQL Enterprise Firewall”.

  • A query rewrite plugin examines statements received by MySQL Server and possibly rewrites them before the server executes them. See Section 5.5.4, “The Rewriter Query Rewrite Plugin”

  • Version Tokens enables creation of and synchronization around server tokens that applications can use to prevent accessing incorrect or out-of-date data. Version Tokens is based on a plugin library that implements a version_tokens plugin and a set of user-defined functions. See Section 5.5.5, “Version Tokens”.

  • Keyring plugins provide secure storage for sensitive information. See Section 6.4.4, “The MySQL Keyring”.

  • X Plugin extends MySQL Server to be able to function as a document store. Running X Plugin enables MySQL Server to communicate with clients using the X Protocol, which is designed to expose the ACID compliant storage abilities of MySQL as a document store. See Section 19.7, “X Plugin”.

  • Test framework plugins test server services. For information about these plugins, see the Plugins for Testing Plugin Services section of the MySQL Server Doxygen documentation, available at https://dev.mysql.com/doc/index-other.html.

The following sections describe how to install and uninstall plugins, and how to determine at runtime which plugins are installed and obtain information about them. For information about writing plugins, see Section 28.2, “The MySQL Plugin API”.

5.5.1 Installing and Uninstalling Plugins

Server plugins must be loaded into the server before they can be used. MySQL supports plugin loading at server startup and runtime. It is also possible to control the activation state of loaded plugins at startup, and to unload them at runtime.

While a plugin is loaded, information about it is available from the INFORMATION_SCHEMA.PLUGINS table and the SHOW PLUGINS statement. See Section 5.5.2, “Obtaining Server Plugin Information”.

Installing Plugins

Before a server plugin can be used, it must be installed using one of the following methods. In the descriptions, plugin_name stands for a plugin name such as innodb, csv, or validate_password.

Built-in plugins:

A built-in plugin is known by the server automatically. Normally, the server enables the plugin at startup. Some built-in plugins permit this to be changed with the --plugin_name[=activation_state] option.

Plugins registered in the mysql.plugin system table:

The plugin table in the mysql system database serves as a registry of plugins (other than built-in plugins, which need not be registered). At startup, the server loads each plugin listed in the table. Normally, for a plugin loaded from the mysql.plugin table, the server also enables the plugin. This can be changed with the --plugin_name[=activation_state] option.

If the server is started with the --skip-grant-tables option, it does not consult the mysql.plugin table and does not load the plugins listed there.

Plugins named with command-line options:

A plugin located in a plugin library file can be loaded at server startup with the --plugin-load, --plugin-load-add, or (as of MySQL 5.7.11) --early-plugin-load option. Normally, for a plugin loaded at startup, the server also enables the plugin. This can be changed with the --plugin_name[=activation_state] option.

The --plugin-load and --plugin-load-add options load plugins after built-in plugins and storage engines have initialized during the server startup sequence. The --early-plugin-load option is used to load plugins that must be available prior to initialization of built-in plugins and storage engines.

The value of each plugin-loading option is a semicolon-separated list of name=plugin_library and plugin_library values. Each name is the name of a plugin to load, and plugin_library is the name of the library file that contains the plugin code. If a plugin library is named without any preceding plugin name, the server loads all plugins in the library. The server looks for plugin library files in the directory named by the plugin_dir system variable.

Plugin-loading options do not register any plugin in the mysql.plugin table. For subsequent restarts, the server loads the plugin again only if --plugin-load, --plugin-load-add, or --early-plugin-load is given again. That is, the option produces a one-time plugin-installation operation that persists for a single server invocation.

--plugin-load, --plugin-load-add, and --early-plugin-load enable plugins to be loaded even when --skip-grant-tables is given (which causes the server to ignore the mysql.plugin table). --plugin-load, --plugin-load-add, and --early-plugin-load also enable plugins to be loaded at startup that cannot be loaded at runtime.

The --plugin-load-add option complements the --plugin-load option:

For example, these options:

--plugin-load=x --plugin-load-add=y

are equivalent to this option:

--plugin-load="x;y"

But these options:

--plugin-load-add=y --plugin-load=x

are equivalent to this option:

--plugin-load=x

Plugins installed with the INSTALL PLUGIN statement:

A plugin located in a plugin library file can be loaded at runtime with the INSTALL PLUGIN statement. The statement also registers the plugin in the mysql.plugin table to cause the server to load it on subsequent restarts. For this reason, INSTALL PLUGIN requires the INSERT privilege for the mysql.plugin table.

The plugin library file base name depends on your platform. Common suffixes are .so for Unix and Unix-like systems, .dll for Windows.

Example: The --plugin-load option installs a plugin at server startup. To install a plugin named myplugin from a plugin library file named somepluglib.so, use these lines in a my.cnf file:

[mysqld]
plugin-load=myplugin=somepluglib.so

In this case, the plugin is not registered in mysql.plugin. Restarting the server without the --plugin-load option causes the plugin not to be loaded at startup.

Alternatively, the INSTALL PLUGIN statement causes the server to load the plugin code from the library file at runtime:

INSTALL PLUGIN myplugin SONAME 'somepluglib.so';

INSTALL PLUGIN also causes permanent plugin registration: The plugin is listed in the mysql.plugin table to ensure that the server loads it on subsequent restarts.

Many plugins can be loaded either at server startup or at runtime. However, if a plugin is designed such that it must be loaded and initialized during server startup, attempts to load it at runtime using INSTALL PLUGIN produce an error:

mysql> INSTALL PLUGIN myplugin SONAME 'somepluglib.so';
ERROR 1721 (HY000): Plugin 'myplugin' is marked as not dynamically
installable. You have to stop the server to install it.

In this case, you must use --plugin-load, --plugin-load-add, or --early-plugin-load.

If a plugin is named both using a --plugin-load, --plugin-load-add, or --early-plugin-load option and (as a result of an earlier INSTALL PLUGIN statement) in the mysql.plugin table, the server starts but writes these messages to the error log:

[ERROR] Function 'plugin_name' already exists
[Warning] Couldn't load plugin named 'plugin_name'
with soname 'plugin_object_file'.

Controlling Plugin Activation State

If the server knows about a plugin when it starts (for example, because the plugin is named using a --plugin-load option or is registered in the mysql.plugin table), the server loads and enables the plugin by default. It is possible to control activation state for such a plugin using a --plugin_name[=activation_state] startup option, where plugin_name is the name of the plugin to affect, such as innodb, csv, or validate_password. As with other options, dashes and underscores are interchangeable in option names. Also, activation state values are not case-sensitive. For example, --my_plugin=ON and --my-plugin=on are equivalent.

  • --plugin_name=OFF

    Tells the server to disable the plugin. This may not be possible for certain built-in plugins, such as mysql_native_password.

  • --plugin_name[=ON]

    Tells the server to enable the plugin. (Specifying the option as --plugin_name without a value has the same effect.) If the plugin fails to initialize, the server runs with the plugin disabled.

  • --plugin_name=FORCE

    Tells the server to enable the plugin, but if plugin initialization fails, the server does not start. In other words, this option forces the server to run with the plugin enabled or not at all.

  • --plugin_name=FORCE_PLUS_PERMANENT

    Like FORCE, but in addition prevents the plugin from being unloaded at runtime. If a user attempts to do so with UNINSTALL PLUGIN, an error occurs.

Plugin activation states are visible in the LOAD_OPTION column of the INFORMATION_SCHEMA.PLUGINS table.

Suppose that CSV, BLACKHOLE, and ARCHIVE are built-in pluggable storage engines and that you want the server to load them at startup, subject to these conditions: The server is permitted to run if CSV initialization fails, must require that BLACKHOLE initialization succeeds, and should disable ARCHIVE. To accomplish that, use these lines in an option file:

[mysqld]
csv=ON
blackhole=FORCE
archive=OFF

The --enable-plugin_name option format is a synonym for --plugin_name=ON. The --disable-plugin_name and --skip-plugin_name option formats are synonyms for --plugin_name=OFF.

If a plugin is disabled, either explicitly with OFF or implicitly because it was enabled with ON but fails to initialize, aspects of server operation that require the plugin will change. For example, if the plugin implements a storage engine, existing tables for the storage engine become inaccessible, and attempts to create new tables for the storage engine result in tables that use the default storage engine unless the NO_ENGINE_SUBSTITUTION SQL mode is enabled to cause an error to occur instead.

Disabling a plugin may require adjustment to other options. For example, if you start the server using --skip-innodb to disable InnoDB, other innodb_xxx options likely will need to be omitted at startup. In addition, because InnoDB is the default storage engine, it will not start unless you specify another available storage engine with --default_storage_engine. You must also set --default_tmp_storage_engine.

Uninstalling Plugins

At runtime, the UNINSTALL PLUGIN statement disables and uninstalls a plugin known to the server. The statement unloads the plugin and removes it from the mysql.plugin system table, if it is registered there. For this reason, UNINSTALL PLUGIN statement requires the DELETE privilege for the mysql.plugin table. With the plugin no longer registered in the table, the server does not load the plugin automatically for subsequent restarts.

UNINSTALL PLUGIN can unload a plugin regardless of whether it was loaded at runtime with INSTALL PLUGIN or at startup with a plugin-loading option, subject to these conditions:

  • It cannot unload plugins that are built in to the server. These can be identified as those that have a library name of NULL in the output from INFORMATION_SCHEMA.PLUGINS or SHOW PLUGINS.

  • It cannot unload plugins for which the server was started with --plugin_name=FORCE_PLUS_PERMANENT, which prevents plugin unloading at runtime. These can be identified from the LOAD_OPTION column of the INFORMATION_SCHEMA.PLUGINS table.

To uninstall a plugin that currently is loaded at server startup with a plugin-loading option, use this procedure.

  1. Remove any options related to the plugin from the my.cnf file.

  2. Restart the server.

  3. Plugins normally are installed using either a plugin-loading option at startup or with INSTALL PLUGIN at runtime, but not both. However, removing options for a plugin from the my.cnf file may not be sufficient to uninstall it if at some point INSTALL PLUGIN has also been used. If the plugin still appears in the output from INFORMATION_SCHEMA.PLUGINS or SHOW PLUGINS, use UNINSTALL PLUGIN to remove it from the mysql.plugin table. Then restart the server again.

5.5.2 Obtaining Server Plugin Information

There are several ways to determine which plugins are installed in the server:

  • The INFORMATION_SCHEMA.PLUGINS table contains a row for each loaded plugin. Any that have a PLUGIN_LIBRARY value of NULL are built in and cannot be unloaded.

    mysql> SELECT * FROM INFORMATION_SCHEMA.PLUGINS\G
    *************************** 1. row ***************************
               PLUGIN_NAME: binlog
            PLUGIN_VERSION: 1.0
             PLUGIN_STATUS: ACTIVE
               PLUGIN_TYPE: STORAGE ENGINE
       PLUGIN_TYPE_VERSION: 50158.0
            PLUGIN_LIBRARY: NULL
    PLUGIN_LIBRARY_VERSION: NULL
             PLUGIN_AUTHOR: MySQL AB
        PLUGIN_DESCRIPTION: This is a pseudo storage engine to represent the binlog in a transaction
            PLUGIN_LICENSE: GPL
               LOAD_OPTION: FORCE
    ...
    *************************** 10. row ***************************
               PLUGIN_NAME: InnoDB
            PLUGIN_VERSION: 1.0
             PLUGIN_STATUS: ACTIVE
               PLUGIN_TYPE: STORAGE ENGINE
       PLUGIN_TYPE_VERSION: 50158.0
            PLUGIN_LIBRARY: ha_innodb_plugin.so
    PLUGIN_LIBRARY_VERSION: 1.0
             PLUGIN_AUTHOR: Innobase Oy
        PLUGIN_DESCRIPTION: Supports transactions, row-level locking,
                            and foreign keys
            PLUGIN_LICENSE: GPL
               LOAD_OPTION: ON
    ...
    
  • The SHOW PLUGINS statement displays a row for each loaded plugin. Any that have a Library value of NULL are built in and cannot be unloaded.

    mysql> SHOW PLUGINS\G
    *************************** 1. row ***************************
       Name: binlog
     Status: ACTIVE
       Type: STORAGE ENGINE
    Library: NULL
    License: GPL
    ...
    *************************** 10. row ***************************
       Name: InnoDB
     Status: ACTIVE
       Type: STORAGE ENGINE
    Library: ha_innodb_plugin.so
    License: GPL
    ...
    
  • The mysql.plugin table shows which plugins have been registered with INSTALL PLUGIN. The table contains only plugin names and library file names, so it does not provide as much information as the PLUGINS table or the SHOW PLUGINS statement.

5.5.3 MySQL Enterprise Thread Pool

Note

MySQL Enterprise Thread Pool is an extension included in MySQL Enterprise Edition, a commercial product. To learn more about commercial products, https://www.mysql.com/products/.

MySQL Enterprise Edition includes MySQL Enterprise Thread Pool, implemented using a server plugin. The default thread-handling model in MySQL Server executes statements using one thread per client connection. As more clients connect to the server and execute statements, overall performance degrades. The thread pool plugin provides an alternative thread-handling model designed to reduce overhead and improve performance. The plugin implements a thread pool that increases server performance by efficiently managing statement execution threads for large numbers of client connections.

The thread pool addresses several problems of the model that uses one thread per connection:

  • Too many thread stacks make CPU caches almost useless in highly parallel execution workloads. The thread pool promotes thread stack reuse to minimize the CPU cache footprint.

  • With too many threads executing in parallel, context switching overhead is high. This also presents a challenging task to the operating system scheduler. The thread pool controls the number of active threads to keep the parallelism within the MySQL server at a level that it can handle and that is appropriate for the server host on which MySQL is executing.

  • Too many transactions executing in parallel increases resource contention. In InnoDB, this increases the time spent holding central mutexes. The thread pool controls when transactions start to ensure that not too many execute in parallel.

Additional Resources

Section A.15, “MySQL 5.7 FAQ: MySQL Enterprise Thread Pool”

5.5.3.1 Thread Pool Components

The thread pool feature comprises these components:

5.5.3.2 Thread Pool Installation

This section describes how to install MySQL Enterprise Thread Pool. For general information about installing plugins, see Section 5.5.1, “Installing and Uninstalling Plugins”.

To be usable by the server, the plugin library file must be located in the MySQL plugin directory (the directory named by the plugin_dir system variable). If necessary, configure the plugin directory location by setting the value of plugin_dir at server startup.

The plugin library file base name is thread_pool. The file name suffix differs per platform (for example, .so for Unix and Unix-like systems, .dll for Windows).

To enable thread pool capability, load the plugins to be used by starting the server with the --plugin-load-add option. For example, if you name only the plugin library file, the server loads all plugins that it contains (that is, the thread pool plugin and all the INFORMATION_SCHEMA tables). To do this, put these lines in the server my.cnf file (adjust the .so suffix for your platform as necessary):

[mysqld]
plugin-load-add=thread_pool.so

That is equivalent to loading all thread pool plugins by naming them individually:

[mysqld]
plugin-load-add=thread_pool=thread_pool.so
plugin-load-add=tp_thread_state=thread_pool.so
plugin-load-add=tp_thread_group_state=thread_pool.so
plugin-load-add=tp_thread_group_stats=thread_pool.so

If desired, you can load individual plugins from the library file. To load the thread pool plugin but not the INFORMATION_SCHEMA tables, use an option like this:

[mysqld]
plugin-load-add=thread_pool=thread_pool.so

To load the thread pool plugin and only the TP_THREAD_STATE INFORMATION_SCHEMA table, use options like this:

[mysqld]
plugin-load-add=thread_pool=thread_pool.so
plugin-load-add=tp_thread_state=thread_pool.so
Note

If you do not load all the INFORMATION_SCHEMA tables, some or all MySQL Enterprise Monitor thread pool graphs will be empty.

To verify plugin installation, examine the INFORMATION_SCHEMA.PLUGINS table or use the SHOW PLUGINS statement (see Section 5.5.2, “Obtaining Server Plugin Information”). For example:

mysql> SELECT PLUGIN_NAME, PLUGIN_STATUS
       FROM INFORMATION_SCHEMA.PLUGINS
       WHERE PLUGIN_NAME LIKE 'thread%' OR PLUGIN_NAME LIKE 'tp%';
+-----------------------+---------------+
| PLUGIN_NAME           | PLUGIN_STATUS |
+-----------------------+---------------+
| thread_pool           | ACTIVE        |
| TP_THREAD_STATE       | ACTIVE        |
| TP_THREAD_GROUP_STATE | ACTIVE        |
| TP_THREAD_GROUP_STATS | ACTIVE        |
+-----------------------+---------------+

If the server loads the thread pool plugin successfully, it sets the thread_handling system variable to loaded-dynamically.

If a plugin fails to initialize, check the server error log for diagnostic messages.

5.5.3.3 Thread Pool Operation

The thread pool consists of a number of thread groups, each of which manages a set of client connections. As connections are established, the thread pool assigns them to thread groups in round-robin fashion.

The number of thread groups is configurable using the thread_pool_size system variable. The default number of groups is 16. For guidelines on setting this variable, see Section 5.5.3.4, “Thread Pool Tuning”.

The maximum number of threads per group is 4096 (or 4095 on some systems where one thread is used internally).

The thread pool separates connections and threads, so there is no fixed relationship between connections and the threads that execute statements received from those connections. This differs from the default thread-handling model that associates one thread with one connection such that a given thread executes all statements from its connection.

The thread pool tries to ensure a maximum of one thread executing in each group at any time, but sometimes permits more threads to execute temporarily for best performance. The algorithm works in the following manner:

  • Each thread group has a listener thread that listens for incoming statements from the connections assigned to the group. When a statement arrives, the thread group either begins executing it immediately or queues it for later execution:

    • Immediate execution occurs if the statement is the only one received and no statements are queued or currently executing.

    • Queuing occurs if the statement cannot begin executing immediately.

  • If immediate execution occurs, execution is performed by the listener thread. (This means that temporarily no thread in the group is listening.) If the statement finishes quickly, the executing thread returns to listening for statements. Otherwise, the thread pool considers the statement stalled and starts another thread as a listener thread (creating it if necessary). To ensure that no thread group becomes blocked by stalled statements, the thread pool has a background thread that regularly monitors thread group states.

    By using the listening thread to execute a statement that can begin immediately, there is no need to create an additional thread if the statement finishes quickly. This ensures the most efficient execution possible in the case of a low number of concurrent threads.

    When the thread pool plugin starts, it creates one thread per group (the listener thread), plus the background thread. Additional threads are created as necessary to execute statements.

  • The value of the thread_pool_stall_limit system variable determines the meaning of finishes quickly in the previous item. The default time before threads are considered stalled is 60ms but can be set to a maximum of 6s. This parameter is configurable to enable you to strike a balance appropriate for the server work load. Short wait values permit threads to start more quickly. Short values are also better for avoiding deadlock situations. Long wait values are useful for workloads that include long-running statements, to avoid starting too many new statements while the current ones execute.

  • The thread pool focuses on limiting the number of concurrent short-running statements. Before an executing statement reaches the stall time, it prevents other statements from beginning to execute. If the statement executes past the stall time, it is permitted to continue but no longer prevents other statements from starting. In this way, the thread pool tries to ensure that in each thread group there is never more than one short-running statement, although there might be multiple long-running statements. It is undesirable to let long-running statements prevent other statements from executing because there is no limit on the amount of waiting that might be necessary. For example, on a replication master, a thread that is sending binary log events to a slave effectively runs forever.

  • A statement becomes blocked if it encounters a disk I/O operation or a user level lock (row lock or table lock). The block would cause the thread group to become unused, so there are callbacks to the thread pool to ensure that the thread pool can immediately start a new thread in this group to execute another statement. When a blocked thread returns, the thread pool permits it to restart immediately.

  • There are two queues, a high-priority queue and a low-priority queue. The first statement in a transaction goes to the low-priority queue. Any following statements for the transaction go to the high-priority queue if the transaction is ongoing (statements for it have begun executing), or to the low-priority queue otherwise. Queue assignment can be affected by enabling the thread_pool_high_priority_connection system variable, which causes all queued statements for a session to go into the high-priority queue.

    Statements for a nontransactional storage engine, or a transactional engine if autocommit is enabled, are treated as low-priority statements because in this case each statement is a transaction. Thus, given a mix of statements for InnoDB and MyISAM tables, the thread pool prioritizes those for InnoDB over those for MyISAM unless autocommit is enabled. With autocommit enabled, all statements will be low priority.

  • When the thread group selects a queued statement for execution, it first looks in the high-priority queue, then in the low-priority queue. If a statement is found, it is removed from its queue and begins to execute.

  • If a statement stays in the low-priority queue too long, the thread pool moves to the high-priority queue. The value of the thread_pool_prio_kickup_timer system variable controls the time before movement. For each thread group, a maximum of one statement per 10ms or 100 per second will be moved from the low-priority queue to the high-priority queue.

  • The thread pool reuses the most active threads to obtain a much better use of CPU caches. This is a small adjustment that has a great impact on performance.

  • While a thread executes a statement from a user connection, Performance Schema instrumentation accounts thread activity to the user connection. Otherwise, Performance Schema accounts activity to the thread pool.

Here are examples of conditions under which a thread group might have multiple threads started to execute statements:

  • One thread begins executing a statement, but runs long enough to be considered stalled. The thread group permits another thread to begin executing another statement even through the first thread is still executing.

  • One thread begins executing a statement, then becomes blocked and reports this back to the thread pool. The thread group permits another thread to begin executing another statement.

  • One thread begins executing a statement, becomes blocked, but does not report back that it is blocked because the block does not occur in code that has been instrumented with thread pool callbacks. In this case, the thread appears to the thread group to be still running. If the block lasts long enough for the statement to be considered stalled, the group permits another thread to begin executing another statement.

The thread pool is designed to be scalable across an increasing number of connections. It is also designed to avoid deadlocks that can arise from limiting the number of actively executing statements. It is important that threads that do not report back to the thread pool do not prevent other statements from executing and thus cause the thread pool to become deadlocked. Examples of such statements follow:

  • Long-running statements. These would lead to all resources used by only a few statements and they could prevent all others from accessing the server.

  • Binary log dump threads that read the binary log and send it to slaves. This is a kind of long-running statement that runs for a very long time, and that should not prevent other statements from executing.

  • Statements blocked on a row lock, table lock, sleep, or any other blocking activity that has not been reported back to the thread pool by MySQL Server or a storage engine.

In each case, to prevent deadlock, the statement is moved to the stalled category when it does not complete quickly, so that the thread group can permit another statement to begin executing. With this design, when a thread executes or becomes blocked for an extended time, the thread pool moves the thread to the stalled category and for the rest of the statement's execution, it does not prevent other statements from executing.

The maximum number of threads that can occur is the sum of max_connections and thread_pool_size. This can happen in a situation where all connections are in execution mode and an extra thread is created per group to listen for more statements. This is not necessarily a state that happens often, but it is theoretically possible.

5.5.3.4 Thread Pool Tuning

This section provides guidelines on setting thread pool system variables for best performance, measured using a metric such as transactions per second.

thread_pool_size is the most important parameter controlling thread pool performance. It can be set only at server startup. Our experience in testing the thread pool indicates the following:

  • If the primary storage engine is InnoDB, the optimal thread_pool_size setting is likely to be between 16 and 36, with the most common optimal values tending to be from 24 to 36. We have not seen any situation where the setting has been optimal beyond 36. There may be special cases where a value smaller than 16 is optimal.

    For workloads such as DBT2 and Sysbench, the optimum for InnoDB seems to be usually around 36. For very write-intensive workloads, the optimal setting can sometimes be lower.

  • If the primary storage engine is MyISAM, the thread_pool_size setting should be fairly low. Optimal performance is often seen with values from 4 to 8. Higher values tend to have a slightly negative but not dramatic impact on performance.

Another system variable, thread_pool_stall_limit, is important for handling of blocked and long-running statements. If all calls that block the MySQL Server are reported to the thread pool, it would always know when execution threads are blocked. However, this may not always be true. For example, blocks could occur in code that has not been instrumented with thread pool callbacks. For such cases, the thread pool must be able to identify threads that appear to be blocked. This is done by means of a timeout, the length of which can be tuned using the thread_pool_stall_limit system variable. This parameter ensures that the server does not become completely blocked. The value of thread_pool_stall_limit has an upper limit of 6 seconds to prevent the risk of a deadlocked server.

thread_pool_stall_limit also enables the thread pool to handle long-running statements. If a long-running statement was permitted to block a thread group, all other connections assigned to the group would be blocked and unable to start execution until the long-running statement completed. In the worst case, this could take hours or even days.

The value of thread_pool_stall_limit should be chosen such that statements that execute longer than its value are considered stalled. Stalled statements generate a lot of extra overhead since they involve extra context switches and in some cases even extra thread creations. On the other hand, setting the thread_pool_stall_limit parameter too high means that long-running statements will block a number of short-running statements for longer than necessary. Short wait values permit threads to start more quickly. Short values are also better for avoiding deadlock situations. Long wait values are useful for workloads that include long-running statements, to avoid starting too many new statements while the current ones execute.

Suppose a server executes a workload where 99.9% of the statements complete within 100ms even when the server is loaded, and the remaining statements take between 100ms and 2 hours fairly evenly spread. In this case, it would make sense to set thread_pool_stall_limit to 10 (meaning 100ms). The default value of 60ms is suitable for servers that primarily execute very simple statements.

The thread_pool_stall_limit parameter can be changed at runtime to enable you to strike a balance appropriate for the server work load. Assuming that the TP_THREAD_GROUP_STATS table is enabled, you can use the following query to determine the fraction of executed statements that stalled:

SELECT SUM(STALLED_QUERIES_EXECUTED) / SUM(QUERIES_EXECUTED)
FROM INFORMATION_SCHEMA.TP_THREAD_GROUP_STATS;

This number should be as low as possible. To decrease the likelihood of statements stalling, increase the value of thread_pool_stall_limit.

When a statement arrives, what is the maximum time it can be delayed before it actually starts executing? Suppose that the following conditions apply:

In the worst case, the 10 high-priority statements represent 10 transactions that continue executing for a long time. Thus, in the worst case, no statements will be moved to the high-priority queue because it will always already contain statements awaiting execution. After 10 seconds, the new statement is eligible to be moved to the high-priority queue. However, before it can be moved, all the statements before it must be moved as well. This could take another 2 seconds because a maximum of 100 statements per second are moved to the high-priority queue. Now when the statement reaches the high-priority queue, there could potentially be many long-running statements ahead of it. In the worst case, every one of those will become stalled and it will take 1 second for each statement before the next statement is retrieved from the high-priority queue. Thus, in this scenario, it will take 222 seconds before the new statement starts executing.

This example shows a worst case for an application. How to handle it depends on the application. If the application has high requirements for the response time, it should most likely throttle users at a higher level itself. Otherwise, it can use the thread pool configuration parameters to set some kind of a maximum waiting time.

5.5.4 The Rewriter Query Rewrite Plugin

MySQL supports query rewrite plugins that can examine and possibly modify SQL statements received by the server before the server executes them. See Query Rewrite Plugins.

MySQL distributions include a postparse query rewrite plugin named Rewriter and scripts for installing the plugin and its associated components. These components work together to provide SELECT rewriting capability:

  • A server-side plugin named Rewriter examines SELECT statements and may rewrite them, based on its in-memory cache of rewrite rules. Standalone SELECT statements and SELECT statements in prepared statements are subject to rewriting. SELECT statements occurring within view definitions or stored programs are not subject to rewriting.

  • The Rewriter plugin uses a database named query_rewrite containing a table named rewrite_rules. The table provides persistent storage for the rules that the plugin uses to decide whether to rewrite statements. Users communicate with the plugin by modifying the set of rules stored in this table. The plugin communicates with users by setting the message column of table rows.

  • The query_rewrite database contains a stored procedure named flush_rewrite_rules() that loads the contents of the rules table into the plugin.

  • A user-defined function named load_rewrite_rules() is used by the flush_rewrite_rules() stored procedure.

  • The Rewriter plugin exposes system variables that enable plugin configuration and status variables that provide runtime operational information.

The following sections describe how to install and use the Rewriter plugin, and provide reference information for its associated components.

5.5.4.1 Installing or Uninstalling the Rewriter Query Rewrite Plugin

Note

If installed, the Rewriter plugin involves some overhead even when disabled. To avoid this overhead, do not install the plugin unless you plan to use it.

To install or uninstall the Rewriter query rewrite plugin, choose the appropriate script located in the share directory of your MySQL installation:

  • install_rewriter.sql: Choose this script to install the Rewriter plugin and its associated components.

  • uninstall_rewriter.sql: Choose this script to uninstall the Rewriter plugin and its associated components.

Run the chosen script as follows:

shell> mysql -u root -p < install_rewriter.sql
Enter password: (enter root password here)

The example here uses the install_rewriter.sql installation script. Substitute uninstall_rewriter.sql if you are uninstalling the plugin.

Running an installation script should install and enable the plugin. To verify that, connect to the server and execute this statement:

mysql> SHOW GLOBAL VARIABLES LIKE 'rewriter_enabled';
+------------------+-------+
| Variable_name    | Value |
+------------------+-------+
| rewriter_enabled | ON    |
+------------------+-------+

For usage instructions, see Section 5.5.4.2, “Using the Rewriter Query Rewrite Plugin”. For reference information, see Section 5.5.4.3, “Rewriter Query Rewrite Plugin Reference”.

5.5.4.2 Using the Rewriter Query Rewrite Plugin

To enable or disable the plugin, enable or disable the rewriter_enabled system variable. By default, the Rewriter plugin is enabled when you install it (see Section 5.5.4.1, “Installing or Uninstalling the Rewriter Query Rewrite Plugin”). To set the initial plugin state explicitly, you can set the variable at server startup. For example, to enable the plugin in an option file, use these lines:

[mysqld]
rewriter_enabled=ON

It is also possible to enable or disable the plugin at runtime:

SET GLOBAL rewriter_enabled = ON;
SET GLOBAL rewriter_enabled = OFF;

Assuming that the Rewriter plugin is enabled, it examines and possibly modifies each SELECT statement received by the server. The plugin determines whether to rewrite statements based on its in-memory cache of rewriting rules, which are loaded from the rewrite_rules table in the query_rewrite database.

Adding Rewrite Rules

To add rules for the Rewriter plugin, add rows to the rewrite_rules table, then invoke the flush_rewrite_rules() stored procedure to load the rules from the table into the plugin. The following example creates a simple rule to match statements that select a single literal value:

INSERT INTO query_rewrite.rewrite_rules (pattern, replacement)
VALUES('SELECT ?', 'SELECT ? + 1');

The resulting table contents look like this:

mysql> SELECT * FROM query_rewrite.rewrite_rules\G
*************************** 1. row ***************************
                id: 1
           pattern: SELECT ?
  pattern_database: NULL
       replacement: SELECT ? + 1
           enabled: YES
           message: NULL
    pattern_digest: NULL
normalized_pattern: NULL

The rule specifies a pattern template indicating which SELECT statements to match, and a replacement template indicating how to rewrite matching statements. However, adding the rule to the rewrite_rules table is not sufficient to cause the Rewriter plugin to use the rule. You must invoke flush_rewrite_rules() to load the table contents into the plugin in-memory cache:

mysql> CALL query_rewrite.flush_rewrite_rules();
Tip

If your rewrite rules seem not to be working properly, make sure that you have reloaded the rules table by calling flush_rewrite_rules().

When the plugin reads each rule from the rules table, it computes a normalized (statement digest) form from the pattern and a digest hash value, and uses them to update the normalized_pattern and pattern_digest columns:

mysql> SELECT * FROM query_rewrite.rewrite_rules\G
*************************** 1. row ***************************
                id: 1
           pattern: SELECT ?
  pattern_database: NULL
       replacement: SELECT ? + 1
           enabled: YES
           message: NULL
    pattern_digest: 46b876e64cd5c41009d91c754921f1d4
normalized_pattern: select ?

For information about statement digesting, normalized statements, and digest hash values, see Section 25.10, “Performance Schema Statement Digests”.

If a rule cannot be loaded due to some error, calling flush_rewrite_rules() produces an error:

mysql> CALL query_rewrite.flush_rewrite_rules();
ERROR 1644 (45000): Loading of some rule(s) failed.

When this occurs, the plugin writes an error message to the message column of the rule row to communicate the problem. Check the rewrite_rules table for rows with non-NULL message column values to see what problems exist.

Patterns use the same syntax as prepared statements (see Section 13.5.1, “PREPARE Statement”). Within a pattern template, ? characters act as parameter markers that match data values. Parameter markers can be used only where data values should appear, not for SQL keywords, identifiers, and so forth. The ? characters should not be enclosed within quotation marks.

Like the pattern, the replacement can contain ? characters. For a statement that matches a pattern template, the plugin rewrites it, replacing ? parameter markers in the replacement using data values matched by the corresponding markers in the pattern. The result is a complete statement string. The plugin asks the server to parse it, and returns the result to the server as the representation of the rewritten statement.

After adding and loading the rule, check whether rewriting occurs according to whether statements match the rule pattern:

mysql> SELECT PI();
+----------+
| PI()     |
+----------+
| 3.141593 |
+----------+
1 row in set (0.01 sec)

mysql> SELECT 10;
+--------+
| 10 + 1 |
+--------+
|     11 |
+--------+
1 row in set, 1 warning (0.00 sec)

No rewriting occurs for the first SELECT statement, but does for the second. The second statement illustrates that when the Rewriter plugin rewrites a statement, it produces a warning message. To view the message, use SHOW WARNINGS:

mysql> SHOW WARNINGS\G
*************************** 1. row ***************************
  Level: Note
   Code: 1105
Message: Query 'SELECT 10' rewritten to 'SELECT 10 + 1' by a query rewrite plugin

To enable or disable an existing rule, modify its enabled column and reload the table into the plugin. To disable rule 1:

UPDATE query_rewrite.rewrite_rules SET enabled = 'NO' WHERE id = 1;
CALL query_rewrite.flush_rewrite_rules();

This enables you to deactivate a rule without removing it from the table.

To re-enable rule 1:

UPDATE query_rewrite.rewrite_rules SET enabled = 'YES' WHERE id = 1;
CALL query_rewrite.flush_rewrite_rules();

The rewrite_rules table contains a pattern_database column that Rewriter uses for matching table names that are not qualified with a database name:

  • Qualified table names in statements match qualified names in the pattern if corresponding database and table names are identical.

  • Unqualified table names in statements match unqualified names in the pattern only if the default database is the same as pattern_database and the table names are identical.

Suppose that a table named appdb.users has a column named id and that applications are expected to select rows from the table using a query of one of these forms, where the second can be used only if appdb is the default database:

SELECT * FROM users WHERE appdb.id = id_value;
SELECT * FROM users WHERE id = id_value;

Suppose also that the id column is renamed to user_id (perhaps the table must be modified to add another type of ID and it is necessary to indicate more specifically what type of ID the id column represents).

The change means that applications must refer to user_id rather than id in the WHERE clause. But if there are old applications that cannot be written to change the SELECT queries they generate, they will no longer work properly. The Rewriter plugin can solve this problem. To match and rewrite statements whether or not they qualify the table name, add the following two rules and reload the rules table:

INSERT INTO query_rewrite.rewrite_rules
    (pattern, replacement) VALUES(
    'SELECT * FROM appdb.users WHERE id = ?',
    'SELECT * FROM appdb.users WHERE user_id = ?'
    );
INSERT INTO query_rewrite.rewrite_rules
    (pattern, replacement, pattern_database) VALUES(
    'SELECT * FROM users WHERE id = ?',
    'SELECT * FROM users WHERE user_id = ?',
    'appdb'
    );
CALL query_rewrite.flush_rewrite_rules();

Rewriter uses the first rule to match statements that use the qualified table name. It uses the second to match statements that used the unqualified name, but only if the default database is appdb (the value in pattern_database).

How Statement Matching Works

The Rewriter plugin uses statement digests and digest hash values to match incoming statements against rewrite rules in stages. The max_digest_length system variable determines the size of the buffer used for computing statement digests. Larger values enable computation of digests that distinguish longer statements. Smaller values use less memory but increase the likelihood of longer statements colliding with the same digest value.

The plugin matches each statement to the rewrite rules as follows:

  1. Compute the statement digest hash value and compare it to the rule digest hash values. This is subject to false positives, but serves as a quick rejection test.

  2. If the statement digest hash value matches any pattern digest hash values, match the normalized (statement digest) form of the statement to the normalized form of the matching rule patterns.

  3. If the normalized statement matches a rule, compare the literal values in the statement and the pattern. A ? character in the pattern matches any literal value in the statement. If the statement prepares a SELECT statement, ? in the pattern also matches ? in the statement. Otherwise, corresponding literals must be the same.

If multiple rules match a statement, it is nondeterministic which one the plugin uses to rewrite the statement.

If a pattern contains more markers than the replacement, the plugin discards excess data values. If a pattern contains fewer markers than the replacement, it is an error. The plugin notices this when the rules table is loaded, writes an error message to the message column of the rule row to communicate the problem, and sets the Rewriter_reload_error status variable to ON.

Rewriting Prepared Statements

Prepared statements are rewritten at parse time (that is, when they are prepared), not when they are executed later.

Prepared statements differ from nonprepared statements in that they may contain ? characters as parameter markers. To match a ? in a prepared statement, a Rewriter pattern must contain ? in the same location. Suppose that a rewrite rule has this pattern:

SELECT ?, 3

The following table shows several prepared SELECT statements and whether the rule pattern matches them.

Prepared Statement Whether Pattern Matches Statement
PREPARE s AS 'SELECT 3, 3' Yes
PREPARE s AS 'SELECT ?, 3' Yes
PREPARE s AS 'SELECT 3, ?' No
PREPARE s AS 'SELECT ?, ?' No
Rewriter Plugin Operational Information

The Rewriter plugin makes information available about its operation by means of several status variables:

mysql> SHOW GLOBAL STATUS LIKE 'Rewriter%';
+-----------------------------------+-------+
| Variable_name                     | Value |
+-----------------------------------+-------+
| Rewriter_number_loaded_rules      | 1     |
| Rewriter_number_reloads           | 5     |
| Rewriter_number_rewritten_queries | 1     |
| Rewriter_reload_error             | ON    |
+-----------------------------------+-------+

For descriptions of these variables, see Section 5.5.4.3.4, “Rewriter Query Rewrite Plugin Status Variables”.

When you load the rules table by calling the flush_rewrite_rules() stored procedure, if an error occurs for some rule, the CALL statement produces an error, and the plugin sets the Rewriter_reload_error status variable to ON:

mysql> CALL query_rewrite.flush_rewrite_rules();
ERROR 1644 (45000): Loading of some rule(s) failed.

mysql> SHOW GLOBAL STATUS LIKE 'Rewriter_reload_error';
+-----------------------+-------+
| Variable_name         | Value |
+-----------------------+-------+
| Rewriter_reload_error | ON    |
+-----------------------+-------+

In this case, check the rewrite_rules table for rows with non-NULL message column values to see what problems exist.

Rewriter Plugin Use of Character Sets

When the rewrite_rules table is loaded into the Rewriter plugin, the plugin interprets statements using the current global value of the character_set_client system variable. If the global character_set_client value is changed subsequently, the rules table must be reloaded.

A client must have a session character_set_client value identical to what the global value was when the rules table was loaded or rule matching will not work for that client.

5.5.4.3 Rewriter Query Rewrite Plugin Reference

The following discussion serves as a reference to these components associated with the Rewriter query rewrite plugin:

  • The Rewriter rules table in the query_rewrite database

  • Rewriter procedures and functions

  • Rewriter system and status variables

5.5.4.3.1 Rewriter Query Rewrite Plugin Rules Table

The rewrite_rules table in the query_rewrite database provides persistent storage for the rules that the Rewriter plugin uses to decide whether to rewrite statements.

Users communicate with the plugin by modifying the set of rules stored in this table. The plugin communicates information to users by setting the table's message column.

Note

The rules table is loaded into the plugin by the flush_rewrite_rules stored procedure. Unless that procedure has been called following the most recent table modification, the table contents do not necessarily correspond to the set of rules the plugin is using.

The rewrite_rules table has these columns:

  • id

    The rule ID. This column is the table primary key. You can use the ID to uniquely identify any rule.

  • pattern

    The template that indicates the pattern for statements that the rule matches. Use ? to represent parameter markers that match data values.

  • pattern_database

    The database used to match unqualified table names in statements. Qualified table names in statements match qualified names in the pattern if corresponding database and table names are identical. Unqualified table names in statements match unqualified names in the pattern only if the default database is the same as pattern_database and the table names are identical.

  • replacement

    The template that indicates how to rewrite statements matching the pattern column value. Use ? to represent parameter markers that match data values. In rewritten statements, the plugin replaces ? parameter markers in replacement using data values matched by the corresponding markers in pattern.

  • enabled

    Whether the rule is enabled. Load operations (performed by invoking the flush_rewrite_rules() stored procedure) load the rule from the table into the Rewriter in-memory cache only if this column is YES.

    This column makes it possible to deactivate a rule without removing it: Set the column to a value other than YES and reload the table into the plugin.

  • message

    The plugin uses this column for communicating with users. If no error occurs when the rules table is loaded into memory, the plugin sets the message column to NULL. A non-NULL value indicates an error and the column contents are the error message. Errors can occur under these circumstances:

    • Either the pattern or the replacement is an incorrect SQL statement that produces syntax errors.

    • The replacement contains more ? parameter markers than the pattern.

    If a load error occurs, the plugin also sets the Rewriter_reload_error status variable to ON.

  • pattern_digest

    This column is used for debugging and diagnostics. If the column exists when the rules table is loaded into memory, the plugin updates it with the pattern digest. This column may be useful if you are trying to determine why some statement fails to be rewritten.

  • normalized_pattern

    This column is used for debugging and diagnostics. If the column exists when the rules table is loaded into memory, the plugin updates it with the normalized form of the pattern. This column may be useful if you are trying to determine why some statement fails to be rewritten.

5.5.4.3.2 Rewriter Query Rewrite Plugin Procedures and Functions

Rewriter plugin operation uses a stored procedure that loads the rules table into its in-memory cache, and a helper user-defined function (UDF). Under normal operation, users invoke only the stored procedure. The UDF is intended to be invoked by the stored procedure, not directly by users.

  • flush_rewrite_rules()

    This stored procedure uses the load_rewrite_rules() UDF to load the contents of the rewrite_rules table into the Rewriter in-memory cache. After loading the table, it also clears the query cache.

    Calling flush_rewrite_rules() implies COMMIT.

    Invoke this procedure after you modify the rules table to cause the plugin to update its cache from the new table contents. If any errors occur, the plugin sets the message column for the appropriate rule rows in the table and sets the Rewriter_reload_error status variable to ON.

  • load_rewrite_rules()

    This UDF is a helper routine used by the flush_rewrite_rules() stored procedure.

5.5.4.3.3 Rewriter Query Rewrite Plugin System Variables

The Rewriter query rewrite plugin supports the following system variables. These variables are available only if the plugin is installed (see Section 5.5.4.1, “Installing or Uninstalling the Rewriter Query Rewrite Plugin”).

5.5.4.3.4 Rewriter Query Rewrite Plugin Status Variables

The Rewriter query rewrite plugin supports the following status variables. These variables are available only if the plugin is installed (see Section 5.5.4.1, “Installing or Uninstalling the Rewriter Query Rewrite Plugin”).

  • Rewriter_number_loaded_rules

    The number of rewrite plugin rewrite rules successfully loaded from the rewrite_rules table into memory for use by the Rewriter plugin.

  • Rewriter_number_reloads

    The number of times the rewrite_rules table has been loaded into the in-memory cache used by the Rewriter plugin.

  • Rewriter_number_rewritten_queries

    The number of queries rewritten by the Rewriter query rewrite plugin since it was loaded.

  • Rewriter_reload_error

    Whether an error occurred the most recent time that the rewrite_rules table was loaded into the in-memory cache used by the Rewriter plugin. If the value is OFF, no error occurred. If the value is ON, an error occurred; check the message column of the rewriter_rules table for error messages.

5.5.5 Version Tokens

MySQL includes Version Tokens, a feature that enables creation of and synchronization around server tokens that applications can use to prevent accessing incorrect or out-of-date data.

The Version Tokens interface has these characteristics:

  • Version tokens are pairs consisting of a name that serves as a key or identifier, plus a value.

  • Version tokens can be locked. An application can use token locks to indicate to other cooperating applications that tokens are in use and should not be modified.

  • Version token lists are established per server (for example, to specify the server assignment or operational state). In addition, an application that communicates with a server can register its own list of tokens that indicate the state it requires the server to be in. An SQL statement sent by the application to a server not in the required state produces an error. This is a signal to the application that it should seek a different server in the required state to receive the SQL statement.

The following sections describe the components of Version Tokens, discuss how to install and use it, and provide reference information for its components.

5.5.5.1 Version Tokens Components

Version Tokens is based on a plugin library that implements these components:

  • A server-side plugin named version_tokens holds the list of version tokens associated with the server and subscribes to notifications for statement execution events. The version_tokens plugin uses the audit plugin API to monitor incoming statements from clients and matches each client's session-specific version token list against the server version token list. If there is a match, the plugin lets the statement through and the server continues to process it. Otherwise, the plugin returns an error to the client and the statement fails.

  • A set of user-defined functions (UDFs) provides an SQL-level API for manipulating and inspecting the list of server version tokens maintained by the plugin. The SUPER privilege is required to call any of the Version Token UDFs.

  • A system variable enables clients to specify the list of version tokens that register the required server state. If the server has a different state when a client sends a statement, the client receives an error.

5.5.5.2 Installing or Uninstalling Version Tokens

Note

If installed, Version Tokens involves some overhead. To avoid this overhead, do not install it unless you plan to use it.

This section describes how to install or uninstall Version Tokens, which is implemented in a plugin library file containing a plugin and user-defined functions (UDFs). For general information about installing or uninstalling plugins and UDFs, see Section 5.5.1, “Installing and Uninstalling Plugins”, and Section 5.6.1, “Installing and Uninstalling User-Defined Functions”.

To be usable by the server, the plugin library file must be located in the MySQL plugin directory (the directory named by the plugin_dir system variable). If necessary, configure the plugin directory location by setting the value of plugin_dir at server startup.

The plugin library file base name is version_tokens. The file name suffix differs per platform (for example, .so for Unix and Unix-like systems, .dll for Windows).

To install the Version Tokens plugin and UDFs, use the INSTALL PLUGIN and CREATE FUNCTION statements (adjust the .so suffix for your platform as necessary):

INSTALL PLUGIN version_tokens SONAME 'version_token.so';
CREATE FUNCTION version_tokens_set RETURNS STRING
  SONAME 'version_token.so';
CREATE FUNCTION version_tokens_show RETURNS STRING
  SONAME 'version_token.so';
CREATE FUNCTION version_tokens_edit RETURNS STRING
  SONAME 'version_token.so';
CREATE FUNCTION version_tokens_delete RETURNS STRING
  SONAME 'version_token.so';
CREATE FUNCTION version_tokens_lock_shared RETURNS INT
  SONAME 'version_token.so';
CREATE FUNCTION version_tokens_lock_exclusive RETURNS INT
  SONAME 'version_token.so';
CREATE FUNCTION version_tokens_unlock RETURNS INT
  SONAME 'version_token.so';

You must install the UDFs to manage the server's version token list, but you must also install the plugin because the UDFs will not work correctly without it.

If the plugin and UDFs are used on a master replication server, install them on all slave servers as well to avoid replication problems.

Once installed as just described, the plugin and UDFs remain installed until uninstalled. To remove them, use the UNINSTALL PLUGIN and DROP FUNCTION statements:

UNINSTALL PLUGIN version_tokens;
DROP FUNCTION version_tokens_set;
DROP FUNCTION version_tokens_show;
DROP FUNCTION version_tokens_edit;
DROP FUNCTION version_tokens_delete;
DROP FUNCTION version_tokens_lock_shared;
DROP FUNCTION version_tokens_lock_exclusive;
DROP FUNCTION version_tokens_unlock;

5.5.5.3 Using Version Tokens

Before using Version Tokens, install it according to the instructions provided at Section 5.5.5.2, “Installing or Uninstalling Version Tokens”.

A scenario in which Version Tokens can be useful is a system that accesses a collection of MySQL servers but needs to manage them for load balancing purposes by monitoring them and adjusting server assignments according to load changes. Such a system comprises these components:

  • The collection of MySQL servers to be managed.

  • An administrative or management application that communicates with the servers and organizes them into high-availability groups. Groups serve different purposes, and servers within each group may have different assignments. Assignment of a server within a certain group can change at any time.

  • Client applications that access the servers to retrieve and update data, choosing servers according to the purposes assigned them. For example, a client should not send an update to a read-only server.

Version Tokens permit server access to be managed according to assignment without requiring clients to repeatedly query the servers about their assignments:

  • The management application performs server assignments and establishes version tokens on each server to reflect its assignment. The application caches this information to provide a central access point to it.

    If at some point the management application needs to change a server assignment (for example, to change it from permitting writes to read only), it changes the server's version token list and updates its cache.

  • To improve performance, client applications obtain cache information from the management application, enabling them to avoid having to retrieve information about server assignments for each statement. Based on the type of statements it will issue (for example, reads versus writes), a client selects an appropriate server and connects to it.

  • In addition, the client sends to the server its own client-specific version tokens to register the assignment it requires of the server. For each statement sent by the client to the server, the server compares its own token list with the client token list. If the server token list contains all tokens present in the client token list with the same values, there is a match and the server executes the statement.

    On the other hand, perhaps the management application has changed the server assignment and its version token list. In this case, the new server assignment may now be incompatible with the client requirements. A token mismatch between the server and client token lists occurs and the server returns an error in reply to the statement. This is an indication to the client to refresh its version token information from the management application cache, and to select a new server to communicate with.

The client-side logic for detecting version token errors and selecting a new server can be implemented different ways:

  • The client can handle all version token registration, mismatch detection, and connection switching itself.

  • The logic for those actions can be implemented in a connector that manages connections between clients and MySQL servers. Such a connector might handle mismatch error detection and statement resending itself, or it might pass the error to the application and leave it to the application to resend the statement.

The following example illustrates the preceding discussion in more concrete form.

When Version Tokens initializes on a given server, the server's version token list is empty. Token list maintenance is performed by calling user-defined functions (UDFs). The SUPER privilege is required to call any of the Version Token UDFs, so token list modification is expected to be done by a management or administrative application that has that privilege.

Suppose that a management application communicates with a set of servers that are queried by clients to access employee and product databases (named emp and prod, respectively). All servers are permitted to process data retrieval statements, but only some of them are permitted to make database updates. To handle this on a database-specific basis, the management application establishes a list of version tokens on each server. In the token list for a given server, token names represent database names and token values are read or write depending on whether the database must be used in read-only fashion or whether it can take reads and writes.

Client applications register a list of version tokens they require the server to match by setting a system variable. Variable setting occurs on a client-specific basis, so different clients can register different requirements. By default, the client token list is empty, which matches any server token list. When a client sets its token list to a nonempty value, matching may succeed or fail, depending on the server version token list.

To define the version token list for a server, the management application calls the version_tokens_set() UDF. (There are also UDFs for modifying and displaying the token list, described later.) For example, the application might send these statements to a group of three servers:

Server 1:

mysql> SELECT version_tokens_set('emp=read;prod=read');
+------------------------------------------+
| version_tokens_set('emp=read;prod=read') |
+------------------------------------------+
| 2 version tokens set.                    |
+------------------------------------------+

Server 2:

mysql> SELECT version_tokens_set('emp=write;prod=read');
+-------------------------------------------+
| version_tokens_set('emp=write;prod=read') |
+-------------------------------------------+
| 2 version tokens set.                     |
+-------------------------------------------+

Server 3:

mysql> SELECT version_tokens_set('emp=read;prod=write');
+-------------------------------------------+
| version_tokens_set('emp=read;prod=write') |
+-------------------------------------------+
| 2 version tokens set.                     |
+-------------------------------------------+

The token list in each case is specified as a semicolon-separated list of name=value pairs. The resulting token list values result in these server assingments:

  • Any server accepts reads for either database.

  • Only server 2 accepts updates for the emp database.

  • Only server 3 accepts updates for the prod database.

In addition to assigning each server a version token list, the management application also maintains a cache that reflects the server assignments.

Before communicating with the servers, a client application contacts the management application and retrieves information about server assignments. Then the client selects a server based on those assignments. Suppose that a client wants to perform both reads and writes on the emp database. Based on the preceding assignments, only server 2 qualifies. The client connects to server 2 and registers its server requirements there by setting its version_tokens_session system variable:

mysql> SET @@SESSION.version_tokens_session = 'emp=write';

For subsequent statements sent by the client to server 2, the server compares its own version token list to the client list to check whether they match. If so, statements execute normally:

mysql> UPDATE emp.employee SET salary = salary * 1.1 WHERE id = 4981;
Query OK, 1 row affected (0.07 sec)
Rows matched: 1  Changed: 1  Warnings: 0

mysql> SELECT last_name, first_name FROM emp.employee WHERE id = 4981;
+-----------+------------+
| last_name | first_name |
+-----------+------------+
| Smith     | Abe        |
+-----------+------------+
1 row in set (0.01 sec)

Discrepancies between the server and client version token lists can occur two ways:

As long as the assignment of server 2 does not change, the client continues to use it for reads and writes. But suppose that the management application wants to change server assignments so that writes for the emp database must be sent to server 1 instead of server 2. To do this, it uses version_tokens_edit() to modify the emp token value on the two servers (and updates its cache of server assignments):

Server 1:

mysql> SELECT version_tokens_edit('emp=write');
+----------------------------------+
| version_tokens_edit('emp=write') |
+----------------------------------+
| 1 version tokens updated.        |
+----------------------------------+

Server 2:

mysql> SELECT version_tokens_edit('emp=read');
+---------------------------------+
| version_tokens_edit('emp=read') |
+---------------------------------+
| 1 version tokens updated.       |
+---------------------------------+

version_tokens_edit() modifies the named tokens in the server token list and leaves other tokens unchanged.

The next time the client sends a statement to server 2, its own token list no longer matches the server token list and an error occurs:

mysql> UPDATE emp.employee SET salary = salary * 1.1 WHERE id = 4982;
ERROR 3136 (42000): Version token mismatch for emp. Correct value read

In this case, the client should contact the management application to obtain updated information about server assignments, select a new server, and send the failed statement to the new server.

Note

Each client must cooperate with Version Tokens by sending only statements in accordance with the token list that it registers with a given server. For example, if a client registers a token list of 'emp=read', there is nothing in Version Tokens to prevent the client from sending updates for the emp database. The client itself must refrain from doing so.

For each statement received from a client, the server implicitly uses locking, as follows:

  • Take a shared lock for each token named in the client token list (that is, in the version_tokens_session value)

  • Perform the comparison between the server and client token lists

  • Execute the statement or produce an error depending on the comparison result

  • Release the locks

The server uses shared locks so that comparisons for multiple sessions can occur without blocking, while preventing changes to the tokens for any session that attempts to acquire an exclusive lock before it manipulates tokens of the same names in the server token list.

The preceding example uses only a few of the user-defined included in the Version Tokens plugin library, but there are others. One set of UDFs permits the server's list of version tokens to be manipulated and inspected. Another set of UDFs permits version tokens to be locked and unlocked.

These UDFs permit the server's list of version tokens to be created, changed, removed, and inspected:

  • version_tokens_set() completely replaces the current list and assigns a new list. The argument is a semicolon-separated list of name=value pairs.

  • version_tokens_edit() enables partial modifications to the current list. It can add new tokens or change the values of existing tokens. The argument is a semicolon-separated list of name=value pairs.

  • version_tokens_delete() deletes tokens from the current list. The argument is a semicolon-separated list of token names.

  • version_tokens_show() displays the current token list. It takes no argument.

Each of those functions, if successful, returns a binary string indicating what action occurred. The following example establishes the server token list, modifies it by adding a new token, deletes some tokens, and displays the resulting token list:

mysql> SELECT version_tokens_set('tok1=a;tok2=b');
+-------------------------------------+
| version_tokens_set('tok1=a;tok2=b') |
+-------------------------------------+
| 2 version tokens set.               |
+-------------------------------------+
mysql> SELECT version_tokens_edit('tok3=c');
+-------------------------------+
| version_tokens_edit('tok3=c') |
+-------------------------------+
| 1 version tokens updated.     |
+-------------------------------+
mysql> SELECT version_tokens_delete('tok2;tok1');
+------------------------------------+
| version_tokens_delete('tok2;tok1') |
+------------------------------------+
| 2 version tokens deleted.          |
+------------------------------------+
mysql> SELECT version_tokens_show();
+-----------------------+
| version_tokens_show() |
+-----------------------+
| tok3=c;               |
+-----------------------+

Warnings occur if a token list is malformed:

mysql> SELECT version_tokens_set('tok1=a; =c');
+----------------------------------+
| version_tokens_set('tok1=a; =c') |
+----------------------------------+
| 1 version tokens set.            |
+----------------------------------+
1 row in set, 1 warning (0.00 sec)

mysql> SHOW WARNINGS\G
*************************** 1. row ***************************
  Level: Warning
   Code: 42000
Message: Invalid version token pair encountered. The list provided
         is only partially updated.
1 row in set (0.00 sec)

As mentioned previously, version tokens are defined using a semicolon-separated list of name=value pairs. Consider this invocation of version_tokens_set():

mysql> SELECT version_tokens_set('tok1=b;;; tok2= a = b ; tok1 = 1\'2 3"4')
+---------------------------------------------------------------+
| version_tokens_set('tok1=b;;; tok2= a = b ; tok1 = 1\'2 3"4') |
+---------------------------------------------------------------+
| 3 version tokens set.                                         |
+---------------------------------------------------------------+

Version Tokens interprets the argument as follows:

  • Whitespace around names and values is ignored. Whitespace within names and values is permitted. (For version_tokens_delete(), which takes a list of names without values, whitespace around names is ignored.)

  • There is no quoting mechanism.

  • Order of tokens is not significant except that if a token list contains multiple instances of a given token name, the last value takes precedence over earlier values.

Given those rules, the preceding version_tokens_set() call results in a token list with two tokens: tok1 has the value 1'2 3"4, and tok2 has the value a = b. To verify this, call version_tokens_show():

mysql> SELECT version_tokens_show();
+--------------------------+
| version_tokens_show()    |
+--------------------------+
| tok2=a = b;tok1=1'2 3"4; |
+--------------------------+

If the token list contains two tokens, why did version_tokens_set() return the value 3 version tokens set? That occurred because the original token list contained two definitions for tok1, and the second definition replaced the first.

The Version Tokens token-manipulation UDFs place these constraints on token names and values:

  • Token names cannot contain = or ; characters and have a maximum length of 64 characters.

  • Token values cannot contain ; characters. Length of values is constrained by the value of the max_allowed_packet system variable.

  • Version Tokens treats token names and values as binary strings, so comparisons are case-sensitive.

Version Tokens also includes a set of UDFs enabling tokens to be locked and unlocked:

Each locking function returns nonzero for success. Otherwise, an error occurs:

mysql> SELECT version_tokens_lock_shared('lock1', 'lock2', 0);
+-------------------------------------------------+
| version_tokens_lock_shared('lock1', 'lock2', 0) |
+-------------------------------------------------+
|                                               1 |
+-------------------------------------------------+

mysql> SELECT version_tokens_lock_shared(NULL, 0);
ERROR 3131 (42000): Incorrect locking service lock name '(null)'.

Locking using Version Tokens locking functions is advisory; applications must agree to cooperate.

It is possible to lock nonexisting token names. This does not create the tokens.

Note

Version Tokens locking functions are based on the locking service described at Section 28.3.1, “The Locking Service”, and thus have the same semantics for shared and exclusive locks. (Version Tokens uses the locking service routines built into the server, not the locking service UDF interface, so those UDFs need not be installed to use Version Tokens.) Locks acquired by Version Tokens use a locking service namespace of version_token_locks. Locking service locks can be monitored using the Performance Schema, so this is also true for Version Tokens locks. For details, see Section 28.3.1.2.3, “Locking Service Monitoring”.

For the Version Tokens locking functions, token name arguments are used exactly as specified. Surrounding whitespace is not ignored and = and ; characters are permitted. This is because Version Tokens simply passes the token names to be locked as is to the locking service.

5.5.5.4 Version Tokens Reference

The following discussion serves as a reference to these Version Tokens components:

Version Tokens Functions

The Version Tokens plugin library includes several user-defined functions. One set of UDFs permits the server's list of version tokens to be manipulated and inspected. Another set of UDFs permits version tokens to be locked and unlocked. The SUPER privilege is required to invoke any Version Tokens UDF.

The following UDFs permit the server's list of version tokens to be created, changed, removed, and inspected. Interpretation of name_list and token_list arguments (including whitespace handling) occurs as described in Section 5.5.5.3, “Using Version Tokens”, which provides details about the syntax for specifying tokens, as well as additional examples.

  • version_tokens_delete(name_list)

    Deletes tokens from the server's list of version tokens using the name_list argument and returns a binary string that indicates the outcome of the operation. name_list is a semicolon-separated list of version token names to delete.

    mysql> SELECT version_tokens_delete('tok1;tok3');
    +------------------------------------+
    | version_tokens_delete('tok1;tok3') |
    +------------------------------------+
    | 2 version tokens deleted.          |
    +------------------------------------+
    

    An argument of NULL is treated as an empty string, which has no effect on the token list.

    version_tokens_delete() deletes the tokens named in its argument, if they exist. (It is not an error to delete nonexisting tokens.) To clear the token list entirely without knowing which tokens are in the list, pass NULL or a string containing no tokens to version_tokens_set():

    mysql> SELECT version_tokens_set(NULL);
    +------------------------------+
    | version_tokens_set(NULL)     |
    +------------------------------+
    | Version tokens list cleared. |
    +------------------------------+
    mysql> SELECT version_tokens_set('');
    +------------------------------+
    | version_tokens_set('')       |
    +------------------------------+
    | Version tokens list cleared. |
    +------------------------------+
    
  • version_tokens_edit(token_list)

    Modifies the server's list of version tokens using the token_list argument and returns a binary string that indicates the outcome of the operation. token_list is a semicolon-separated list of name=value pairs specifying the name of each token to be defined and its value. If a token exists, its value is updated with the given value. If a token does not exist, it is created with the given value. If the argument is NULL or a string containing no tokens, the token list remains unchanged.

    mysql> SELECT version_tokens_set('tok1=value1;tok2=value2');
    +-----------------------------------------------+
    | version_tokens_set('tok1=value1;tok2=value2') |
    +-----------------------------------------------+
    | 2 version tokens set.                         |
    +-----------------------------------------------+
    mysql> SELECT version_tokens_edit('tok2=new_value2;tok3=new_value3');
    +--------------------------------------------------------+
    | version_tokens_edit('tok2=new_value2;tok3=new_value3') |
    +--------------------------------------------------------+
    | 2 version tokens updated.                              |
    +--------------------------------------------------------+
    
  • version_tokens_set(token_list)

    Replaces the server's list of version tokens with the tokens defined in the token_list argument and returns a binary string that indicates the outcome of the operation. token_list is a semicolon-separated list of name=value pairs specifying the name of each token to be defined and its value. If the argument is NULL or a string containing no tokens, the token list is cleared.

    mysql> SELECT version_tokens_set('tok1=value1;tok2=value2');
    +-----------------------------------------------+
    | version_tokens_set('tok1=value1;tok2=value2') |
    +-----------------------------------------------+
    | 2 version tokens set.                         |
    +-----------------------------------------------+
    
  • version_tokens_show()

    Returns the server's list of version tokens as a binary string containing a semicolon-separated list of name=value pairs.

    mysql> SELECT version_tokens_show();
    +--------------------------+
    | version_tokens_show()    |
    +--------------------------+
    | tok2=value2;tok1=value1; |
    +--------------------------+
    

The following UDFs permit version tokens to be locked and unlocked:

  • version_tokens_lock_exclusive(token_name[, token_name] ..., timeout)

    Acquires exclusive locks on one or more version tokens, specified by name as strings, timing out with an error if the locks are not acquired within the given timeout value.

    mysql> SELECT version_tokens_lock_exclusive('lock1', 'lock2', 10);
    +-----------------------------------------------------+
    | version_tokens_lock_exclusive('lock1', 'lock2', 10) |
    +-----------------------------------------------------+
    |                                                   1 |
    +-----------------------------------------------------+
    
  • version_tokens_lock_shared(token_name[, token_name] ..., timeout)

    Acquires shared locks on one or more version tokens, specified by name as strings, timing out with an error if the locks are not acquired within the given timeout value.

    mysql> SELECT version_tokens_lock_shared('lock1', 'lock2', 10);
    +--------------------------------------------------+
    | version_tokens_lock_shared('lock1', 'lock2', 10) |
    +--------------------------------------------------+
    |                                                1 |
    +--------------------------------------------------+
    
  • version_tokens_unlock()

    Releases all locks that were acquired within the current session using version_tokens_lock_exclusive() and version_tokens_lock_shared().

    mysql> SELECT version_tokens_unlock();
    +-------------------------+
    | version_tokens_unlock() |
    +-------------------------+
    |                       1 |
    +-------------------------+
    

The locking functions share these characteristics:

  • The return value is nonzero for success. Otherwise, an error occurs.

  • Token names are strings.

  • In contrast to argument handling for the UDFs that manipulate the server token list, whitespace surrounding token name arguments is not ignored and = and ; characters are permitted.

  • It is possible to lock nonexisting token names. This does not create the tokens.

  • Timeout values are nonnegative integers representing the time in seconds to wait to acquire locks before timing out with an error. If the timeout is 0, there is no waiting and the function produces an error if locks cannot be acquired immediately.

  • Version Tokens locking functions are based on the locking service described at Section 28.3.1, “The Locking Service”.

Version Tokens System Variables

Version Tokens supports the following system variables. These variables are unavailable unless the Version Tokens plugin is installed (see Section 5.5.5.2, “Installing or Uninstalling Version Tokens”).

System variables:

  • version_tokens_session

    Property Value
    Command-Line Format --version-tokens-session=value
    System Variable version_tokens_session
    Scope Global, Session
    Dynamic Yes
    Type String
    Default Value NULL

    The session value of this variable specifies the client version token list and indicates the tokens that the client session requires the server version token list to have.

    If the version_tokens_session variable is NULL (the default) or has an empty value, any server version token list matches. (In effect, an empty value disables matching requirements.)

    If the version_tokens_session variable has a nonempty value, any mismatch between its value and the server version token list results in an error for any statement the session sends to the server. A mismatch occurs under these conditions:

    It is not a mismatch for the server version token list to include a token not named in the version_tokens_session value.

    Suppose that a management application has set the server token list as follows:

    mysql> SELECT version_tokens_set('tok1=a;tok2=b;tok3=c');
    +--------------------------------------------+
    | version_tokens_set('tok1=a;tok2=b;tok3=c') |
    +--------------------------------------------+
    | 3 version tokens set.                      |
    +--------------------------------------------+
    

    A client registers the tokens it requires the server to match by setting its version_tokens_session value. Then, for each subsequent statement sent by the client, the server checks its token list against the client version_tokens_session value and produces an error if there is a mismatch:

    mysql> SET @@SESSION.version_tokens_session = 'tok1=a;tok2=b';
    mysql> SELECT 1;
    +---+
    | 1 |
    +---+
    | 1 |
    +---+
    
    mysql> SET @@SESSION.version_tokens_session = 'tok1=b';
    mysql> SELECT 1;
    ERROR 3136 (42000): Version token mismatch for tok1. Correct value a
    

    The first SELECT succeeds because the client tokens tok1 and tok2 are present in the server token list and each token has the same value in the server list. The second SELECT fails because, although tok1 is present in the server token list, it has a different value than specified by the client.

    At this point, any statement sent by the client fails, unless the server token list changes such that it matches again. Suppose that the management application changes the server token list as follows:

    mysql> SELECT version_tokens_edit('tok1=b');
    +-------------------------------+
    | version_tokens_edit('tok1=b') |
    +-------------------------------+
    | 1 version tokens updated.     |
    +-------------------------------+
    mysql> SELECT version_tokens_show();
    +-----------------------+
    | version_tokens_show() |
    +-----------------------+
    | tok3=c;tok1=b;tok2=b; |
    +-----------------------+
    

    Now the client version_tokens_session value matches the server token list and the client can once again successfully execute statements:

    mysql> SELECT 1;
    +---+
    | 1 |
    +---+
    | 1 |
    +---+
    
  • version_tokens_session_number

    Property Value
    Command-Line Format --version-tokens-session-number=#
    System Variable version_tokens_session_number
    Scope Global, Session
    Dynamic No
    Type Integer
    Default Value 0

    This variable is for internal use.

5.6 MySQL Server User-Defined Functions

MySQL Server enables user-defined functions (UDFs) to be created and loaded into the server to extend server capabilities. Server capabilities can be implemented in whole or in part using UDFs. In addition, you can write your own UDFs.

The following sections describe how to install and uninstall UDFs, and how to determine at runtime which UDFs are installed and obtain information about them. For information about writing UDFs, see Section 28.4, “Adding Functions to MySQL”.

5.6.1 Installing and Uninstalling User-Defined Functions

User-defined functions (UDFs) must be loaded into the server before they can be used. MySQL supports UDF loading at runtime.

To load a UDF, use the CREATE FUNCTION statement. For example:

CREATE FUNCTION metaphon
  RETURNS STRING
  SONAME 'udf_example.so';

The UDF file base name depends on your platform. Common suffixes are .so for Unix and Unix-like systems, .dll for Windows.

While a UDF is loaded, information about it is available from the mysql.func system table. See Section 5.6.2, “Obtaining User-Defined Function Information”.

CREATE FUNCTION also registers the UDF in the mysql.func system table to cause the server to load it on subsequent restarts. For this reason, CREATE FUNCTION requires the INSERT privilege for the mysql system database.

To remove a UDF, use the DROP FUNCTION statement. For example:

DROP FUNCTION metaphon;

DROP FUNCTION unloads the UDF and removes it from the mysql.func system table. For this reason, DROP FUNCTION requires the DELETE privilege for the mysql system database. With the UDF no longer registered in the table, the server does not load the UDF automatically for subsequent restarts.

You cannnot use CREATE FUNCTION to reinstall a function that has previously been installed. To reinstall a function, first remove it with DROP FUNCTION, then install it again with CREATE FUNCTION. You would need to do this, for example, if you upgrade to a new version of MySQL that provides an updated implementation of the function, or you recompile a new version of a function that you have written. Otherwise, the server continues to use the old version.

If the server is started with the --skip-grant-tables option, it does not consult the mysql.func table and does not load the UDFs listed there.

5.6.2 Obtaining User-Defined Function Information

The func table in the mysql system database shows which UDFs have been registered with CREATE FUNCTION:

SELECT * FROM mysql.func;

The func table has these columns:

  • name

    The UDF name as referred to in SQL statements.

  • ret

    The UDF return value type. Permitted values are 0 (STRING), 1 (REAL), 2 (INTEGER), 3 (ROW), or 4 (DECIMAL).

  • dl

    The name of the UDF library file containing the executable UDF code. The file is located in the directory named by the plugin_dir system variable.

  • type

    The UDF type, either function (scalar) or aggregate.

5.7 Running Multiple MySQL Instances on One Machine

In some cases, you might want to run multiple instances of MySQL on a single machine. You might want to test a new MySQL release while leaving an existing production setup undisturbed. Or you might want to give different users access to different mysqld servers that they manage themselves. (For example, you might be an Internet Service Provider that wants to provide independent MySQL installations for different customers.)

It is possible to use a different MySQL server binary per instance, or use the same binary for multiple instances, or any combination of the two approaches. For example, you might run a server from MySQL 5.6 and one from MySQL 5.7, to see how different versions handle a given workload. Or you might run multiple instances of the current production version, each managing a different set of databases.

Whether or not you use distinct server binaries, each instance that you run must be configured with unique values for several operating parameters. This eliminates the potential for conflict between instances. Parameters can be set on the command line, in option files, or by setting environment variables. See Section 4.2.2, “Specifying Program Options”. To see the values used by a given instance, connect to it and execute a SHOW VARIABLES statement.

The primary resource managed by a MySQL instance is the data directory. Each instance should use a different data directory, the location of which is specified using the --datadir=dir_name option. For methods of configuring each instance with its own data directory, and warnings about the dangers of failing to do so, see Section 5.7.1, “Setting Up Multiple Data Directories”.

In addition to using different data directories, several other options must have different values for each server instance:

  • --port=port_num

    --port controls the port number for TCP/IP connections. Alternatively, if the host has multiple network addresses, you can set the bind_address system variable to cause each server to listen to a different address.

  • --socket={file_name|pipe_name}

    --socket controls the Unix socket file path on Unix or the named-pipe name on Windows. On Windows, it is necessary to specify distinct pipe names only for those servers configured to permit named-pipe connections.

  • --shared-memory-base-name=name

    This option is used only on Windows. It designates the shared-memory name used by a Windows server to permit clients to connect using shared memory. It is necessary to specify distinct shared-memory names only for those servers configured to permit shared-memory connections.

  • --pid-file=file_name

    This option indicates the path name of the file in which the server writes its process ID.

If you use the following log file options, their values must differ for each server:

For further discussion of log file options, see Section 5.4, “MySQL Server Logs”.

To achieve better performance, you can specify the following option differently for each server, to spread the load between several physical disks:

Having different temporary directories also makes it easier to determine which MySQL server created any given temporary file.

If you have multiple MySQL installations in different locations, you can specify the base directory for each installation with the --basedir=dir_name option. This causes each instance to automatically use a different data directory, log files, and PID file because the default for each of those parameters is relative to the base directory. In that case, the only other options you need to specify are the --socket and --port options. Suppose that you install different versions of MySQL using tar file binary distributions. These install in different locations, so you can start the server for each installation using the command bin/mysqld_safe under its corresponding base directory. mysqld_safe determines the proper --basedir option to pass to mysqld, and you need specify only the --socket and --port options to mysqld_safe.

As discussed in the following sections, it is possible to start additional servers by specifying appropriate command options or by setting environment variables. However, if you need to run multiple servers on a more permanent basis, it is more convenient to use option files to specify for each server those option values that must be unique to it. The --defaults-file option is useful for this purpose.

5.7.1 Setting Up Multiple Data Directories

Each MySQL Instance on a machine should have its own data directory. The location is specified using the --datadir=dir_name option.

There are different methods of setting up a data directory for a new instance:

  • Create a new data directory.

  • Copy an existing data directory.

The following discussion provides more detail about each method.

Warning

Normally, you should never have two servers that update data in the same databases. This may lead to unpleasant surprises if your operating system does not support fault-free system locking. If (despite this warning) you run multiple servers using the same data directory and they have logging enabled, you must use the appropriate options to specify log file names that are unique to each server. Otherwise, the servers try to log to the same files.

Even when the preceding precautions are observed, this kind of setup works only with MyISAM and MERGE tables, and not with any of the other storage engines. Also, this warning against sharing a data directory among servers always applies in an NFS environment. Permitting multiple MySQL servers to access a common data directory over NFS is a very bad idea. The primary problem is that NFS is the speed bottleneck. It is not meant for such use. Another risk with NFS is that you must devise a way to ensure that two or more servers do not interfere with each other. Usually NFS file locking is handled by the lockd daemon, but at the moment there is no platform that performs locking 100% reliably in every situation.

Create a New Data Directory

With this method, the data directory will be in the same state as when you first install MySQL. It will have the default set of MySQL accounts and no user data.

On Unix, initialize the data directory. See Section 2.10, “Postinstallation Setup and Testing”.

On Windows, the data directory is included in the MySQL distribution:

  • MySQL Zip archive distributions for Windows contain an unmodified data directory. You can unpack such a distribution into a temporary location, then copy it data directory to where you are setting up the new instance.

  • Windows MSI package installers create and set up the data directory that the installed server will use, but also create a pristine template data directory named data under the installation directory. After an installation has been performed using an MSI package, the template data directory can be copied to set up additional MySQL instances.

Copy an Existing Data Directory

With this method, any MySQL accounts or user data present in the data directory are carried over to the new data directory.

  1. Stop the existing MySQL instance using the data directory. This must be a clean shutdown so that the instance flushes any pending changes to disk.

  2. Copy the data directory to the location where the new data directory should be.

  3. Copy the my.cnf or my.ini option file used by the existing instance. This serves as a basis for the new instance.

  4. Modify the new option file so that any pathnames referring to the original data directory refer to the new data directory. Also, modify any other options that must be unique per instance, such as the TCP/IP port number and the log files. For a list of parameters that must be unique per instance, see Section 5.7, “Running Multiple MySQL Instances on One Machine”.

  5. Start the new instance, telling it to use the new option file.

5.7.2 Running Multiple MySQL Instances on Windows

You can run multiple servers on Windows by starting them manually from the command line, each with appropriate operating parameters, or by installing several servers as Windows services and running them that way. General instructions for running MySQL from the command line or as a service are given in Section 2.3, “Installing MySQL on Microsoft Windows”. The following sections describe how to start each server with different values for those options that must be unique per server, such as the data directory. These options are listed in Section 5.7, “Running Multiple MySQL Instances on One Machine”.

5.7.2.1 Starting Multiple MySQL Instances at the Windows Command Line

The procedure for starting a single MySQL server manually from the command line is described in Section 2.3.4.6, “Starting MySQL from the Windows Command Line”. To start multiple servers this way, you can specify the appropriate options on the command line or in an option file. It is more convenient to place the options in an option file, but it is necessary to make sure that each server gets its own set of options. To do this, create an option file for each server and tell the server the file name with a --defaults-file option when you run it.

Suppose that you want to run one instance of mysqld on port 3307 with a data directory of C:\mydata1, and another instance on port 3308 with a data directory of C:\mydata2. Use this procedure:

  1. Make sure that each data directory exists, including its own copy of the mysql database that contains the grant tables.

  2. Create two option files. For example, create one file named C:\my-opts1.cnf that looks like this:

    [mysqld]
    datadir = C:/mydata1
    port = 3307

    Create a second file named C:\my-opts2.cnf that looks like this:

    [mysqld]
    datadir = C:/mydata2
    port = 3308
  3. Use the --defaults-file option to start each server with its own option file:

    C:\> C:\mysql\bin\mysqld --defaults-file=C:\my-opts1.cnf
    C:\> C:\mysql\bin\mysqld --defaults-file=C:\my-opts2.cnf
    

    Each server starts in the foreground (no new prompt appears until the server exits later), so you will need to issue those two commands in separate console windows.

To shut down the servers, connect to each using the appropriate port number:

C:\> C:\mysql\bin\mysqladmin --port=3307 --host=127.0.0.1 --user=root --password shutdown
C:\> C:\mysql\bin\mysqladmin --port=3308 --host=127.0.0.1 --user=root --password shutdown

Servers configured as just described permit clients to connect over TCP/IP. If your version of Windows supports named pipes and you also want to permit named-pipe connections, specify options that enable the named pipe and specify its name. Each server that supports named-pipe connections must use a unique pipe name. For example, the C:\my-opts1.cnf file might be written like this:

[mysqld]
datadir = C:/mydata1
port = 3307
enable-named-pipe
socket = mypipe1

Modify C:\my-opts2.cnf similarly for use by the second server. Then start the servers as described previously.

A similar procedure applies for servers that you want to permit shared-memory connections. Enable such connections by starting the server with the shared_memory system variable enabled and specify a unique shared-memory name for each server by setting the shared_memory_base_name system variable.

5.7.2.2 Starting Multiple MySQL Instances as Windows Services

On Windows, a MySQL server can run as a Windows service. The procedures for installing, controlling, and removing a single MySQL service are described in Section 2.3.4.8, “Starting MySQL as a Windows Service”.

To set up multiple MySQL services, you must make sure that each instance uses a different service name in addition to the other parameters that must be unique per instance.

For the following instructions, suppose that you want to run the mysqld server from two different versions of MySQL that are installed at C:\mysql-5.5.9 and C:\mysql-5.7.30, respectively. (This might be the case if you are running 5.5.9 as your production server, but also want to conduct tests using 5.7.30.)

To install MySQL as a Windows service, use the --install or --install-manual option. For information about these options, see Section 2.3.4.8, “Starting MySQL as a Windows Service”.

Based on the preceding information, you have several ways to set up multiple services. The following instructions describe some examples. Before trying any of them, shut down and remove any existing MySQL services.

  • Approach 1: Specify the options for all services in one of the standard option files. To do this, use a different service name for each server. Suppose that you want to run the 5.5.9 mysqld using the service name of mysqld1 and the 5.7.30 mysqld using the service name mysqld2. In this case, you can use the [mysqld1] group for 5.5.9 and the [mysqld2] group for 5.7.30. For example, you can set up C:\my.cnf like this:

    # options for mysqld1 service
    [mysqld1]
    basedir = C:/mysql-5.5.9
    port = 3307
    enable-named-pipe
    socket = mypipe1
    
    # options for mysqld2 service
    [mysqld2]
    basedir = C:/mysql-5.7.30
    port = 3308
    enable-named-pipe
    socket = mypipe2

    Install the services as follows, using the full server path names to ensure that Windows registers the correct executable program for each service:

    C:\> C:\mysql-5.5.9\bin\mysqld --install mysqld1
    C:\> C:\mysql-5.7.30\bin\mysqld --install mysqld2
    

    To start the services, use the services manager, or NET START or SC START with the appropriate service names:

    C:\> SC START mysqld1
    C:\> SC START mysqld2
    

    To stop the services, use the services manager, or use NET STOP or SC STOP with the appropriate service names:

    C:\> SC STOP mysqld1
    C:\> SC STOP mysqld2
    
  • Approach 2: Specify options for each server in separate files and use --defaults-file when you install the services to tell each server what file to use. In this case, each file should list options using a [mysqld] group.

    With this approach, to specify options for the 5.5.9 mysqld, create a file C:\my-opts1.cnf that looks like this:

    [mysqld]
    basedir = C:/mysql-5.5.9
    port = 3307
    enable-named-pipe
    socket = mypipe1

    For the 5.7.30 mysqld, create a file C:\my-opts2.cnf that looks like this:

    [mysqld]
    basedir = C:/mysql-5.7.30
    port = 3308
    enable-named-pipe
    socket = mypipe2

    Install the services as follows (enter each command on a single line):

    C:\> C:\mysql-5.5.9\bin\mysqld --install mysqld1
               --defaults-file=C:\my-opts1.cnf
    C:\> C:\mysql-5.7.30\bin\mysqld --install mysqld2
               --defaults-file=C:\my-opts2.cnf
    

    When you install a MySQL server as a service and use a --defaults-file option, the service name must precede the option.

    After installing the services, start and stop them the same way as in the preceding example.

To remove multiple services, use SC DELETE mysqld_service_name for each one. Alternatively, use mysqld --remove for each one, specifying a service name following the --remove option. If the service name is the default (MySQL), you can omit it when using mysqld --remove.

5.7.3 Running Multiple MySQL Instances on Unix

Note

The discussion here uses mysqld_safe to launch multiple instances of MySQL. For MySQL installation using an RPM distribution, server startup and shutdown is managed by systemd on several Linux platforms. On these platforms, mysqld_safe is not installed because it is unnecessary. For information about using systemd to handle multiple MySQL instances, see Section 2.5.10, “Managing MySQL Server with systemd”.

One way is to run multiple MySQL instances on Unix is to compile different servers with different default TCP/IP ports and Unix socket files so that each one listens on different network interfaces. Compiling in different base directories for each installation also results automatically in a separate, compiled-in data directory, log file, and PID file location for each server.

Assume that an existing 5.6 server is configured for the default TCP/IP port number (3306) and Unix socket file (/tmp/mysql.sock). To configure a new 5.7.30 server to have different operating parameters, use a CMake command something like this:

shell> cmake . -DMYSQL_TCP_PORT=port_number \
             -DMYSQL_UNIX_ADDR=file_name \
             -DCMAKE_INSTALL_PREFIX=/usr/local/mysql-5.7.30

Here, port_number and file_name must be different from the default TCP/IP port number and Unix socket file path name, and the CMAKE_INSTALL_PREFIX value should specify an installation directory different from the one under which the existing MySQL installation is located.

If you have a MySQL server listening on a given port number, you can use the following command to find out what operating parameters it is using for several important configurable variables, including the base directory and Unix socket file name:

shell> mysqladmin --host=host_name --port=port_number variables

With the information displayed by that command, you can tell what option values not to use when configuring an additional server.

If you specify localhost as the host name, mysqladmin defaults to using a Unix socket file connection rather than TCP/IP. To explicitly specify the connection protocol, use the --protocol={TCP|SOCKET|PIPE|MEMORY} option.

You need not compile a new MySQL server just to start with a different Unix socket file and TCP/IP port number. It is also possible to use the same server binary and start each invocation of it with different parameter values at runtime. One way to do so is by using command-line options:

shell> mysqld_safe --socket=file_name --port=port_number

To start a second server, provide different --socket and --port option values, and pass a --datadir=dir_name option to mysqld_safe so that the server uses a different data directory.

Alternatively, put the options for each server in a different option file, then start each server using a --defaults-file option that specifies the path to the appropriate option file. For example, if the option files for two server instances are named /usr/local/mysql/my.cnf and /usr/local/mysql/my.cnf2, start the servers like this: command:

shell> mysqld_safe --defaults-file=/usr/local/mysql/my.cnf
shell> mysqld_safe --defaults-file=/usr/local/mysql/my.cnf2

Another way to achieve a similar effect is to use environment variables to set the Unix socket file name and TCP/IP port number:

shell> MYSQL_UNIX_PORT=/tmp/mysqld-new.sock
shell> MYSQL_TCP_PORT=3307
shell> export MYSQL_UNIX_PORT MYSQL_TCP_PORT
shell> mysqld --initialize --user=mysql
...set root password...
shell> mysqld_safe --datadir=/path/to/datadir &

This is a quick way of starting a second server to use for testing. The nice thing about this method is that the environment variable settings apply to any client programs that you invoke from the same shell. Thus, connections for those clients are automatically directed to the second server.

Section 4.9, “Environment Variables”, includes a list of other environment variables you can use to affect MySQL programs.

On Unix, the mysqld_multi script provides another way to start multiple servers. See Section 4.3.4, “mysqld_multi — Manage Multiple MySQL Servers”.

5.7.4 Using Client Programs in a Multiple-Server Environment

To connect with a client program to a MySQL server that is listening to different network interfaces from those compiled into your client, you can use one of the following methods:

  • Start the client with --host=host_name --port=port_number to connect using TCP/IP to a remote server, with --host=127.0.0.1 --port=port_number to connect using TCP/IP to a local server, or with --host=localhost --socket=file_name to connect to a local server using a Unix socket file or a Windows named pipe.

  • Start the client with --protocol=TCP to connect using TCP/IP, --protocol=SOCKET to connect using a Unix socket file, --protocol=PIPE to connect using a named pipe, or --protocol=MEMORY to connect using shared memory. For TCP/IP connections, you may also need to specify --host and --port options. For the other types of connections, you may need to specify a --socket option to specify a Unix socket file or Windows named-pipe name, or a --shared-memory-base-name option to specify the shared-memory name. Shared-memory connections are supported only on Windows.

  • On Unix, set the MYSQL_UNIX_PORT and MYSQL_TCP_PORT environment variables to point to the Unix socket file and TCP/IP port number before you start your clients. If you normally use a specific socket file or port number, you can place commands to set these environment variables in your .login file so that they apply each time you log in. See Section 4.9, “Environment Variables”.

  • Specify the default Unix socket file and TCP/IP port number in the [client] group of an option file. For example, you can use C:\my.cnf on Windows, or the .my.cnf file in your home directory on Unix. See Section 4.2.2.2, “Using Option Files”.

  • In a C program, you can specify the socket file or port number arguments in the mysql_real_connect() call. You can also have the program read option files by calling mysql_options(). See Section 27.7.6, “C API Function Descriptions”.

  • If you are using the Perl DBD::mysql module, you can read options from MySQL option files. For example:

    $dsn = "DBI:mysql:test;mysql_read_default_group=client;"
            . "mysql_read_default_file=/usr/local/mysql/data/my.cnf";
    $dbh = DBI->connect($dsn, $user, $password);

    See Section 27.9, “MySQL Perl API”.

    Other programming interfaces may provide similar capabilities for reading option files.

5.8 Tracing mysqld Using DTrace

Support for DTrace is deprecated in MySQL 5.7 and is removed in MySQL 8.0.

The DTrace probes in the MySQL server are designed to provide information about the execution of queries within MySQL and the different areas of the system being utilized during that process. The organization and triggering of the probes means that the execution of an entire query can be monitored with one level of probes (query-start and query-done) but by monitoring other probes you can get successively more detailed information about the execution of the query in terms of the locks used, sort methods and even row-by-row and storage-engine level execution information.

The DTrace probes are organized so that you can follow the entire query process, from the point of connection from a client, through the query execution, row-level operations, and back out again. You can think of the probes as being fired within a specific sequence during a typical client connect/execute/disconnect sequence, as shown in the following figure.

Figure 5.1 DTrace Probe Sequence

Example of a DTrace probe sequence during a typical client connect, execute, disconnect sequence.

Global information is provided in the arguments to the DTrace probes at various levels. Global information, that is, the connection ID and user/host and where relevant the query string, is provided at key levels (connection-start, command-start, query-start, and query-exec-start). As you go deeper into the probes, it is assumed either you are only interested in the individual executions (row-level probes provide information on the database and table name only), or that you will combine the row-level probes with the notional parent probes to provide the information about a specific query. Examples of this will be given as the format and arguments of each probe are provided.

MySQL includes support for DTrace probes on these platforms:

  • Solaris 10 Update 5 (Solaris 5/08) on SPARC, x86 and x86_64 platforms

  • OS X 10.4 and higher

  • Oracle Linux 6 and higher with UEK kernel (as of MySQL 5.7.5)

Enabling the probes should be automatic on these platforms. To explicitly enable or disable the probes during building, use the -DENABLE_DTRACE=1 or -DENABLE_DTRACE=0 option to CMake.

If a non-Solaris platform includes DTrace support, building mysqld on that platform will include DTrace support.

Additional Resources

5.8.1 mysqld DTrace Probe Reference

MySQL supports the following static probes, organized into groups of functionality.

Table 5.5 MySQL DTrace Probes

Group Probes
Connection connection-start, connection-done
Command command-start, command-done
Query query-start, query-done
Query Parsing query-parse-start, query-parse-done
Query Cache query-cache-hit, query-cache-miss
Query Execution query-exec-start, query-exec-done
Row Level insert-row-start, insert-row-done
update-row-start, update-row-done
delete-row-start, delete-row-done
Row Reads read-row-start, read-row-done
Index Reads index-read-row-start, index-read-row-done
Lock handler-rdlock-start, handler-rdlock-done
handler-wrlock-start, handler-wrlock-done
handler-unlock-start, handler-unlock-done
Filesort filesort-start, filesort-done
Statement select-start, select-done
insert-start, insert-done
insert-select-start, insert-select-done
update-start, update-done
multi-update-start, multi-update-done
delete-start, delete-done
multi-delete-start, multi-delete-done
Network net-read-start, net-read-done, net-write-start, net-write-done
Keycache keycache-read-start, keycache-read-block, keycache-read-done, keycache-read-hit, keycache-read-miss, keycache-write-start, keycache-write-block, keycache-write-done

Note

When extracting the argument data from the probes, each argument is available as argN, starting with arg0. To identify each argument within the definitions they are provided with a descriptive name, but you must access the information using the corresponding argN parameter.

5.8.1.1 Connection Probes

The connection-start and connection-done probes enclose a connection from a client, regardless of whether the connection is through a socket or network connection.

connection-start(connectionid, user, host)
connection-done(status, connectionid)
  • connection-start: Triggered after a connection and successful login/authentication have been completed by a client. The arguments contain the connection information:

    • connectionid: An unsigned long containing the connection ID. This is the same as the process ID shown as the Id value in the output from SHOW PROCESSLIST.

    • user: The username used when authenticating. The value will be blank for the anonymous user.

    • host: The host of the client connection. For a connection made using Unix sockets, the value will be blank.

  • connection-done: Triggered just as the connection to the client has been closed. The arguments are:

    • status: The status of the connection when it was closed. A logout operation will have a value of 0; any other termination of the connection has a nonzero value.

    • connectionid: The connection ID of the connection that was closed.

The following D script will quantify and summarize the average duration of individual connections, and provide a count, dumping the information every 60 seconds:

#!/usr/sbin/dtrace -s


mysql*:::connection-start
{
  self->start = timestamp;
}

mysql*:::connection-done
/self->start/
{
  @ = quantize(((timestamp - self->start)/1000000));
  self->start = 0;
}

tick-60s
{
  printa(@);
}

When executed on a server with a large number of clients you might see output similar to this:

  1  57413                        :tick-60s

           value  ------------- Distribution ------------- count
              -1 |                                         0
               0 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ 30011
               1 |                                         59
               2 |                                         5
               4 |                                         20
               8 |                                         29
              16 |                                         18
              32 |                                         27
              64 |                                         30
             128 |                                         11
             256 |                                         10
             512 |                                         1
            1024 |                                         6
            2048 |                                         8
            4096 |                                         9
            8192 |                                         8
           16384 |                                         2
           32768 |                                         1
           65536 |                                         1
          131072 |                                         0
          262144 |                                         1
524288 |                                         0        

5.8.1.2 Command Probes

The command probes are executed before and after a client command is executed, including any SQL statement that might be executed during that period. Commands include operations such as the initialization of the DB, use of the COM_CHANGE_USER operation (supported by the MySQL protocol), and manipulation of prepared statements. Many of these commands are used only by the MySQL client API from various connectors such as PHP and Java.

command-start(connectionid, command, user, host)
command-done(status)
  • command-start: Triggered when a command is submitted to the server.

    • connectionid: The connection ID of the client executing the command.

    • command: An integer representing the command that was executed. Possible values are shown in the following table.

      Value Name Description
      00 COM_SLEEP Internal thread state
      01 COM_QUIT Close connection
      02 COM_INIT_DB Select database (USE ...)
      03 COM_QUERY Execute a query
      04 COM_FIELD_LIST Get a list of fields
      05 COM_CREATE_DB Create a database (deprecated)
      06 COM_DROP_DB Drop a database (deprecated)
      07 COM_REFRESH Refresh connection
      08 COM_SHUTDOWN Shutdown server
      09 COM_STATISTICS Get statistics
      10 COM_PROCESS_INFO Get processes (SHOW PROCESSLIST)
      11 COM_CONNECT Initialize connection
      12 COM_PROCESS_KILL Kill process
      13 COM_DEBUG Get debug information
      14 COM_PING Ping
      15 COM_TIME Internal thread state
      16 COM_DELAYED_INSERT Internal thread state
      17 COM_CHANGE_USER Change user
      18 COM_BINLOG_DUMP Used by a replication slave or mysqlbinlog to initiate a binary log read
      19 COM_TABLE_DUMP Used by a replication slave to get the master table information
      20 COM_CONNECT_OUT Used by a replication slave to log a connection to the server
      21 COM_REGISTER_SLAVE Used by a replication slave during registration
      22 COM_STMT_PREPARE Prepare a statement
      23 COM_STMT_EXECUTE Execute a statement
      24 COM_STMT_SEND_LONG_DATA Used by a client when requesting extended data
      25 COM_STMT_CLOSE Close a prepared statement
      26 COM_STMT_RESET Reset a prepared statement
      27 COM_SET_OPTION Set a server option
      28 COM_STMT_FETCH Fetch a prepared statement
    • user: The user executing the command.

    • host: The client host.

  • command-done: Triggered when the command execution completes. The status argument contains 0 if the command executed successfully, or 1 if the statement was terminated before normal completion.

The command-start and command-done probes are best used when combined with the statement probes to get an idea of overall execution time.

5.8.1.3 Query Probes

The query-start and query-done probes are triggered when a specific query is received by the server and when the query has been completed and the information has been successfully sent to the client.

query-start(query, connectionid, database, user, host)
query-done(status)
  • query-start: Triggered after the query string has been received from the client. The arguments are:

    • query: The full text of the submitted query.

    • connectionid: The connection ID of the client that submitted the query. The connection ID equals the connection ID returned when the client first connects and the Id value in the output from SHOW PROCESSLIST.

    • database: The database name on which the query is being executed.

    • user: The username used to connect to the server.

    • host: The hostname of the client.

  • query-done: Triggered once the query has been executed and the information has been returned to the client. The probe includes a single argument, status, which returns 0 when the query is successfully executed and 1 if there was an error.

You can get a simple report of the execution time for each query using the following D script:

#!/usr/sbin/dtrace -s

#pragma D option quiet

dtrace:::BEGIN
{
   printf("%-20s %-20s %-40s %-9s\n", "Who", "Database", "Query", "Time(ms)");
}

mysql*:::query-start
{
   self->query = copyinstr(arg0);
   self->connid = arg1;
   self->db    = copyinstr(arg2);
   self->who   = strjoin(copyinstr(arg3),strjoin("@",copyinstr(arg4)));
   self->querystart = timestamp;
}

mysql*:::query-done
{
   printf("%-20s %-20s %-40s %-9d\n",self->who,self->db,self->query,
          (timestamp - self->querystart) / 1000000);
}

When executing the above script you should get a basic idea of the execution time of your queries:

shell> ./query.d
Who                  Database             Query                                    Time(ms)
root@localhost       test                 select * from t1 order by i limit 10     0
root@localhost       test                 set global query_cache_size=0            0
root@localhost       test                 select * from t1 order by i limit 10     776
root@localhost       test                 select * from t1 order by i limit 10     773
root@localhost       test                 select * from t1 order by i desc limit 10 795 

5.8.1.4 Query Parsing Probes

The query parsing probes are triggered before the original SQL statement is parsed and when the parsing of the statement and determination of the execution model required to process the statement has been completed:

query-parse-start(query)
query-parse-done(status)
  • query-parse-start: Triggered just before the statement is parsed by the MySQL query parser. The single argument, query, is a string containing the full text of the original query.

  • query-parse-done: Triggered when the parsing of the original statement has been completed. The status is an integer describing the status of the operation. A 0 indicates that the query was successfully parsed. A 1 indicates that the parsing of the query failed.

For example, you could monitor the execution time for parsing a given query using the following D script:

#!/usr/sbin/dtrace -s

#pragma D option quiet

mysql*:::query-parse-start
{
   self->parsestart = timestamp;
   self->parsequery = copyinstr(arg0);
}

mysql*:::query-parse-done
/arg0 == 0/
{
   printf("Parsing %s: %d microseconds\n", self->parsequery,((timestamp - self->parsestart)/1000));
}

mysql*:::query-parse-done
/arg0 != 0/
{
   printf("Error parsing %s: %d microseconds\n", self->parsequery,((timestamp - self->parsestart)/1000));
}

In the above script a predicate is used on query-parse-done so that different output is generated based on the status value of the probe.

When running the script and monitoring the execution:

shell> ./query-parsing.d
Error parsing select from t1 join (t2) on (t1.i = t2.i) order by t1.s,t1.i limit 10: 36 ms
Parsing select * from t1 join (t2) on (t1.i = t2.i) order by t1.s,t1.i limit 10: 176 ms

5.8.1.5 Query Cache Probes

The query cache probes are fired when executing any query. The query-cache-hit query is triggered when a query exists in the query cache and can be used to return the query cache information. The arguments contain the original query text and the number of rows returned from the query cache for the query. If the query is not within the query cache, or the query cache is not enabled, then the query-cache-miss probe is triggered instead.

query-cache-hit(query, rows)
query-cache-miss(query)
  • query-cache-hit: Triggered when the query has been found within the query cache. The first argument, query, contains the original text of the query. The second argument, rows, is an integer containing the number of rows in the cached query.

  • query-cache-miss: Triggered when the query is not found within the query cache. The first argument, query, contains the original text of the query.

The query cache probes are best combined with a probe on the main query so that you can determine the differences in times between using or not using the query cache for specified queries. For example, in the following D script, the query and query cache information are combined into the information output during monitoring:

#!/usr/sbin/dtrace -s

#pragma D option quiet

dtrace:::BEGIN
{
   printf("%-20s %-20s %-40s %2s %-9s\n", "Who", "Database", "Query", "QC", "Time(ms)");
}

mysql*:::query-start
{
   self->query = copyinstr(arg0);
   self->connid = arg1;
   self->db    = copyinstr(arg2);
   self->who   = strjoin(copyinstr(arg3),strjoin("@",copyinstr(arg4)));
   self->querystart = timestamp;
   self->qc = 0;
}

mysql*:::query-cache-hit
{
   self->qc = 1;
}

mysql*:::query-cache-miss
{
   self->qc = 0;
}

mysql*:::query-done
{
   printf("%-20s %-20s %-40s %-2s %-9d\n",self->who,self->db,self->query,(self->qc ? "Y" : "N"),
          (timestamp - self->querystart) / 1000000);
}

When executing the script you can see the effects of the query cache. Initially the query cache is disabled. If you set the query cache size and then execute the query multiple times you should see that the query cache is being used to return the query data:

shell> ./query-cache.d
root@localhost       test                 select * from t1 order by i limit 10     N  1072
root@localhost                            set global query_cache_size=262144       N  0
root@localhost       test                 select * from t1 order by i limit 10     N  781
root@localhost       test                 select * from t1 order by i limit 10     Y  0 

5.8.1.6 Query Execution Probes

The query execution probe is triggered when the actual execution of the query starts, after the parsing and checking the query cache but before any privilege checks or optimization. By comparing the difference between the start and done probes you can monitor the time actually spent servicing the query (instead of just handling the parsing and other elements of the query).

query-exec-start(query, connectionid, database, user, host, exec_type)
query-exec-done(status)
Note

The information provided in the arguments for query-start and query-exec-start are almost identical and designed so that you can choose to monitor either the entire query process (using query-start) or only the execution (using query-exec-start) while exposing the core information about the user, client, and query being executed.

  • query-exec-start: Triggered when the execution of a individual query is started. The arguments are:

    • query: The full text of the submitted query.

    • connectionid: The connection ID of the client that submitted the query. The connection ID equals the connection ID returned when the client first connects and the Id value in the output from SHOW PROCESSLIST.

    • database: The database name on which the query is being executed.

    • user: The username used to connect to the server.

    • host: The hostname of the client.

    • exec_type: The type of execution. Execution types are determined based on the contents of the query and where it was submitted. The values for each type are shown in the following table.

      Value Description
      0 Executed query from sql_parse, top-level query.
      1 Executed prepared statement
      2 Executed cursor statement
      3 Executed query in stored procedure
  • query-exec-done: Triggered when the execution of the query has completed. The probe includes a single argument, status, which returns 0 when the query is successfully executed and 1 if there was an error.

5.8.1.7 Row-Level Probes

The *row-{start,done} probes are triggered each time a row operation is pushed down to a storage engine. For example, if you execute an INSERT statement with 100 rows of data, then the insert-row-start and insert-row-done probes will be triggered 100 times each, for each row insert.

insert-row-start(database, table)
insert-row-done(status)

update-row-start(database, table)
update-row-done(status)

delete-row-start(database, table)
delete-row-done(status)
  • insert-row-start: Triggered before a row is inserted into a table.

  • insert-row-done: Triggered after a row is inserted into a table.

  • update-row-start: Triggered before a row is updated in a table.

  • update-row-done: Triggered before a row is updated in a table.

  • delete-row-start: Triggered before a row is deleted from a table.

  • delete-row-done: Triggered before a row is deleted from a table.

The arguments supported by the probes are consistent for the corresponding start and done probes in each case:

  • database: The database name.

  • table: The table name.

  • status: The status; 0 for success or 1 for failure.

Because the row-level probes are triggered for each individual row access, these probes can be triggered many thousands of times each second, which may have a detrimental effect on both the monitoring script and MySQL. The DTrace environment should limit the triggering on these probes to prevent the performance being adversely affected. Either use the probes sparingly, or use counter or aggregation functions to report on these probes and then provide a summary when the script terminates or as part of a query-done or query-exec-done probes.

The following example script summarizes the duration of each row operation within a larger query:

#!/usr/sbin/dtrace -s

#pragma D option quiet

dtrace:::BEGIN
{
   printf("%-2s %-10s %-10s %9s %9s %-s \n",
          "St", "Who", "DB", "ConnID", "Dur ms", "Query");
}

mysql*:::query-start
{
   self->query = copyinstr(arg0);
   self->who   = strjoin(copyinstr(arg3),strjoin("@",copyinstr(arg4)));
   self->db    = copyinstr(arg2);
   self->connid = arg1;
   self->querystart = timestamp;
   self->rowdur = 0;
}

mysql*:::query-done
{
   this->elapsed = (timestamp - self->querystart) /1000000;
   printf("%2d %-10s %-10s %9d %9d %s\n",
          arg0, self->who, self->db,
          self->connid, this->elapsed, self->query);
}

mysql*:::query-done
/ self->rowdur /
{
   printf("%34s %9d %s\n", "", (self->rowdur/1000000), "-> Row ops");
}

mysql*:::insert-row-start
{
   self->rowstart = timestamp;
}

mysql*:::delete-row-start
{
   self->rowstart = timestamp;
}

mysql*:::update-row-start
{
   self->rowstart = timestamp;
}

mysql*:::insert-row-done
{
   self->rowdur += (timestamp-self->rowstart);
}

mysql*:::delete-row-done
{
   self->rowdur += (timestamp-self->rowstart);
}

mysql*:::update-row-done
{
   self->rowdur += (timestamp-self->rowstart);
}

Running the above script with a query that inserts data into a table, you can monitor the exact time spent performing the raw row insertion:

St Who        DB            ConnID    Dur ms Query
 0 @localhost test              13     20767 insert into t1(select * from t2)
4827 -> Row ops

5.8.1.8 Read Row Probes

The read row probes are triggered at a storage engine level each time a row read operation occurs. These probes are specified within each storage engine (as opposed to the *row-start probes which are in the storage engine interface). These probes can therefore be used to monitor individual storage engine row-level operations and performance. Because these probes are triggered around the storage engine row read interface, they may be hit a significant number of times during a basic query.

read-row-start(database, table, scan_flag)
read-row-done(status)
  • read-row-start: Triggered when a row is read by the storage engine from the specified database and table. The scan_flag is set to 1 (true) when the read is part of a table scan (that is, a sequential read), or 0 (false) when the read is of a specific record.

  • read-row-done: Triggered when a row read operation within a storage engine completes. The status returns 0 on success, or a positive value on failure.

5.8.1.9 Index Probes

The index probes are triggered each time a row is read using one of the indexes for the specified table. The probe is triggered within the corresponding storage engine for the table.

index-read-row-start(database, table)
index-read-row-done(status)
  • index-read-row-start: Triggered when a row is read by the storage engine from the specified database and table.

  • index-read-row-done: Triggered when an indexed row read operation within a storage engine completes. The status returns 0 on success, or a positive value on failure.

5.8.1.10 Lock Probes

The lock probes are called whenever an external lock is requested by MySQL for a table using the corresponding lock mechanism on the table as defined by the table's engine type. There are three different types of lock, the read lock, write lock, and unlock operations. Using the probes you can determine the duration of the external locking routine (that is, the time taken by the storage engine to implement the lock, including any time waiting for another lock to become free) and the total duration of the lock/unlock process.

handler-rdlock-start(database, table)
handler-rdlock-done(status)

handler-wrlock-start(database, table)
handler-wrlock-done(status)

handler-unlock-start(database, table)
handler-unlock-done(status)
  • handler-rdlock-start: Triggered when a read lock is requested on the specified database and table.

  • handler-wrlock-start: Triggered when a write lock is requested on the specified database and table.

  • handler-unlock-start: Triggered when an unlock request is made on the specified database and table.

  • handler-rdlock-done: Triggered when a read lock request completes. The status is 0 if the lock operation succeeded, or >0 on failure.

  • handler-wrlock-done: Triggered when a write lock request completes. The status is 0 if the lock operation succeeded, or >0 on failure.

  • handler-unlock-done: Triggered when an unlock request completes. The status is 0 if the unlock operation succeeded, or >0 on failure.

You can use arrays to monitor the locking and unlocking of individual tables and then calculate the duration of the entire table lock using the following script:

#!/usr/sbin/dtrace -s

#pragma D option quiet

mysql*:::handler-rdlock-start
{
   self->rdlockstart = timestamp;
   this->lockref = strjoin(copyinstr(arg0),strjoin("@",copyinstr(arg1)));
   self->lockmap[this->lockref] = self->rdlockstart;
   printf("Start: Lock->Read   %s.%s\n",copyinstr(arg0),copyinstr(arg1));
}

mysql*:::handler-wrlock-start
{
   self->wrlockstart = timestamp;
   this->lockref = strjoin(copyinstr(arg0),strjoin("@",copyinstr(arg1)));
   self->lockmap[this->lockref] = self->rdlockstart;
   printf("Start: Lock->Write  %s.%s\n",copyinstr(arg0),copyinstr(arg1));
}

mysql*:::handler-unlock-start
{
   self->unlockstart = timestamp;
   this->lockref = strjoin(copyinstr(arg0),strjoin("@",copyinstr(arg1)));
   printf("Start: Lock->Unlock %s.%s (%d ms lock duration)\n",
          copyinstr(arg0),copyinstr(arg1),
          (timestamp - self->lockmap[this->lockref])/1000000);
}

mysql*:::handler-rdlock-done
{
   printf("End:   Lock->Read   %d ms\n",
          (timestamp - self->rdlockstart)/1000000);
}

mysql*:::handler-wrlock-done
{
   printf("End:   Lock->Write  %d ms\n",
          (timestamp - self->wrlockstart)/1000000);
}

mysql*:::handler-unlock-done
{
   printf("End:   Lock->Unlock %d ms\n",
          (timestamp - self->unlockstart)/1000000);
}

When executed, you should get information both about the duration of the locking process itself, and of the locks on a specific table:

Start: Lock->Read   test.t2
End:   Lock->Read   0 ms
Start: Lock->Unlock test.t2 (25743 ms lock duration)
End:   Lock->Unlock 0 ms
Start: Lock->Read   test.t2
End:   Lock->Read   0 ms
Start: Lock->Unlock test.t2 (1 ms lock duration)
End:   Lock->Unlock 0 ms
Start: Lock->Read   test.t2
End:   Lock->Read   0 ms
Start: Lock->Unlock test.t2 (1 ms lock duration)
End:   Lock->Unlock 0 ms
Start: Lock->Read   test.t2
End:   Lock->Read   0 ms

5.8.1.11 Filesort Probes

The filesort probes are triggered whenever a filesort operation is applied to a table. For more information on filesort and the conditions under which it occurs, see Section 8.2.1.14, “ORDER BY Optimization”.

filesort-start(database, table)
filesort-done(status, rows)
  • filesort-start: Triggered when the filesort operation starts on a table. The two arguments to the probe, database and table, will identify the table being sorted.

  • filesort-done: Triggered when the filesort operation completes. Two arguments are supplied, the status (0 for success, 1 for failure), and the number of rows sorted during the filesort process.

An example of this is in the following script, which tracks the duration of the filesort process in addition to the duration of the main query:

#!/usr/sbin/dtrace -s

#pragma D option quiet

dtrace:::BEGIN
{
   printf("%-2s %-10s %-10s %9s %18s %-s \n",
          "St", "Who", "DB", "ConnID", "Dur microsec", "Query");
}

mysql*:::query-start
{
   self->query = copyinstr(arg0);
   self->who   = strjoin(copyinstr(arg3),strjoin("@",copyinstr(arg4)));
   self->db    = copyinstr(arg2);
   self->connid = arg1;
   self->querystart = timestamp;
   self->filesort = 0;
   self->fsdb = "";
   self->fstable = "";
}

mysql*:::filesort-start
{
  self->filesort = timestamp;
  self->fsdb = copyinstr(arg0);
  self->fstable = copyinstr(arg1);
}

mysql*:::filesort-done
{
   this->elapsed = (timestamp - self->filesort) /1000;
   printf("%2d %-10s %-10s %9d %18d Filesort on %s\n",
          arg0, self->who, self->fsdb,
          self->connid, this->elapsed, self->fstable);
}

mysql*:::query-done
{
   this->elapsed = (timestamp - self->querystart) /1000;
   printf("%2d %-10s %-10s %9d %18d %s\n",
          arg0, self->who, self->db,
          self->connid, this->elapsed, self->query);
}

Executing a query on a large table with an ORDER BY clause that triggers a filesort, and then creating an index on the table and then repeating the same query, you can see the difference in execution speed:

St Who        DB            ConnID       Dur microsec Query
 0 @localhost test              14           11335469 Filesort on t1
 0 @localhost test              14           11335787 select * from t1 order by i limit 100
 0 @localhost test              14          466734378 create index t1a on t1 (i)
0 @localhost test              14              26472 select * from t1 order by i limit 100

5.8.1.12 Statement Probes

The individual statement probes are provided to give specific information about different statement types. For the start probes the string of the query is provided as the only argument. Depending on the statement type, the information provided by the corresponding done probe will differ. For all done probes the status of the operation (0 for success, >0 for failure) is provided. For SELECT, INSERT, INSERT ... (SELECT FROM ...), DELETE, and DELETE FROM t1,t2 operations the number of rows affected is returned.

For UPDATE and UPDATE t1,t2 ... statements the number of rows matched and the number of rows actually changed is provided. This is because the number of rows actually matched by the corresponding WHERE clause, and the number of rows changed can differ. MySQL does not update the value of a row if the value already matches the new setting.

select-start(query)
select-done(status,rows)

insert-start(query)
insert-done(status,rows)

insert-select-start(query)
insert-select-done(status,rows)

update-start(query)
update-done(status,rowsmatched,rowschanged)

multi-update-start(query)
multi-update-done(status,rowsmatched,rowschanged)

delete-start(query)
delete-done(status,rows)

multi-delete-start(query)
multi-delete-done(status,rows)
  • select-start: Triggered before a SELECT statement.

  • select-done: Triggered at the end of a SELECT statement.

  • insert-start: Triggered before a INSERT statement.

  • insert-done: Triggered at the end of an INSERT statement.

  • insert-select-start: Triggered before an INSERT ... SELECT statement.

  • insert-select-done: Triggered at the end of an INSERT ... SELECT statement.

  • update-start: Triggered before an UPDATE statement.

  • update-done: Triggered at the end of an UPDATE statement.

  • multi-update-start: Triggered before an UPDATE statement involving multiple tables.

  • multi-update-done: Triggered at the end of an UPDATE statement involving multiple tables.

  • delete-start: Triggered before a DELETE statement.

  • delete-done: Triggered at the end of a DELETE statement.

  • multi-delete-start: Triggered before a DELETE statement involving multiple tables.

  • multi-delete-done: Triggered at the end of a DELETE statement involving multiple tables.

The arguments for the statement probes are:

  • query: The query string.

  • status: The status of the query. 0 for success, and >0 for failure.

  • rows: The number of rows affected by the statement. This returns the number rows found for SELECT, the number of rows deleted for DELETE, and the number of rows successfully inserted for INSERT.

  • rowsmatched: The number of rows matched by the WHERE clause of an UPDATE operation.

  • rowschanged: The number of rows actually changed during an UPDATE operation.

You use these probes to monitor the execution of these statement types without having to monitor the user or client executing the statements. A simple example of this is to track the execution times:

#!/usr/sbin/dtrace -s

#pragma D option quiet

dtrace:::BEGIN
{
   printf("%-60s %-8s %-8s %-8s\n", "Query", "RowsU", "RowsM", "Dur (ms)");
}

mysql*:::update-start, mysql*:::insert-start,
mysql*:::delete-start, mysql*:::multi-delete-start,
mysql*:::multi-delete-done, mysql*:::select-start,
mysql*:::insert-select-start, mysql*:::multi-update-start
{
    self->query = copyinstr(arg0);
    self->querystart = timestamp;
}

mysql*:::insert-done, mysql*:::select-done,
mysql*:::delete-done, mysql*:::multi-delete-done, mysql*:::insert-select-done
/ self->querystart /
{
    this->elapsed = ((timestamp - self->querystart)/1000000);
    printf("%-60s %-8d %-8d %d\n",
           self->query,
           0,
           arg1,
           this->elapsed);
    self->querystart = 0;
}

mysql*:::update-done, mysql*:::multi-update-done
/ self->querystart /
{
    this->elapsed = ((timestamp - self->querystart)/1000000);
    printf("%-60s %-8d %-8d %d\n",
           self->query,
           arg1,
           arg2,
           this->elapsed);
    self->querystart = 0;
}

When executed you can see the basic execution times and rows matches:

Query                                                        RowsU    RowsM    Dur (ms)
select * from t2                                             0        275      0
insert into t2 (select * from t2)                            0        275      9
update t2 set i=5 where i > 75                               110      110      8
update t2 set i=5 where i < 25                               254      134      12
delete from t2 where i < 5                                   0        0        0

Another alternative is to use the aggregation functions in DTrace to aggregate the execution time of individual statements together:

#!/usr/sbin/dtrace -s

#pragma D option quiet


mysql*:::update-start, mysql*:::insert-start,
mysql*:::delete-start, mysql*:::multi-delete-start,
mysql*:::multi-delete-done, mysql*:::select-start,
mysql*:::insert-select-start, mysql*:::multi-update-start
{
    self->querystart = timestamp;
}

mysql*:::select-done
{
        @statements["select"] = sum(((timestamp - self->querystart)/1000000));
}

mysql*:::insert-done, mysql*:::insert-select-done
{
        @statements["insert"] = sum(((timestamp - self->querystart)/1000000));
}

mysql*:::update-done, mysql*:::multi-update-done
{
        @statements["update"] = sum(((timestamp - self->querystart)/1000000));
}

mysql*:::delete-done, mysql*:::multi-delete-done
{
        @statements["delete"] = sum(((timestamp - self->querystart)/1000000));
}

tick-30s
{
        printa(@statements);
}

The script just shown aggregates the times spent doing each operation, which could be used to help benchmark a standard suite of tests.

 delete                                                            0
  update                                                            0
  insert                                                           23
  select                                                         2484

  delete                                                            0
  update                                                            0
  insert                                                           39
  select                                                        10744

  delete                                                            0
  update                                                           26
  insert                                                           56
  select                                                        10944

  delete                                                            0
  update                                                           26
  insert                                                         2287
select                                                        15985

5.8.1.13 Network Probes

The network probes monitor the transfer of information from the MySQL server and clients of all types over the network. The probes are defined as follows:

net-read-start()
net-read-done(status, bytes)
net-write-start(bytes)
net-write-done(status)
  • net-read-start: Triggered when a network read operation is started.

  • net-read-done: Triggered when the network read operation completes. The status is an integer representing the return status for the operation, 0 for success and 1 for failure. The bytes argument is an integer specifying the number of bytes read during the process.

  • net-start-bytes: Triggered when data is written to a network socket. The single argument, bytes, specifies the number of bytes written to the network socket.

  • net-write-done: Triggered when the network write operation has completed. The single argument, status, is an integer representing the return status for the operation, 0 for success and 1 for failure.

You can use the network probes to monitor the time spent reading from and writing to network clients during execution. The following D script provides an example of this. Both the cumulative time for the read or write is calculated, and the number of bytes. Note that the dynamic variable size has been increased (using the dynvarsize option) to cope with the rapid firing of the individual probes for the network reads/writes.

#!/usr/sbin/dtrace -s

#pragma D option quiet
#pragma D option dynvarsize=4m

dtrace:::BEGIN
{
   printf("%-2s %-30s %-10s %9s %18s %-s \n",
          "St", "Who", "DB", "ConnID", "Dur microsec", "Query");
}

mysql*:::query-start
{
   self->query = copyinstr(arg0);
   self->who   = strjoin(copyinstr(arg3),strjoin("@",copyinstr(arg4)));
   self->db    = copyinstr(arg2);
   self->connid = arg1;
   self->querystart = timestamp;
   self->netwrite = 0;
   self->netwritecum = 0;
   self->netwritebase = 0;
   self->netread = 0;
   self->netreadcum = 0;
   self->netreadbase = 0;
}

mysql*:::net-write-start
{
   self->netwrite += arg0;
   self->netwritebase = timestamp;
}

mysql*:::net-write-done
{
   self->netwritecum += (timestamp - self->netwritebase);
   self->netwritebase = 0;
}

mysql*:::net-read-start
{
   self->netreadbase = timestamp;
}

mysql*:::net-read-done
{
   self->netread += arg1;
   self->netreadcum += (timestamp - self->netreadbase);
   self->netreadbase = 0;
}

mysql*:::query-done
{
   this->elapsed = (timestamp - self->querystart) /1000000;
   printf("%2d %-30s %-10s %9d %18d %s\n",
          arg0, self->who, self->db,
          self->connid, this->elapsed, self->query);
   printf("Net read: %d bytes (%d ms) write: %d bytes (%d ms)\n",
               self->netread, (self->netreadcum/1000000),
               self->netwrite, (self->netwritecum/1000000));
}

When executing the above script on a machine with a remote client, you can see that approximately a third of the time spent executing the query is related to writing the query results back to the client.

St Who                            DB            ConnID       Dur microsec Query
 0 root@::ffff:198.51.100.108      test              31               3495 select * from t1 limit 1000000
Net read: 0 bytes (0 ms) write: 10000075 bytes (1220 ms)

5.8.1.14 Keycache Probes

The keycache probes are triggered when using the index key cache used with the MyISAM storage engine. Probes exist to monitor when data is read into the keycache, cached key data is written from the cache into a cached file, or when accessing the keycache.

Keycache usage indicates when data is read or written from the index files into the cache, and can be used to monitor how efficient the memory allocated to the keycache is being used. A high number of keycache reads across a range of queries may indicate that the keycache is too small for size of data being accessed.

keycache-read-start(filepath, bytes, mem_used, mem_free)
keycache-read-block(bytes)
keycache-read-hit()
keycache-read-miss()
keycache-read-done(mem_used, mem_free)
keycache-write-start(filepath, bytes, mem_used, mem_free)
keycache-write-block(bytes)
keycache-write-done(mem_used, mem_free)

When reading data from the index files into the keycache, the process first initializes the read operation (indicated by keycache-read-start), then loads blocks of data (keycache-read-block), and then the read block is either matches the data being identified (keycache-read-hit) or more data needs to be read (keycache-read-miss). Once the read operation has completed, reading stops with the keycache-read-done.

Data will be read from the index file into the keycache only when the specified key is not already within the keycache.

  • keycache-read-start: Triggered when the keycache read operation is started. Data is read from the specified filepath, reading the specified number of bytes. The mem_used and mem_avail indicate memory currently used by the keycache and the amount of memory available within the keycache.

  • keycache-read-block: Triggered when the keycache reads a block of data, of the specified number of bytes, from the index file into the keycache.

  • keycache-read-hit: Triggered when the block of data read from the index file matches the key data requested.

  • keycache-read-miss: Triggered when the block of data read from the index file does not match the key data needed.

  • keycache-read-done: Triggered when the keycache read operation has completed. The mem_used and mem_avail indicate memory currently used by the keycache and the amount of memory available within the keycache.

Keycache writes occur when the index information is updated during an INSERT, UPDATE, or DELETE operation, and the cached key information is flushed back to the index file.

  • keycache-write-start: Triggered when the keycache write operation is started. Data is written to the specified filepath, reading the specified number of bytes. The mem_used and mem_avail indicate memory currently used by the keycache and the amount of memory available within the keycache.

  • keycache-write-block: Triggered when the keycache writes a block of data, of the specified number of bytes, to the index file from the keycache.

  • keycache-write-done: Triggered when the keycache write operation has completed. The mem_used and mem_avail indicate memory currently used by the keycache and the amount of memory available within the keycache.