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 8.9.2, “Tuning Server Parameters”.

Section 14.6.3, “InnoDB Startup Options and System Variables”, lists InnoDB system variables.

Section 17.3.4.3, “MySQL Cluster System Variables”, lists system variables which are specific to MySQL Cluster.

For information on server system variables specific to replication, see Section 16.1.3, “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 server's init_connect system variable:

SET GLOBAL init_connect='SET autocommit=0';

The init_connect variable can also be set on the command line or in an option file. To set the variable as just shown using an option file, include these lines:

[mysqld]
init_connect='SET autocommit=0'

The content of init_connect is not executed for users that have the SUPER privilege.

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 19.2.2, “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.

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.

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 cannot be used as a client character set, which means that it also does not work for SET NAMES or SET CHARACTER SET.

character_set_connection

The character set for the current connection. Used for literals that do not have an explicit character set specification 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'. This variable was added in MySQL 5.1.6.

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.

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 1 (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 0.

This variable can take three integer values.

See also Section 8.7.3, “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 as of MySQL 5.1.23 and 5 seconds before that.

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.

datadir

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

date_format

This variable is unused.

datetime_format

This variable is unused.

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

This variable was added in MySQL 5.1.7.

debug_sync

This variable is the user interface to the Debug Sync facility. Use of Debug Sync requires that MySQL be configured with the --enable-debug-sync option (see Section 2.11.4, “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.

This variable was added in MySQL 5.1.41.

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 option (for example, --myisam-recover=BACKUP,FORCE). See Section 5.1.3, “Server Command Options”, and Section 14.5.1, “MyISAM Startup Options”.

delayed_insert_limit

After inserting delayed_insert_limit delayed rows, the INSERT DELAYED handler thread checks whether there are any SELECT statements pending. If so, it permits them to execute before continuing to insert delayed rows.

delayed_insert_timeout

How many seconds an INSERT DELAYED handler thread should wait for INSERT statements before terminating.

delayed_queue_size

This is a per-table limit on the number of rows to queue when handling INSERT DELAYED statements. If the queue becomes full, any client that issues an INSERT DELAYED statement waits until there is room in the queue again.

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

engine_condition_pushdown

The engine condition pushdown optimization enables processing for certain comparisons to be “pushed down” to the storage engine level for more efficient execution. For more information, see Section 8.3.1.5, “Engine Condition Pushdown Optimization”.

Engine condition pushdown is used only by the NDBCLUSTER storage engine. Enabling this optimization on a MySQL Server acting as a MySQL Cluster SQL node causes WHERE conditions on unindexed columns to be evaluated on the cluster's data nodes and only the rows that match to be sent back to the SQL node that issued the query. This greatly reduces the amount of cluster data that must be sent over the network, increasing the efficiency with which results are returned.

The engine_condition_pushdown variable controls whether engine condition pushdown is enabled. By default, this variable is ON (1). Setting it to OFF (0) disables pushdown.

error_count

The number of errors that resulted from the last statement that generated messages. This variable is read only. See Section 13.7.5.18, “SHOW ERRORS Syntax”.

event_scheduler

This variable indicates the status of the Event Scheduler; as of MySQL 5.1.12, 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.

This variable was added in MySQL 5.1.6.

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”.

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. 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 14.6.4.4, “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 FULLTEXT index.

ft_min_word_len

The minimum length of the word to be included in a 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. 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 (--log before MySQL 5.1.29). 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. The general_log variable was added in MySQL 5.1.12.

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 (--log before MySQL 5.1.29). This variable was added in MySQL 5.1.12.

group_concat_max_len

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

have_archive

YES if mysqld supports ARCHIVE tables, NO if not. This variable was removed in MySQL 5.1.14.

have_blackhole_engine

YES if mysqld supports BLACKHOLE tables, NO if not. This variable was removed in MySQL 5.1.14.

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_community_features

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 13.7.5.33, “SHOW PROFILES Syntax”.

This variable was added in MySQL 5.1.24. It is renamed to have_profiling in MySQL 5.5.

have_crypt

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

have_csv

YES if mysqld supports CSV tables, NO if not.

This variable is deprecated and is removed in MySQL 5.6. Use SHOW ENGINES instead.

have_dynamic_loading

YES if mysqld supports dynamic loading of plugins, NO if not. This variable was added in MySQL 5.1.10.

have_example_engine

YES if mysqld supports EXAMPLE tables, NO if not. This variable was removed in MySQL 5.1.14.

have_federated_engine

YES if mysqld supports FEDERATED tables, NO if not. This variable was removed in MySQL 5.1.14.

have_geometry

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

have_innodb

YES if mysqld supports InnoDB tables. DISABLED if --skip-innodb is used.

This variable is deprecated and is removed in MySQL 5.6. Use SHOW ENGINES instead.

have_isam

In MySQL 5.1, this variable appears only for reasons of backward compatibility. It is always NO because ISAM tables are no longer supported. This variable was removed in MySQL 5.1.7.

have_merge_engine

YES if mysqld supports MERGE tables. DISABLED if --skip-merge is used. This variable was removed in MySQL 5.1.3.

have_openssl

YES if mysqld supports SSL connections, NO if not. As of MySQL 5.1.17, this variable is an alias for have_ssl.

have_partitioning

YES if mysqld supports partitioning. Added in MySQL 5.1.1 as have_partition_engine and renamed to have_partioning in 5.1.6.

This variable is deprecated and is removed in MySQL 5.6. Use SHOW ENGINES instead.

have_query_cache

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

have_row_based_replication

YES if the server can perform replication using row-based binary logging. If the value is NO, the server can use only statement-based logging. See Section 16.1.2, “Replication Formats”. This variable was added in MySQL 5.1.5 and removed in 5.1.15.

have_raid

In MySQL 5.1, this variable appears only for reasons of backward compatibility. It is always NO because RAID tables are no longer supported. This variable was removed in MySQL 5.1.7.

have_rtree_keys

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

have_ssl

YES if mysqld supports SSL connections, NO if not. DISABLED indicates that the server was compiled with SSL support, but but was not started with the appropriate --ssl-xxx options. For more information, see Section 6.3.6.2, “Configuring MySQL for SSL”.

This variable was added in MySQL 5.1.17. Before that, use have_openssl.

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, and on Windows for support of data directory symlinks. If the server is started with the --skip-symbolic-links option, the value is DISABLED.

hostname

The server sets this variable to the server host name at startup. This variable was added in MySQL 5.1.17.

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.

init_connect

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 example, each client session begins by default with autocommit mode enabled. There is no global autocommit system variable to specify that autocommit should be disabled by default, but init_connect can be used to achieve the same effect:

SET GLOBAL init_connect='SET autocommit=0';

The init_connect variable can also be set on the command line or in an option file. To set the variable as just shown using an option file, include these lines:

[mysqld]
init_connect='SET autocommit=0'

The content of init_connect is not executed for users that have the SUPER privilege. 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.

init_file

The name of the file specified with the --init-file option when you start the server. This should be a file containing SQL statements that you want the server to execute when it starts. Each statement must be on a single line and should not include comments. No statement terminator such as ;, \g, or \G should be given at the end of each statement.

Note that the --init-file option is unavailable if MySQL was configured with the --disable-grant-options option. See Section 2.11.4, “MySQL Source-Configuration Options”.

innodb_xxx

InnoDB system variables are listed in Section 14.6.3, “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 in MySQL 5.5 and higher, where 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

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.

join_buffer_size

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. 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 to a larger setting 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.

The maximum permissible setting for join_buffer_size is 4GB. As of MySQL 5.1.23, values larger than 4GB are permitted for 64-bit platforms (except 64-bit Windows, for which large values are truncated to 4GB with a warning).

keep_files_on_create

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.

This variable was added in MySQL 5.1.23.

key_buffer_size

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 on 32-bit platforms. As of MySQL 5.1.23, values larger than 4GB are permitted for 64-bit platforms, except 64-bit Windows prior to MySQL 5.1.31, for which large values are truncated to 4GB with a warning. As of MySQL 5.1.31, values larger than 4GB are also permitted for 64-bit Windows. 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 8.3.2.1, “Speed of 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 Syntax”.) 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 8.6.1, “The MyISAM Key Cache”.

key_cache_age_threshold