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
Scope:Global
Dynamic:yes

When this variable is enabled, administrative statements will be logged to the slow query log. 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.

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_type
Command Line:Yes
Config File:Yes
Scope:Global
Dynamic:Yes
Variable Type:Enumerated
Default Value:session
Range:session, query

Specifies semantic of log_slow_rate_limit - session or query.

variable log_slow_rate_limit
Command Line:Yes
Config File:Yes
Scope:Global, session
Dynamic:Yes
Default Value:1
Range:1-1000

Behavior of this variable depends from log_slow_rate_type.

Specifies that only a fraction of session/query should be logged. Logging is enabled for every nth session/query. By default, n is 1, so logging is enabled for every session/query. Please note: when log_slow_rate_type is 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.
  • When log_slow_rate_type is session, 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. Note that this feature will not work well if your application uses any type of connection pooling or persistent connections. Note that you change log_slow_rate_limit in session mode, you should reconnect for get effect.
  • When log_slow_rate_type is query, this option lets you log just some queries for later analysis. For example, if you set the value to 100, then one percent of queries will be logged.

Note that every query has global unique query_id and every connection can has it own (session) log_slow_rate_limit.

Decision “log or no” calculated in following manner:

  • if log_slow_rate_limit is 1 - log every query
  • If log_slow_rate_limit > 1 - randomly log every 1/log_slow_rate_limit query.

This allows flexible setup logging behavior.

For example, if you set the value to 100, then one percent of sessions/queries will be logged. In Percona Server 5.5.34-32.0 information about the log_slow_rate_limit has been added to the slow query log. This means that if the log_slow_rate_limit is effective it will be reflected in the slow query log for each written query. Example of the output looks like this:

Log_slow_rate_type: query  Log_slow_rate_limit: 10

Prior to 5.5.37-35.0 implementation of the log_slow_rate_type set to query with log_slow_rate_limit feature would log every nth query deterministically. With the current implementation each query has a non-deterministic probability of 1/n to get logged.”

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

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
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.

Note

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

variable log_slow_verbosity
Version Info:
  • 5.5.8-20.0 – Added profiling and profiling_use_getrusage
Command Line:

Yes

Config File:

Yes

Scope:

Global, session

Dynamic:

Yes

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.
  • query_plan: Log information about the query’s execution plan.
  • innodb: Log InnoDB statistics.
  • minimal: Equivalent to enabling just microtime.
  • standard: Equivalent to enabling microtime,innodb.
  • full: Equivalent to all other values OR’ed together without the profiling and profiling_use_getrusage options.
  • 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 or standard. To turn all options on, set the option to full.

variable slow_query_log_timestamp_always
Version Info:
  • 5.5.10-20.1 – Introduced (renamed from use_global_log_slow_control)
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.

variable slow_query_log_timestamp_precision
Version Info:
  • 5.5.10-20.1 – Introduced (renamed from slow_query_log_microseconds_timestamp)
Command Line:

Yes

Config File:

Yes

Scope:

Global

Dynamic:

Yes

Variable Type:

Enumerated

Default Value:

second

Range:

second, microsecond

Normally, entries to the slow query log are in seconds precision, in this format:

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

If slow_query_log_timestamp_precision =microsecond, entries to the slow query log are in microsecond precision, in this format:

# Time: 090402 9:23:36.123456 # User@Host: XXX @ XXX [10.X.X.X]
variable slow_query_log_use_global_control
Command Line:

Yes

Config File:

Yes

Scope:

Global

Dynamic:

Yes

Default Value:

None

Version Info:
  • 5.5.10-20.1 – Introduced (renamed from log_slow_timestamp_every)

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 from log_slow_timestamp_every since 5.5.10-20.1.

variable slow_query_log_always_write_time
Command Line:Yes
Config File:Yes
Scope:Global
Dynamic:Yes
Default Value:10 (seconds)

This variable can be used to specify the query execution time after which the query will be written to the slow query log. It can be used to specify an additional execution time threshold for the slow query log, that, when exceeded, will cause a query to be logged unconditionally, that is, log_slow_rate_limit will not apply to it.

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
# 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: 0.962742  Lock_time: 0.000202  Rows_sent: 4  Rows_examined: 1543719  Rows_affected: 0  Rows_read: 4

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: Yes  Full_join: No  Tmp_table: No  Tmp_table_on_disk: No
# Filesort: No  Filesort_on_disk: 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: 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

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.

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