The Ultimate Guide to High Availability with MariaDB
This is a read-only copy of the MariaDB Knowledgebase generated on 2024-11-24. For the latest, interactive version please visit https://mariadb.com/kb/.

Slow Query Log Overview

Contents

  1. Enabling the Slow Query Log
  2. Configuring the Slow Query Log Filename
  3. Choosing the Slow Query Log Output Destination
    1. Writing the Slow Query Log to a File
    2. Writing the Slow Query Log to a Table
  4. Disabling the Slow Query Log for a Session
  5. Disabling the Slow Query Log for Specific Statements
  6. Configuring the Slow Query Log Time
  7. Logging Queries That Don't Use Indexes
  8. Excluding Queries That Examine Fewer Than a Minimum Row Limit
  9. Logging Slow Administrative Statements
  10. Enabling the Slow Query Log for Specific Criteria
  11. Throttling the Slow Query Log
  12. Configuring the Slow Query Log Verbosity
  13. Viewing the Slow Query Log
  14. Variables Related to the Slow Query Log
  15. Rotating the Slow Query Log on Unix and Linux

The slow query log is a record of SQL queries that took a long time to perform.

Note that, if your queries contain user's passwords, the slow query log may contain passwords too. Thus, it should be protected.

The number of rows affected by the slow query are also recorded in the slow query log.

Enabling the Slow Query Log

The slow query log is disabled by default.

To enable the slow query log, set the slow_query_log system variable (or, from MariaDB 10.11, log_slow_query) to 1. It can be changed dynamically with SET GLOBAL. For example:

SET GLOBAL slow_query_log=1;

It can also be set in a server option group in an option file prior to starting up the server. For example:

[mariadb]
...
slow_query_log

Configuring the Slow Query Log Filename

By default, the slow query log is written to ${hostname}-slow.log in the datadir directory. However, this can be changed.

One way to configure the slow query log filename is to set the slow_query_log_file system variable (or, from MariaDB 10.11, log_slow_query_file). It can be changed dynamically with SET GLOBAL. For example:

SET GLOBAL slow_query_log_file='mariadb-slow.log';

It can also be set in a server option group in an option file prior to starting up the server. For example:

[mariadb]
...
slow_query_log
slow_query_log_file=mariadb-slow.log

If it is a relative path, then the slow_query_log_file is relative to the datadir directory.

However, the slow_query_log_file system variable can also be an absolute path. For example:

[mariadb]
...
slow_query_log
slow_query_log_file=/var/log/mysql/mariadb-slow.log

Another way to configure the slow query log filename is to set the log-basename option, which configures MariaDB to use a common prefix for all log files (e.g. slow query log, general query log, error log, binary logs, etc.). The slow query log filename will be built by adding -slow.log to this prefix. This option cannot be set dynamically. It can be set in a server option group in an option file prior to starting up the server. For example:

[mariadb]
...
log-basename=mariadb
slow_query_log

The log-basename cannot be an absolute path. The log file name is relative to the datadir directory.

Choosing the Slow Query Log Output Destination

The slow query log can either be written to a file on disk, or it can be written to the slow_log table in the mysql database. To choose the slow query log output destination, set the log_output system variable.

Writing the Slow Query Log to a File

The slow query log is output to a file by default. However, it can be explicitly chosen by setting the log_output system variable to FILE. It can be changed dynamically with SET GLOBAL. For example:

SET GLOBAL log_output='FILE';

It can also be set in a server option group in an option file prior to starting up the server. For example:

[mariadb]
...
log_output=FILE
slow_query_log
slow_query_log_file=slow-queries.log

Writing the Slow Query Log to a Table

The slow query log can either be written to the slow_log table in the mysql database by setting the log_output system variable to TABLE. It can be changed dynamically with SET GLOBAL. For example:

SET GLOBAL log_output='TABLE';

It can also be set in a server option group in an option file prior to starting up the server. For example:

[mariadb]
...
log_output=TABLE
slow_query_log

Some rows in this table might look like this:

SELECT * FROM mysql.slow_log\G
...
*************************** 2. row ***************************
    start_time: 2014-11-11 07:56:28.721519
     user_host: root[root] @ localhost []
    query_time: 00:00:12.000215
     lock_time: 00:00:00.000000
     rows_sent: 1
 rows_examined: 0
            db: test
last_insert_id: 0
     insert_id: 0
     server_id: 1
      sql_text: SELECT SLEEP(12)
     thread_id: 74
...

See Writing logs into tables for more information.

Disabling the Slow Query Log for a Session

A user can disable logging to the slow query log for a connection by setting the slow_query_log system variable (or, from MariaDB 10.11, log_slow_query) to 0. For example:

SET SESSION slow_query_log=0;

Disabling the Slow Query Log for Specific Statements

It is possible to disable logging to the slow query log for specific types of statements by setting the log_slow_disabled_statements system variable. This option cannot be set dynamically. It can be set in a server option group in an option file prior to starting up the server. For example:

[mariadb]
...
log_output=FILE
general_log
general_log_file=queries.log
log_slow_disabled_statements='admin,call,slave,sp'

Configuring the Slow Query Log Time

The time that defines a slow query can be configured by setting the long_query_time system variable (or, from MariaDB 10.11, log_slow_query_time). It uses a units of seconds, with an optional milliseconds component. The default value is 10. It can be changed dynamically with SET GLOBAL. For example:

SET GLOBAL long_query_time=5.0;

It can also be set in a server option group in an option file prior to starting up the server. For example:

[mariadb]
...
log_output=FILE
slow_query_log
slow_query_log_file=slow-queries.log
long_query_time=5.0

Logging Queries That Don't Use Indexes

It can be beneficial to log queries that don't use indexes to the slow query log, since queries that don't use indexes can usually be optimized either by adding an index or by doing a slight rewrite. The slow query log can be configured to log queries that don't use indexes regardless of their execution time by adding the option "not_using_index" to log_slow_filter or setting the log_queries_not_using_indexes system variable to 1. It can be changed dynamically with SET GLOBAL. Some examples:

SET @@log_slow_filter=concat(@@log_slow_filter,",not_using_index");
SET GLOBAL log_queries_not_using_indexes=ON;

It can also be set in a server option group in an option file prior to starting up the server. For example:

[mariadb]
...
log_output=FILE
slow_query_log
slow_query_log_file=slow-queries.log
long_query_time=5.0
log_queries_not_using_indexes=ON

As a significant number of queries can run quickly even without indexes, you can use the min_examined_row_limit system variable (or, from MariaDB 10.11, log_slow_min_examined_row_limit) with log_queries_not_using_indexes to limit the logged queries to those having a material impact on the server.

Excluding Queries That Examine Fewer Than a Minimum Row Limit

It can be beneficial to exclude queries that examine fewer than a minimum number of rows from the log. This can be done by setting the min_examined_row_limit system variable, or, from MariaDB 10.11, log_slow_min_examined_row_limit. It can be changed dynamically with SET GLOBAL. For example:

SET GLOBAL min_examined_row_limit=100000;

It can also be set in a server option group in an option file prior to starting up the server. For example:

[mariadb]
...
log_output=FILE
slow_query_log
slow_query_log_file=slow-queries.log
long_query_time=5.0
min_examined_row_limit=100000

Logging Slow Administrative Statements

By default, the Slow Query Log logs administrative statements. To disable logging of administrative statements, remove "admin" from the log_slow_filter system variable or alternatively set the log_slow_admin_statements system variable to OFF. The Slow Query Log considers the following statements administrative: ALTER TABLE, ANALYZE TABLE, CHECK TABLE, CREATE INDEX, DROP INDEX, OPTIMIZE TABLE, and REPAIR TABLE. In MariaDB 10.3 and later, this also includes ALTER SEQUENCE statements.

You can dynamically enable this feature using a SET GLOBAL statement and setting it for just the current connection with LOCAL. Some examples:

SET SESSION log_slow_filter=replace(@@log_slow_filter,"admin","");
SET GLOBAL log_slow_admin_statements=ON;

It can also be set in a server option group in an option file prior to starting up the server. For example:

[mariadb]
...
log_output=FILE
slow_query_log
slow_query_log_file=slow-queries.log
long_query_time=5.0
log_slow_admin_statements=ON

Enabling the Slow Query Log for Specific Criteria

It is possible to enable logging to the slow query log for queries that meet specific criteria by configuring the log_slow_filter system variable. It can be changed dynamically with SET GLOBAL. For example:

SET GLOBAL log_slow_filter='filesort,filesort_on_disk,tmp_table,tmp_table_on_disk';

It can also be set in a server option group in an option file prior to starting up the server. For example:

[mariadb]
...
log_output=FILE
slow_query_log
slow_query_log_file=slow-queries.log
long_query_time=5.0
log_slow_filter=filesort,filesort_on_disk,tmp_table,tmp_table_on_disk

You can find all options for log_slow_filter at log_slow_filter system variable or at Slow Query Log Extended Statistics.

Throttling the Slow Query Log

The slow query log can create a lot of I/O, so it can be beneficial to throttle it in some cases. The slow query log can be throttled by configuring the log_slow_rate_limit system variable. It can be changed dynamically with SET GLOBAL. For example:

SET GLOBAL log_slow_rate_limit=5;

It can also be set in a server option group in an option file prior to starting up the server. For example:

[mariadb]
...
log_output=FILE
slow_query_log
slow_query_log_file=slow-queries.log
long_query_time=5.0
log_slow_rate_limit=5

Configuring the Slow Query Log Verbosity

There are a few optional pieces of information that can be included in the slow query log for each query. This optional information can be included by configuring the log_slow_verbosity system variable. It can be changed dynamically with SET GLOBAL. For example:

SET GLOBAL log_slow_verbosity='full';

It can also be set in a server option group in an option file prior to starting up the server. For example:

[mariadb]
...
log_output=FILE
slow_query_log
slow_query_log_file=slow-queries.log
long_query_time=5.0
log_slow_verbosity=query_plan,explain,engine

It is possible to have EXPLAIN output printed in the slow query log.

Viewing the Slow Query Log

Slow query logs written to file can be viewed with any text editor, or you can use the mariadb-dumpslow tool to ease the process by summarizing the information.

Queries that you find in the log are key queries to try to optimize by constructing a more efficient query or by making better use of indexes.

For queries that appear in the log that cannot be optimized in the above ways, perhaps because they are simply very large selects, due to slow hardware, or very high lock/cpu/io contention, using shard/clustering/load balancing solutions, better hardware, or stats tables may help to improve these queries.

Slow query logs written to table can be viewed by querying the slow_log table.

Rotating the Slow Query Log on Unix and Linux

Unix and Linux distributions offer the logrotate utility, which makes it very easy to rotate log files. See Rotating Logs on Unix and Linux for more information on how to use this utility to rotate the slow query log.

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