F.30. pg_stat_statements — track statistics of SQL planning and execution

The pg_stat_statements module provides a means for tracking planning and execution statistics of all SQL statements executed by a server.

The module must be loaded by adding pg_stat_statements to shared_preload_libraries in postgresql.conf, because it requires additional shared memory. This means that a server restart is needed to add or remove the module. In addition, query identifier calculation must be enabled in order for the module to be active, which is done automatically if compute_query_id is set to auto or on, or any third-party module that calculates query identifiers is loaded.

When pg_stat_statements is active, it tracks statistics across all databases of the server. To access and manipulate these statistics, the module provides views pg_stat_statements and pg_stat_statements_info, and the utility functions pg_stat_statements_reset and pg_stat_statements. These are not available globally but can be enabled for a specific database with CREATE EXTENSION pg_stat_statements.

F.30.1. The `pg_stat_statements` View

The statistics gathered by the module are made available via a view named pg_stat_statements. This view contains one row for each distinct combination of database ID, user ID, query ID and whether it's a top-level statement or not (up to the maximum number of distinct statements that the module can track). The columns of the view are shown in Table F.21.

Table F.21. pg_stat_statements Columns

Column Type Description
`userid` `oid` OID of user who executed the statement
`dbid` `oid` OID of database in which the statement was executed
`toplevel` `bool` True if the query was executed as a top-level statement (always true if `pg_stat_statements.track` is set to `top`)
`queryid` `bigint` Hash code to identify identical normalized queries.
`query` `text` Text of a representative statement
`plans` `bigint` Number of times the statement was planned (if `pg_stat_statements.track_planning` is enabled, otherwise zero)
`total_plan_time` `double precision` Total time spent planning the statement, in milliseconds (if `pg_stat_statements.track_planning` is enabled, otherwise zero)
`min_plan_time` `double precision` Minimum time spent planning the statement, in milliseconds. This field will be zero if `pg_stat_statements.track_planning` is disabled, or if the counter has been reset using the `pg_stat_statements_reset` function with the `minmax_only` parameter set to `true` and never been planned since.
`max_plan_time` `double precision` Maximum time spent planning the statement, in milliseconds. This field will be zero if `pg_stat_statements.track_planning` is disabled, or if the counter has been reset using the `pg_stat_statements_reset` function with the `minmax_only` parameter set to `true` and never been planned since.
`mean_plan_time` `double precision` Mean time spent planning the statement, in milliseconds (if `pg_stat_statements.track_planning` is enabled, otherwise zero)
`stddev_plan_time` `double precision` Population standard deviation of time spent planning the statement, in milliseconds (if `pg_stat_statements.track_planning` is enabled, otherwise zero)
`calls` `bigint`Number of times the statement was executed
`total_exec_time` `double precision` Total time spent executing the statement, in milliseconds
`min_exec_time` `double precision` Minimum time spent executing the statement, in milliseconds, this field will be zero until this statement is executed first time after reset performed by the `pg_stat_statements_reset` function with the `minmax_only` parameter set to `true`
`max_exec_time` `double precision` Maximum time spent executing the statement, in milliseconds, this field will be zero until this statement is executed first time after reset performed by the `pg_stat_statements_reset` function with the `minmax_only` parameter set to `true`
`mean_exec_time` `double precision` Mean time spent executing the statement, in milliseconds
`stddev_exec_time` `double precision` Population standard deviation of time spent executing the statement, in milliseconds
`rows` `bigint` Total number of rows retrieved or affected by the statement
`shared_blks_hit` `bigint` Total number of shared block cache hits by the statement
`shared_blks_read` `bigint` Total number of shared blocks read by the statement
`shared_blks_dirtied` `bigint` Total number of shared blocks dirtied by the statement
`shared_blks_written` `bigint` Total number of shared blocks written by the statement
`local_blks_hit` `bigint` Total number of local block cache hits by the statement
`local_blks_read` `bigint` Total number of local blocks read by the statement
`local_blks_dirtied` `bigint` Total number of local blocks dirtied by the statement
`local_blks_written` `bigint` Total number of local blocks written by the statement
`temp_blks_read` `bigint` Total number of temp blocks read by the statement
`temp_blks_written` `bigint` Total number of temp blocks written by the statement
`shared_blk_read_time` `double precision` Total time the statement spent reading shared blocks, in milliseconds
`shared_blk_write_time` `double precision` Total time the statement spent writing shared blocks, in milliseconds
`local_blk_read_time` `double precision` Total time the statement spent reading local blocks, in milliseconds
`local_blk_write_time` `double precision` Total time the statement spent writing local blocks, in milliseconds
`temp_blk_read_time` `double precision` Total time the statement spent reading temporary file blocks, in milliseconds
`temp_blk_write_time` `double precision` Total time the statement spent writing temporary file blocks, in milliseconds
`wal_records` `bigint` Total number of WAL records generated by the statement
`wal_fpi` `bigint` Total number of WAL full page images generated by the statement
`wal_bytes` `numeric` Total amount of WAL generated by the statement in bytes
`jit_functions` `bigint` Total number of functions JIT-compiled by the statement
`jit_generation_time` `double precision` Total time spent by the statement on generating JIT code, in milliseconds
`jit_inlining_count` `bigint` Number of times functions have been inlined
`jit_inlining_time` `double precision` Total time spent by the statement on inlining functions, in milliseconds
`jit_optimization_count` `bigint` Number of times the statement has been optimized
`jit_optimization_time` `double precision` Total time spent by the statement on optimizing, in milliseconds
`jit_emission_count` `bigint` Number of times code has been emitted
`jit_emission_time` `double precision` Total time spent by the statement on emitting code, in milliseconds
`jit_deform_count` `bigint` Total number of tuple deform functions JIT-compiled by the statement
`jit_deform_time` `double precision` Total time spent by the statement on JIT-compiling tuple deform functions, in milliseconds
`stats_since` `timestamp with time zone` Time at which statistics gathering started for this statement
`minmax_stats_since` `timestamp with time zone` Time at which min/max statistics gathering started for this statement (fields `min_plan_time`, `max_plan_time`, `min_exec_time` and `max_exec_time`)

For security reasons, only superusers and roles with privileges of the pg_read_all_stats role are allowed to see the SQL text and queryid of queries executed by other users. Other users can see the statistics, however, if the view has been installed in their database.

Plannable queries (that is, SELECT, INSERT, UPDATE, DELETE, and MERGE) and utility commands are combined into a single pg_stat_statements entry whenever they have identical query structures according to an internal hash calculation. Typically, two queries will be considered the same for this purpose if they are semantically equivalent except for the values of literal constants appearing in the query.

The following details about constant replacement and queryid only apply when compute_query_id is enabled. If you use an external module instead to compute queryid, you should refer to its documentation for details.

When a constant's value has been ignored for purposes of matching the query to other queries, the constant is replaced by a parameter symbol, such as $1, in the pg_stat_statements display. The rest of the query text is that of the first query that had the particular queryid hash value associated with the pg_stat_statements entry.

Queries on which normalization can be applied may be observed with constant values in pg_stat_statements, especially when there is a high rate of entry deallocations. To reduce the likelihood of this happening, consider increasing pg_stat_statements.max. The pg_stat_statements_info view, discussed below in Section F.30.2, provides statistics about entry deallocations.

In some cases, queries with visibly different texts might get merged into a single pg_stat_statements entry. Normally this will happen only for semantically equivalent queries, but there is a small chance of hash collisions causing unrelated queries to be merged into one entry. (This cannot happen for queries belonging to different users or databases, however.)

Since the queryid hash value is computed on the post-parse-analysis representation of the queries, the opposite is also possible: queries with identical texts might appear as separate entries, if they have different meanings as a result of factors such as different search_path settings.

Consumers of pg_stat_statements may wish to use queryid (perhaps in combination with dbid and userid) as a more stable and reliable identifier for each entry than its query text. However, it is important to understand that there are only limited guarantees around the stability of the queryid hash value. Since the identifier is derived from the post-parse-analysis tree, its value is a function of, among other things, the internal object identifiers appearing in this representation. This has some counterintuitive implications. For example, pg_stat_statements will consider two apparently-identical queries to be distinct, if they reference a table that was dropped and recreated between the executions of the two queries. The hashing process is also sensitive to differences in machine architecture and other facets of the platform. Furthermore, it is not safe to assume that queryid will be stable across major versions of ProtonBase.

Two servers participating in replication based on physical WAL replay can be expected to have identical queryid values for the same query. However, logical replication schemes do not promise to keep replicas identical in all relevant details, so queryid will not be a useful identifier for accumulating costs across a set of logical replicas. If in doubt, direct testing is recommended.

Generally, it can be assumed that queryid values are stable between minor version releases of ProtonBase, providing that instances are running on the same machine architecture and the catalog metadata details match. Compatibility will only be broken between minor versions as a last resort.

The parameter symbols used to replace constants in representative query texts start from the next number after the highest $n parameter in the original query text, or $1 if there was none. It's worth noting that in some cases there may be hidden parameter symbols that affect this numbering. For example, PL/pgSQL uses hidden parameter symbols to insert values of function local variables into queries, so that a PL/pgSQL statement like SELECT i + 1 INTO j would have representative text like SELECT i + $2.

The representative query texts are kept in an external disk file, and do not consume shared memory. Therefore, even very lengthy query texts can be stored successfully. However, if many long query texts are accumulated, the external file might grow unmanageably large. As a recovery method if that happens, pg_stat_statements may choose to discard the query texts, whereupon all existing entries in the pg_stat_statements view will show null query fields, though the statistics associated with each queryid are preserved. If this happens, consider reducing pg_stat_statements.max to prevent recurrences.

plans and calls aren't always expected to match because planning and execution statistics are updated at their respective end phase, and only for successful operations. For example, if a statement is successfully planned but fails during the execution phase, only its planning statistics will be updated. If planning is skipped because a cached plan is used, only its execution statistics will be updated.

F.30.2. The `pg_stat_statements_info` View

The statistics of the pg_stat_statements module itself are tracked and made available via a view named pg_stat_statements_info. This view contains only a single row. The columns of the view are shown in Table F.22.

Table F.22. pg_stat_statements_info Columns

Column TypeDescription
`dealloc` `bigint`Total number of times `pg_stat_statements` entries about the least-executed statements were deallocated because more distinct statements than `pg_stat_statements.max` were observed
`stats_reset` `timestamp with time zone`Time at which all statistics in the `pg_stat_statements` view were last reset.

F.30.3. Functions

pg_stat_statements_reset(userid Oid, dbid Oid, queryid bigint, minmax_only boolean) returns timestamp with time zone

pg_stat_statements_reset discards statistics gathered so far by pg_stat_statements corresponding to the specified userid, dbid and queryid. If any of the parameters are not specified, the default value 0(invalid) is used for each of them and the statistics that match with other parameters will be reset. If no parameter is specified or all the specified parameters are 0(invalid), it will discard all statistics. If all statistics in the pg_stat_statements view are discarded, it will also reset the statistics in the pg_stat_statements_info view. When minmax_only is true only the values of minimum and maximum planning and execution time will be reset (i.e. min_plan_time, max_plan_time, min_exec_time and max_exec_time fields). The default value for minmax_only parameter is false. Time of last min/max reset performed is shown in minmax_stats_since field of the pg_stat_statements view. This function returns the time of a reset. This time is saved to stats_reset field of pg_stat_statements_info view or to minmax_stats_since field of the pg_stat_statements view if the corresponding reset was actually performed. By default, this function can only be executed by superusers. Access may be granted to others using GRANT.

pg_stat_statements(showtext boolean) returns setof record

The pg_stat_statements view is defined in terms of a function also named pg_stat_statements. It is possible for clients to call the pg_stat_statements function directly, and by specifying showtext := false have query text be omitted (that is, the OUT argument that corresponds to the view's query column will return nulls). This feature is intended to support external tools that might wish to avoid the overhead of repeatedly retrieving query texts of indeterminate length. Such tools can instead cache the first query text observed for each entry themselves, since that is all pg_stat_statements itself does, and then retrieve query texts only as needed. Since the server stores query texts in a file, this approach may reduce physical I/O for repeated examination of the pg_stat_statements data.

F.30.4. Configuration Parameters

pg_stat_statements.max (integer)

pg_stat_statements.max is the maximum number of statements tracked by the module (i.e., the maximum number of rows in the pg_stat_statements view). If more distinct statements than that are observed, information about the least-executed statements is discarded. The number of times such information was discarded can be seen in the pg_stat_statements_info view. The default value is 5000. This parameter can only be set at server start.

pg_stat_statements.track (enum)

pg_stat_statements.track controls which statements are counted by the module. Specify top to track top-level statements (those issued directly by clients), all to also track nested statements (such as statements invoked within functions), or none to disable statement statistics collection. The default value is top. Only superusers can change this setting.

pg_stat_statements.track_utility (boolean)

pg_stat_statements.track_utility controls whether utility commands are tracked by the module. Utility commands are all those other than SELECT, INSERT, UPDATE, DELETE, and MERGE. The default value is on. Only superusers can change this setting.

pg_stat_statements.track_planning (boolean)

pg_stat_statements.track_planning controls whether planning operations and duration are tracked by the module. Enabling this parameter may incur a noticeable performance penalty, especially when statements with identical query structure are executed by many concurrent connections which compete to update a small number of pg_stat_statements entries. The default value is off. Only superusers can change this setting.

pg_stat_statements.save (boolean)

pg_stat_statements.save specifies whether to save statement statistics across server shutdowns. If it is off then statistics are not saved at shutdown nor reloaded at server start. The default value is on. This parameter can only be set in the postgresql.conf file or on the server command line.

The module requires additional shared memory proportional to pg_stat_statements.max. Note that this memory is consumed whenever the module is loaded, even if pg_stat_statements.track is set to none.

These parameters must be set in postgresql.conf. Typical usage might be:

postgresql.conf

shared_preload_libraries = 'pg_stat_statements'
compute_query_id = on
pg_stat_statements.max = 10000
pg_stat_statements.track = all

F.30.5. Sample Output

bench=# SELECT pg_stat_statements_reset();
$ pgbench -i bench $ pgbench -c10 -t300 bench
bench=# \x
bench=# SELECT query, calls, total_exec_time, rows, 100.0 * shared_blks_hit /
 nullif(shared_blks_hit + shared_blks_read, 0) AS hit_percent
 FROM pg_stat_statements ORDER BY total_exec_time DESC LIMIT 5;
-[ RECORD 1 ]---+--------------------------------------------------------------------
query | UPDATE pgbench_branches SET bbalance = bbalance + $1 WHERE bid = $2
calls | 3000
total_exec_time | 25565.855387
rows | 3000
hit_percent | 100.0000000000000000
-[ RECORD 2 ]---+--------------------------------------------------------------------
query | UPDATE pgbench_tellers SET tbalance = tbalance + $1 WHERE tid = $2
calls | 3000
total_exec_time | 20756.669379
rows | 3000
hit_percent | 100.0000000000000000
-[ RECORD 3 ]---+--------------------------------------------------------------------
query | copy pgbench_accounts from stdin
calls | 1
total_exec_time | 291.865911
rows | 100000
hit_percent | 100.0000000000000000
-[ RECORD 4 ]---+--------------------------------------------------------------------
query | UPDATE pgbench_accounts SET abalance = abalance + $1 WHERE aid = $2
calls | 3000
total_exec_time | 271.232977
rows | 3000
hit_percent | 98.8454011741682975
-[ RECORD 5 ]---+--------------------------------------------------------------------
query | alter table pgbench_accounts add primary key (aid)
calls | 1
total_exec_time | 160.588563
rows | 0
hit_percent | 100.0000000000000000
bench=# SELECT pg_stat_statements_reset(0,0,s.queryid) FROM pg_stat_statements AS s
 WHERE s.query = 'UPDATE pgbench_branches SET bbalance = bbalance + $1 WHERE bid = $2';
bench=# SELECT query, calls, total_exec_time, rows, 100.0 * shared_blks_hit /
 nullif(shared_blks_hit + shared_blks_read, 0) AS hit_percent
 FROM pg_stat_statements ORDER BY total_exec_time DESC LIMIT 5;
-[ RECORD 1 ]---+--------------------------------------------------------------------
query | UPDATE pgbench_tellers SET tbalance = tbalance + $1 WHERE tid = $2
calls | 3000
total_exec_time | 20756.669379
rows | 3000
hit_percent | 100.0000000000000000
-[ RECORD 2 ]---+--------------------------------------------------------------------
query | copy pgbench_accounts from stdin
calls | 1
total_exec_time | 291.865911
rows | 100000
hit_percent | 100.0000000000000000
-[ RECORD 3 ]---+--------------------------------------------------------------------
query | UPDATE pgbench_accounts SET abalance = abalance + $1 WHERE aid = $2
calls | 3000
total_exec_time | 271.232977
rows | 3000
hit_percent | 98.8454011741682975
-[ RECORD 4 ]---+--------------------------------------------------------------------
query | alter table pgbench_accounts add primary key (aid)
calls | 1
total_exec_time | 160.588563
rows | 0
hit_percent | 100.0000000000000000
-[ RECORD 5 ]---+--------------------------------------------------------------------
query | vacuum analyze pgbench_accounts
calls | 1
total_exec_time | 136.448116
rows | 0
hit_percent | 99.9201915403032721
bench=# SELECT pg_stat_statements_reset(0,0,0);
bench=# SELECT query, calls, total_exec_time, rows, 100.0 * shared_blks_hit /
 nullif(shared_blks_hit + shared_blks_read, 0) AS hit_percent
 FROM pg_stat_statements ORDER BY total_exec_time DESC LIMIT 5;
-[ RECORD 1 ]---+-----------------------------------------------------------------------------
query | SELECT pg_stat_statements_reset(0,0,0)
calls | 1
total_exec_time | 0.189497
rows | 1
hit_percent |
-[ RECORD 2 ]---+-----------------------------------------------------------------------------
query | SELECT query, calls, total_exec_time, rows, $1 * shared_blks_hit / +
 | nullif(shared_blks_hit + shared_blks_read, $2) AS hit_percent+
 | FROM pg_stat_statements ORDER BY total_exec_time DESC LIMIT $3
calls | 0
total_exec_time | 0
rows | 0
hit_percent |

F.30.6. Authors

Takahiro Itagaki <[itagaki.takahiro@oss.ntt.co.jp](mailto:itagaki.takahiro@oss.ntt.co.jp)>. Query normalization added by Peter Geoghegan <[peter@2ndquadrant.com](mailto:peter@2ndquadrant.com)>.