F.29. pg_stat_statements

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.

When pg_stat_statements is loaded, it tracks statistics across all databases of the server. To access and manipulate these statistics, the module provides a view, pg_stat_statements, 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.

The statistics gathered by the module are made available via a view named pg_stat_statements. This view contains one row for each distinct database ID, user ID and query ID (up to the maximum number of distinct statements that the module can track). The columns of the view are shown in .

Table F.21. pg_stat_statements Columns

For security reasons, only superusers and members 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, and DELETE) 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. Utility commands (that is, all other commands) are compared strictly on the basis of their textual query strings, however.

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.

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.

As a rule of thumb, queryid values can be assumed to be stable and comparable only so long as the underlying server version and catalog metadata details stay exactly the same. 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.

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.

pg_stat_statements_reset(userid Oid, dbid Oid, queryid bigint) returns void

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. 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.

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 default value is 5000. This parameter can only be set at server start.

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 and DELETE. The default value is on. 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:

  1. bench=# SELECT pg_stat_statements_reset();
  3. $ pgbench -i bench
  4. $ pgbench -c10 -t300 bench
  6. bench=# \x
  7. bench=# SELECT query, calls, total_time, rows, 100.0 * shared_blks_hit /
  8.                nullif(shared_blks_hit + shared_blks_read, 0) AS hit_percent
  9.           FROM pg_stat_statements ORDER BY total_time DESC LIMIT 5;
  10. -[ RECORD 1 ]--------------------------------------------------------------------
  11. query       | UPDATE pgbench_branches SET bbalance = bbalance + $1 WHERE bid = $2
  12. calls       | 3000
  13. total_time  | 25565.855387
  14. rows        | 3000
  15. hit_percent | 100.0000000000000000
  16. -[ RECORD 2 ]--------------------------------------------------------------------
  18. calls       | 3000
  19. total_time  | 20756.669379
  20. rows        | 3000
  21. hit_percent | 100.0000000000000000
  22. -[ RECORD 3 ]--------------------------------------------------------------------
  23. query       | copy pgbench_accounts from stdin
  24. calls       | 1
  25. total_time  | 291.865911
  26. hit_percent | 100.0000000000000000
  27. -[ RECORD 4 ]--------------------------------------------------------------------
  28. query       | UPDATE pgbench_accounts SET abalance = abalance + $1 WHERE aid = $2
  29. calls       | 3000
  30. total_time  | 271.232977
  31. rows        | 3000
  32. hit_percent | 98.5723926698852723
  33. -[ RECORD 5 ]--------------------------------------------------------------------
  34. query       | alter table pgbench_accounts add primary key (aid)
  35. calls       | 1
  36. total_time  | 160.588563
  37. rows        | 0
  38. hit_percent | 100.0000000000000000
  41. bench=# SELECT pg_stat_statements_reset(0,0,s.queryid) FROM pg_stat_statements AS s
  42.             WHERE s.query = 'UPDATE pgbench_branches SET bbalance = bbalance + $1 WHERE bid = $2';
  44. bench=# SELECT query, calls, total_time, rows, 100.0 * shared_blks_hit /
  45.                nullif(shared_blks_hit + shared_blks_read, 0) AS hit_percent
  46.           FROM pg_stat_statements ORDER BY total_time DESC LIMIT 5;
  47. -[ RECORD 1 ]--------------------------------------------------------------------
  48. query       | UPDATE pgbench_tellers SET tbalance = tbalance + $1 WHERE tid = $2
  49. calls       | 3000
  50. total_time  | 20756.669379
  51. rows        | 3000
  52. hit_percent | 100.0000000000000000
  53. -[ RECORD 2 ]--------------------------------------------------------------------
  54. query       | copy pgbench_accounts from stdin
  55. calls       | 1
  56. total_time  | 291.865911
  57. hit_percent | 100.0000000000000000
  58. -[ RECORD 3 ]--------------------------------------------------------------------
  59. query       | UPDATE pgbench_accounts SET abalance = abalance + $1 WHERE aid = $2
  60. calls       | 3000
  61. total_time  | 271.232977
  62. rows        | 3000
  63. hit_percent | 98.5723926698852723
  64. -[ RECORD 4 ]--------------------------------------------------------------------
  65. query       | alter table pgbench_accounts add primary key (aid)
  66. calls       | 1
  67. total_time  | 160.588563
  68. rows        | 0
  69. hit_percent | 100.0000000000000000
  70. -[ RECORD 5 ]--------------------------------------------------------------------
  71. query       | vacuum analyze pgbench_accounts
  72. calls       | 1
  73. total_time  | 136.448116
  74. rows        | 0
  75. hit_percent | 99.9201915403032721
  77. bench=# SELECT pg_stat_statements_reset(0,0,0);
  79. bench=# SELECT query, calls, total_time, rows, 100.0 * shared_blks_hit /
  80.                nullif(shared_blks_hit + shared_blks_read, 0) AS hit_percent
  81.           FROM pg_stat_statements ORDER BY total_time DESC LIMIT 5;
  82. -[ RECORD 1 ]---------------------------------------
  83. query       | SELECT pg_stat_statements_reset(0,0,0)
  84. calls       | 1
  85. total_time  | 0.189497
  86. rows        | 1
  87. hit_percent |

Takahiro Itagaki <>. Query normalization added by Peter Geoghegan <peter@2ndquadrant.com>.