5.1.4. Server 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 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.

Section 5.1.5, "Using System Variables", discusses the syntax for setting and displaying system variable values.

Section 5.1.5.2, "Dynamic System Variables", lists the variables that can be set at runtime.

Information on tuning system variables can be found in Section 8.11.2, "Tuning Server Parameters".

Section 14.2.6, "InnoDB Startup Options and System Variables", lists InnoDB system variables.

MySQL Cluster System Variables, lists system variables which are specific to MySQL Cluster.

For information on server system variables specific to replication, see Section 16.1.4, "Replication and Binary Logging Options and Variables".

autocommit

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 Syntax".

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

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.

See also Section 18.2.2, "Stored Routines and MySQL Privileges".

back_log

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

The MySQL installation base directory. This variable can be set with the --basedir option. Relative path names for other variables usually are resolved relative to the base directory.

big_tables

If set to 1, all temporary tables are stored on disk rather than in memory. This is a little slower, but the error The table tbl_name is full does not occur for SELECT operations that require a large temporary table. The default value for a new connection is 0 (use in-memory temporary tables). Normally, you should never need to set this variable, because in-memory tables are automatically converted to disk-based tables as required.

bind_address

The value of the --bind-address option.

bulk_insert_buffer_size

MyISAM uses a special tree-like cache to make bulk inserts faster for INSERT ... SELECT, INSERT ... VALUES (...), (...), ..., and LOAD DATA INFILE 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

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 10.1.4, "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:

ucs2, utf16, utf16le, and utf32 cannot be used as a client character set, which means that they also do not work for SET NAMES or SET CHARACTER SET.

character_set_connection

The character set used for literals that do not have a character set introducer and for number-to-string conversion.

character_set_database

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.

character_set_filesystem

The file system character set. This variable is used to interpret string literals that refer to file names, such as in the LOAD DATA INFILE 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 multi-byte 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 'utf8'.

character_set_results

The character set used for returning query results such as result sets or error messages to the client.

character_set_server

The server's default character set.

character_set_system

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

character_sets_dir

The directory where character sets are installed.

collation_connection

The collation of the connection character set.

collation_database

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.

collation_server

The server's default collation.

completion_type

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.

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

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

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.

See also Section 8.10.3, "Concurrent Inserts".

connect_timeout

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

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

datadir

The MySQL data directory. This variable can be set with the --datadir option.

date_format

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

datetime_format

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

debug

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; the SUPER privilege is required, even for the session value.

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       |+---------+

debug_sync

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 option (see Section 2.9.4, "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 by users that have the SUPER privilege to control synchronization points.

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

default_storage_engine

The default storage engine. 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.

default_storage_engine should be used in preference to storage_engine, which is deprecated.

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

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.

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

The default mode value to use for the WEEK() function. See Section 12.7, "Date and Time Functions".

delay_key_write

This option applies only to MyISAM tables. It 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.

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 option (for example, --myisam-recover-options=BACKUP,FORCE). See Section 5.1.3, "Server Command Options", and Section 14.3.1, "MyISAM Startup Options".

delayed_insert_limit

In MySQL 5.7, this system variable is deprecated (because DELAYED inserts are not supported), and will be removed in a future release.

delayed_insert_timeout

In MySQL 5.7, this system variable is deprecated (because DELAYED inserts are not supported), and will be removed in a future release.

delayed_queue_size

In MySQL 5.7, this system variable is deprecated (because DELAYED inserts are not supported), and will be removed in a future release.

disconnect_on_expired_passwords

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 6.3.6, "Password Expiration and Sandbox Mode".

div_precision_increment

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

Whether optimizer JSON output should add end markers.

eq_range_index_dive_limit

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. Set eq_range_index_dive_limit to 0 to disable use of index statistics and always use index dives regardless of N.

For more information, see Section 8.13.1.3, "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 13.7.5.16, "SHOW ERRORS Syntax".

event_scheduler

This variable indicates the status of the Event Scheduler; possible values are ON, OFF, and DISABLED, with the default being OFF. This variable and its effects on the Event Scheduler's operation are discussed in greater detail in the Overview section of the Events chapter.

expire_logs_days

The number of days for automatic binary log file removal. The default is 0, which means "no automatic removal." Possible removals happen at startup and when the binary log is flushed. Log flushing occurs as indicated in Section 5.2, "MySQL Server Logs".

To remove binary log files manually, use the PURGE BINARY LOGS statement. See Section 13.4.1.1, "PURGE BINARY LOGS Syntax".

explicit_defaults_for_timestamp

In MySQL, the TIMESTAMP data type differs in nonstandard ways from other data types:

Those nonstandard behaviors remain the default for TIMESTAMP but as of MySQL 5.6.6 are deprecated and this warning appears at startup:

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

As indicated by the warning, to turn off the nonstandard behaviors, enable the new explicit_defaults_for_timestamp system variable at server startup. With this variable enabled, the server handles TIMESTAMP as follows instead:

external_user

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 6.3.8, "Proxy Users".

flush

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 C.5.4.2, "What to Do If MySQL Keeps Crashing". This variable is set to ON if you start mysqld with the --flush option.

flush_time

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.

foreign_key_checks

If set to 1 (the default), foreign key constraints for InnoDB tables are checked. If set to 0, they are ignored. 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 5.4.5, "InnoDB and 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.

ft_boolean_syntax

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:

ft_max_word_len

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

ft_min_word_len

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

ft_query_expansion_limit

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

ft_stopword_file

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".

general_log

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 default value depends on whether the --general_log option is given. 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

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