Slow Query Log

This feature adds microsecond time resolution and additional statistics to the slow query log output. It lets you enable or disable the slow query log at runtime, adds logging for the slave SQL thread, and adds fine-grained control over what and how much to log into the slow query log.

The ability to log queries with microsecond precision is essential for measuring the work the MySQL server performs. The standard slow query log in MySQL 5.0 has only 1-second granularity, which is too coarse for all but the slowest queries. MySQL 5.1 has microsecond resolution, but does not have the extra information about query execution that is included in the Percona Server.

You can use Percona Toolkit‘s pt-query-digest tool to aggregate similar queries together and report on those that consume the most execution time.

Version Specific Information

Other Information

  • Author / Origin: Maciej Dobrzanski, Percona

System Variables

variable log_slow_admin_statements
Command Line:Yes
Config File:Yes

When this variable is enabled, administrative statements will be logged to the slow query log. This feature has been implemented in the upstream MySQL as a command line option and requires server restart in order to be enabled/disabled. Percona Server has implemented this as global variable, which can be enabled/disabled without restarting the server.

variable log_slow_filter
Command Line:Yes
Config File:Yes
Scope:Global, Session

Filters the slow log by the query’s execution plan. The value is a comma-delimited string, and can contain any combination of the following values:

  • qc_miss: The query was not found in the query cache.
  • full_scan: The query performed a full table scan.
  • full_join: The query performed a full join (a join without indexes).
  • tmp_table: The query created an implicit internal temporary table.
  • tmp_table_on_disk: The query’s temporary table was stored on disk.
  • filesort: The query used a filesort.
  • filesort_on_disk: The filesort was performed on disk.

Values are OR’ed together. If the string is empty, then the filter is disabled. If it is not empty, then queries will only be logged to the slow log if their execution plan matches one of the types of plans present in the filter.

For example, to log only queries that perform a full table scan, set the value to full_scan. To log only queries that use on-disk temporary storage for intermediate results, set the value to tmp_table_on_disk,filesort_on_disk.

variable log_slow_rate_limit
Command Line:Yes
Config File:Yes
Scope:Global, session
Range:1-ULONG_MAX (either 4294967295 or 18446744073709551615, depending on the platform)

Specifies that only a fraction of sessions should be logged. Logging is enabled for every nth session. By default, n is 1, so logging is enabled for every session. Rate limiting is disabled for the replication thread.

Logging all queries might consume I/O bandwidth and cause the log file to grow large. This option lets you log full sessions, so you have complete records of sessions for later analysis; but you can rate-limit the number of sessions that are logged. For example, if you set the value to 100, then one percent of sessions will be logged in their entirety. Note that this feature will not work well if your application uses any type of connection pooling or persistent connections.

variable log_slow_slave_statements
Command Line:Yes
Config File:Yes
Scope:Global, session

Specifies that slow queries replayed by the slave SQL thread on a MySQL slave will be logged. Upstream version of the MySQL server has implemented command line option with same name. Significant difference is that this feature is implemented as variable in Percona Server, that means it can be enabled/disabled dynamically without restarting the server.

To start the logging from the slave thread, you should change the global value: set global log_slow_slave_statements =ON; and then execute: STOP SLAVE; START SLAVE;. This will destroy and recreate the slave SQL thread, so it will see the newly set global value.

To stop the logging from the slave thread, you should just change the global value: set global log_slow_slave_statements =OFF; the logging stops immediately.

variable log_slow_sp_statements
Command Line:Yes
Config File:Yes
Variable Type:Boolean
Default Value:TRUE

If TRUE, statements executed by stored procedures are logged to the slow if it is open.


Support for logging stored proceders doesn’t involve triggers, so they won’t be logged even if this is feature is enabled.

variable log_slow_timestamp_every
Command Line:Yes
Config File:Yes
Variable Type:Boolean
Default Value:FALSE

If TRUE, a timestamp is printed on every slow log record. Multiple records may have the same time.

variable log_slow_verbosity
Command Line:Yes
Config File:Yes
Scope:Global, session
Default Value:microtime

Specifies how much information to include in your slow log. The value is a comma-delimited string, and can contain any combination of the following values:

  • microtime: Log queries with microsecond precision (mandatory).
  • query_plan: Log information about the query’s execution plan (optional).
  • innodb: Log InnoDB statistics (optional).
  • minimal: Equivalent to enabling just microtime.
  • standard: Equivalent to enabling microtime,innodb.
  • full: Equivalent to all other values OR’ed together.

Values are OR’ed together.

For example, to enable microsecond query timing and InnoDB statistics, set this option to microtime,innodb or standard. To turn all options on, set the option to full.

variable profiling_server
Command Line:Yes
Config File:Yes
Variable Type:BOOL
Default Value:OFF

When ON, this variable enables profiling of all queries (in all connections).

variable profiling_use_getrusage
Command Line:Yes
Config File:Yes
Variable Type:BOOL
Default Value:OFF

When ON, this variable enables usage of the getrusage function in profiling. A possible problem is that this function is very expensive, and with profiling_server enabled it can cause performance degradation.

variable slow_query_log_microseconds_timestamp
Command Line:Yes
Config File:Yes
Variable Type:Boolean
Default Value:FALSE

When TRUE, entries to the slow log are done in microsecond precision.

Normally, the slow query log contains output in this format:

# Time: 090402 9:23:36 # User@Host: XXX @ XXX [10.X.X.X]

If TRUE, this variable causes the format to be like this:

# Time: 090402 9:23:36.123456 # User@Host: XXX @ XXX [10.X.X.X]
variable use_global_log_slow_control
Command Line:Yes
Config File:Yes
Default Value:None

Specifies which variables have global scope instead of local. Value is a “flag” variable - you can specify multiple values separated by commas

  • none: All variables use local scope
  • log_slow_filter: Global variable log_slow_filter has effect (instead of local)
  • log_slow_rate_limit: Global variable log_slow_rate_limit has effect (instead of local)
  • log_slow_verbosity: Global variable log_slow_verbosity has effect (instead of local)
  • long_query_time: Global variable long_query_time has effect (instead of local)
  • min_examined_row_limit: Global variable min_examined_row_limit has effect (instead of local)
  • all Global variables has effect (instead of local)
variable use_global_long_query_time
Command Line:Yes
Config File:Yes

If 1 is set, global long_query_time is always used instead of the local long_query_time, and the local long_query_time is updated by global when used. 0 is same as normal behavior. (default 0)

Other Information

Changes to the Log Format

The feature adds more information to the slow log output. Here is a sample log entry:

# User@Host: mailboxer[mailboxer] @  []
# Thread_id: 11167745  Schema: board
# Query_time: 1.009400  Lock_time: 0.000190  Rows_sent: 4  Rows_examined: 1543719  Rows_affected: 0  Rows_read: 4
# Bytes_sent: 278  Tmp_tables: 0  Tmp_disk_tables: 0  Tmp_table_sizes: 0
# InnoDB_trx_id: 1500
# QC_Hit: No  Full_scan: Yes  Full_join: No  Tmp_table: No  Tmp_table_on_disk: No
# Filesort: No  Filesort_on_disk: No  Merge_passes: 0
#   InnoDB_IO_r_ops: 6415  InnoDB_IO_r_bytes: 105103360  InnoDB_IO_r_wait: 0.001279
#   InnoDB_rec_lock_wait: 0.000000  InnoDB_queue_wait: 0.000000
#   InnoDB_pages_distinct: 6430
SET timestamp=1346844943;
SELECT id,title,production_year FROM title WHERE title = 'Bambi';

Another example (log_slow_verbosity =profiling):

# Query_time: 0.962742  Lock_time: 0.000202  Rows_sent: 4  Rows_examined: 1543719  Rows_affected: 0  Rows_read: 4
# Bytes_sent: 278  Tmp_tables: 0  Tmp_disk_tables: 0  Tmp_table_sizes: 0
# Profile_starting: 0.000030 Profile_starting_cpu: 0.000028 Profile_Waiting_for_query_cache_lock: 0.000003
  Profile_Waiting_for_query_cache_lock_cpu: 0.000003 Profile_Waiting_on_query_cache_mutex: 0.000003
  Profile_Waiting_on_query_cache_mutex_cpu: 0.000003 Profile_checking_query_cache_for_query: 0.000076
  Profile_checking_query_cache_for_query_cpu: 0.000076 Profile_checking_permissions: 0.000011
  Profile_checking_permissions_cpu: 0.000011 Profile_Opening_tables: 0.000078 Profile_Opening_tables_cpu: 0.000078
  Profile_System_lock: 0.000022 Profile_System_lock_cpu: 0.000022 Profile_Waiting_for_query_cache_lock: 0.000003
  Profile_Waiting_for_query_cache_lock_cpu: 0.000002 Profile_Waiting_on_query_cache_mutex: 0.000054
  Profile_Waiting_on_query_cache_mutex_cpu: 0.000054 Profile_init: 0.000039 Profile_init_cpu: 0.000040
  Profile_optimizing: 0.000015 Profile_optimizing_cpu: 0.000014 Profile_statistics: 0.000021 Profile_statistics_cpu: 0.000021
  Profile_preparing: 0.000020 Profile_preparing_cpu: 0.000020 Profile_executing: 0.000003 Profile_executing_cpu: 0.000003
  Profile_Sending_data: 0.962324 Profile_Sending_data_cpu: 0.961526 Profile_end: 0.000006 Profile_end_cpu: 0.000005
  Profile_query_end: 0.000004 Profile_query_end_cpu: 0.000004 Profile_closing_tables: 0.000008 Profile_closing_tables_cpu: 0.000008
  Profile_freeing_items: 0.000007 Profile_freeing_items_cpu: 0.000007 Profile_Waiting_for_query_cache_lock: 0.000000
  Profile_Waiting_for_query_cache_lock_cpu: 0.000001 Profile_Waiting_on_query_cache_mutex: 0.000001
  Profile_Waiting_on_query_cache_mutex_cpu: 0.000001 Profile_freeing_items: 0.000017 Profile_freeing_items_cpu: 0.000016
  Profile_Waiting_for_query_cache_lock: 0.000001 Profile_Waiting_for_query_cache_lock_cpu: 0.000001
  Profile_Waiting_on_query_cache_mutex: 0.000000 Profile_Waiting_on_query_cache_mutex_cpu: 0.000001
  Profile_freeing_items: 0.000001 Profile_freeing_items_cpu: 0.000001 Profile_storing_result_in_query_cache: 0.000002
  Profile_storing_result_in_query_cache_cpu: 0.000002 Profile_logging_slow_query: 0.000001 Profile_logging_slow_query_cpu: 0.000001
# Profile_total: 0.962751 Profile_total_cpu: 0.961950
# InnoDB_trx_id: 1700

Connection and Schema Identifier

Each slow log entry now contains a connection identifier, so you can trace all the queries coming from a single connection. This is the same value that is shown in the Id column in SHOW FULL PROCESSLIST or returned from the CONNECTION_ID() function.

Each entry also contains a schema name, so you can trace all the queries whose default database was set to a particular schema.

# Thread_id: 11167745  Schema: board

Microsecond Time Resolution and Extra Row Information

This is the original functionality offered by the microslow feature. Query_time and Lock_time are logged with microsecond resolution.

The feature also adds information about how many rows were examined for SELECT queries, and how many were analyzed and affected for UPDATE, DELETE, and INSERT queries,

# Query_time: 0.000659  Lock_time: 0.000070  Rows_sent: 0  Rows_examined: 30  Rows_affected: 0  Rows_read: 30

Values and context:

  • Rows_examined: Number of rows scanned - SELECT
  • Rows_affected: Number of rows changed - UPDATE, DELETE, INSERT
  • Rows_read: Number of rows read - UPDATE, DELETE, INSERT

Memory Footprint

The feature provides information about the amount of bytes sent for the result of the query and the number of temporary tables created for its execution - differentiated by whether they were created on memory or on disk - with the total number of bytes used by them.

# Bytes_sent: 8053  Tmp_tables: 1  Tmp_disk_tables: 0  Tmp_table_sizes: 950528

Values and context:

  • Bytes_sent: The amount of bytes sent for the result of the query
  • Tmp_tables: Number of temporary tables created on memory for the query
  • Tmp_disk_tables: Number of temporary tables created on disk for the query
  • Tmp_table_sizes: Total Size in bytes for all temporary tables used in the query

Query Plan Information

Each query can be executed in various ways. For example, it may use indexes or do a full table scan, or a temporary table may be needed. These are the things that you can usually see by running EXPLAIN on the query. The feature will now allow you to see the most important facts about the execution in the log file.

# QC_Hit: No  Full_scan: No  Full_join: No  Tmp_table: Yes  Disk_tmp_table: No
# Filesort: Yes  Disk_filesort: No  Merge_passes: 0

The values and their meanings are documented with the log_slow_filter option.

InnoDB Usage Information

The final part of the output is the InnoDB usage statistics. MySQL currently shows many per-session statistics for operations with SHOW SESSION STATUS, but that does not include those of InnoDB, which are always global and shared by all threads. This feature lets you see those values for a given query.

# innodb_IO_r_ops: 1  innodb_IO_r_bytes: 16384  innodb_IO_r_wait: 0.028487
# innodb_rec_lock_wait: 0.000000  innodb_queue_wait: 0.000000
# innodb_pages_distinct: 5


  • innodb_IO_r_ops: Counts the number of page read operations scheduled. The actual number of read operations may be different, but since this can be done asynchronously, there is no good way to measure it.
  • innodb_IO_r_bytes: Similar to innodb_IO_r_ops, but the unit is bytes.
  • innodb_IO_r_wait: Shows how long (in seconds) it took InnoDB to actually read the data from storage.
  • innodb_rec_lock_wait: Shows how long (in seconds) the query waited for row locks.
  • innodb_queue_wait: Shows how long (in seconds) the query spent either waiting to enter the InnoDB queue or inside that queue waiting for execution.
  • innodb_pages_distinct: Counts approximately the number of unique pages the query accessed. The approximation is based on a small hash array representing the entire buffer pool, because it could take a lot of memory to map all the pages. The inaccuracy grows with the number of pages accessed by a query, because there is a higher probability of hash collisions.

If the query did not use InnoDB tables, that information is written into the log instead of the above statistics.

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