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.

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.

F.29.1. 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 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-22.

Table F-22. 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
`queryid`	`bigint`		Internal hash code, computed from the statement's parse tree
`query`	`text`		Text of a representative statement
`calls`	`bigint`		Number of times executed
`total_time`	`double precision`		Total time spent in the statement, in milliseconds
`min_time`	`double precision`		Minimum time spent in the statement, in milliseconds
`max_time`	`double precision`		Maximum time spent in the statement, in milliseconds
`mean_time`	`double precision`		Mean time spent in the statement, in milliseconds
`stddev_time`	`double precision`		Population standard deviation of 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)

For security reasons, non-superusers are not allowed to see the SQL text or queryid 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 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.

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 PostgreSQL.

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 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.

F.29.2. 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.
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.29.3. 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 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 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:

# postgresql.conf
shared_preload_libraries = 'pg_stat_statements'

pg_stat_statements.max = 10000
pg_stat_statements.track = all

F.29.4. 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.29.5. Authors

Takahiro Itagaki <itagaki.takahiro@oss.ntt.co.jp>. Query normalization added by Peter Geoghegan <peter@2ndquadrant.com>.

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Links:

https://www.postgresql.org/docs/9.6/static/pgstatstatements.html

doc_PostgreSQL

2016-10-12 11:33:54

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