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 ‘’mk-query-digest’’ tool to aggregate similar queries together and report on those that consume the most execution time.

Version Specific Information

• 4.1.x:
  • Limited functionality available: only microsecond resolution, no added statistics.
• 5.0.x:
  • Full functionality available.
• 5.1.x:
  • Microsecond resolution included in official MySQL server; this feature adds statistics.
• 5.1.47-11.0:
  • Full functionality of use_global_log_slow_control available, except all option and link between use_global_long_query_time and use_global_log_slow_control =long_query_time available.
• 5.1.47-12.0:
  • Full functionality of use_global_log_slow_control available.
  • Fixed #600684 - log_slow_verbosity =iInnodb doesn’t work on slave (statement-based replication)
• 5.1.53-11.7:
  • Fixed #643149 - Slow query log entries were not being done in the usual parsing format; made parsing difficult. Made compatible.

Other Information

• Author / Origin: Maciej Dobrzanski, Percona

System Variables

variable log_slow_filter

Command Line:Yes Config File:Yes Scope:Global, Session Dynamic:Yes

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 Dynamic:Yes

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 Dynamic:Yes (in 5.1 releases only)

Specifies that queries replayed by the slave SQL thread on a MySQL slave will be logged. The standard MySQL server will not log any queries executed by the slave``s SQL thread.

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 Scope:Global Dynamic:Yes Variable Type:Boolean Default Value:TRUE Range:TRUE/FALSE

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

variable log_slow_timestamp_every

Command Line:Yes Config File:Yes Scope:Global Dynamic:Yes Variable Type:Boolean Default Value:FALSE Range:TRUE/FALSE

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

NOTE: This variable has been renamed to slow_query_log_timestamp_always since 5.5.10-20.1.

variable log_slow_verbosity

Command Line:

Yes

Config File:

Yes

Scope:

Global, session

Dynamic:

Yes

Version Info:

• 5.5.8-20.0 – Added profiling and profiling_use_getrusage

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).
• full: Equivalent to all other values OR``ed together.
• profiling: Enables profiling of all queries in all connections.
• profiling_use_getrusage: Enables usage of the getrusage function.

Values are OR``ed together.

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

variable long_query_time

Command Line:Yes Config File:Yes Scope:Global, session Dynamic:Yes

Specifies the time threshold for filtering queries out of the slow query log. The unit of time is seconds. This option has the same meaning as in a standard MySQL server, with the following changes:

The option accepts fractional values. If set to 0.5, for example, queries longer than 1/2 second will be logged.

If the value is set to 0, then all queries are logged. This is different from the standard MySQL build, where a value of 0 disables logging. Before version 1.01 of this feature, the value was an integer, and the unit of time was microseconds, not seconds.

variable profiling_server

Command Line:Yes Config File:Yes Scope:Global Dynamic:Yes Variable Type:BOOL Default Value:OFF Range:ON/OFF

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

variable profiling_use_getrusage

Command Line:Yes Config File:Yes Scope:Global Dynamic:Yes Variable Type:BOOL Default Value:OFF Range:ON/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 Scope:Global Dynamic:Yes Variable Type:Boolean Default Value:FALSE Range:TRUE/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]

NOTE: This variable has been renamed to slow_query_log_timestamp_precision since 5.5.10-20.1.

variable use_global_slow_control

Command Line:

Yes

Config File:

Yes

Scope:

Global

Dynamic:

Yes

Default Value:

None

Version Info:

• 5.5.10-20.1 – Renamed to slow_query_log_use_global_control

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)

NOTE: This variable has been renamed to slow_query_log_use_global_control since 5.5.10-20.1.

variable use_global_long_query_time

Command Line:Yes Config File:Yes Scope:Global Dynamic: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] @  [192.168.10.165]
# Thread_id: 11167745  Schema: board
# QC_Hit: No  Full_scan: No  Full_join: No  Tmp_table: Yes  Disk_tmp_table: No
# Filesort: Yes  Disk_filesort: No  Merge_passes: 0
# Query_time: 0.000659  Lock_time: 0.000070  Rows_sent: 0  Rows_examined: 30  Rows_affected: 0  Rows_read: 30
#   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
select count(distinct author_id) from art87.article87 force index (forum_id) where forum_id = 240215 and thread_id = ``710575``

Another example (log_slow_verbosity =profiling):

# Query_time: 4.555235  Lock_time: 0.000000  Rows_sent: 1  Rows_examined: 0  Rows_affected: 0  Rows_read: 1
# Profile_starting: 4.554799 Profile_starting_cpu: 0.000000 Profile_checking_permissions: 0.000095 Profile_checking_permissions_cpu: 0.000000 Profile_Opening_tables: 0.000088 Profile_Opening_tables_cpu: 0.000000 Profile_init: 0.000056 Profile_init_cpu: 0.000000 Profile_optimizing: 0.000046 Profile_optimizing_cpu: 0.000000 Profile_executing: 0.000098 Profile_executing_cpu: 0.000000 Profile_end: 0.000049 Profile_end_cpu: 0.000000 Profile_query_end: 0.000045 Profile_query_end_cpu: 0.000000 Profile_freeing_items: 0.000084 Profile_freeing_items_cpu: 0.000000 Profile_logging_slow_query: 0.000045 Profile_logging_slow_query_cpu: 0.000000
# Profile_total: 4.555405 Profile_total_cpu: 0.000000
insert into teee4 select * from teee4 limit 10000000;

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

Values:

• 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.

Related Reading

• http://www.mysqlperformanceblog.com/2009/02/10/impact-of-logging-on-mysql%E2%80%99s-performance/
• log_slow_filter Usage
• Blueprint in Launchpad