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F.28. pg_stat_statements

The pg_stat_statements module provides a means for tracking 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.

F.28.1. The pg_stat_statements View

The statistics gathered by the module are made available via a system view named pg_stat_statements. This view contains one row for each distinct query, database ID, and user ID (up to the maximum number of distinct statements that the module can track). The columns of the view are shown in Table F-20.

Table F-20. pg_stat_statements Columns

Name Type References Description
userid oid pg_authid.oid OID of user who executed the statement
dbid oid pg_database.oid OID of database in which the statement was executed
query text   Text of a representative statement (up to track_activity_query_size bytes)
calls bigint   Number of times executed
total_time double precision   Total time spent in 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
blk_read_time double precision   Total time the statement spent reading blocks, in milliseconds (if track_io_timing is enabled, otherwise zero)
blk_write_time double precision   Total time the statement spent writing blocks, in milliseconds (if track_io_timing is enabled, otherwise zero)

This view, and the function pg_stat_statements_reset, are available only in databases they have been specifically installed into by installing the pg_stat_statements extension. However, statistics are tracked across all databases of the server whenever the pg_stat_statements module is loaded into the server, regardless of presence of the view.

For security reasons, non-superusers are not allowed to see the text of queries executed by other users. They 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 ? in the pg_stat_statements display. The rest of the query text is that of the first query that had the particular 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 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.

F.28.2. Functions

pg_stat_statements_reset() returns void

pg_stat_statements_reset discards all statistics gathered so far by pg_stat_statements. By default, this function can only be executed by superusers.

F.28.3. 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 default value is 1000. 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 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 amounting to about pg_stat_statements.max * track_activity_query_size bytes. 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'

pg_stat_statements.max = 10000
pg_stat_statements.track = all

F.28.4. Sample Output

bench=# SELECT pg_stat_statements_reset();

$ pgbench -i bench
$ pgbench -c10 -t300 bench

bench=# \x
bench=# SELECT query, calls, total_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_time DESC LIMIT 5;
-[ RECORD 1 ]---------------------------------------------------------------------
query       | UPDATE pgbench_branches SET bbalance = bbalance + ? WHERE bid = ?;
calls       | 3000
total_time  | 9609.00100000002
rows        | 2836
hit_percent | 99.9778970000200936
-[ RECORD 2 ]---------------------------------------------------------------------
query       | UPDATE pgbench_tellers SET tbalance = tbalance + ? WHERE tid = ?;
calls       | 3000
total_time  | 8015.156
rows        | 2990
hit_percent | 99.9731126579631345
-[ RECORD 3 ]---------------------------------------------------------------------
query       | copy pgbench_accounts from stdin
calls       | 1
total_time  | 310.624
rows        | 100000
hit_percent | 0.30395136778115501520
-[ RECORD 4 ]---------------------------------------------------------------------
query       | UPDATE pgbench_accounts SET abalance = abalance + ? WHERE aid = ?;
calls       | 3000
total_time  | 271.741999999997
rows        | 3000
hit_percent | 93.7968855088209426
-[ RECORD 5 ]---------------------------------------------------------------------
query       | alter table pgbench_accounts add primary key (aid)
calls       | 1
total_time  | 81.42
rows        | 0
hit_percent | 34.4947735191637631

F.28.5. Authors

Takahiro Itagaki . Query normalization added by Peter Geoghegan .

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