pg\_stat\_statements {#pgstatstatements}
====================

pg\_stat\_statements

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
[???](#guc-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 [???](#guc-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`.

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\_title](#pgstatstatements-columns).

+-----------------------------------------------------------------------+
| Column Type                                                           |
|                                                                       |
| Description                                                           |
+=======================================================================+
| userid `oid` (references [pg\_authid](#catalog-pg-authid).oid)        |
|                                                                       |
| OID of user who executed the statement                                |
+-----------------------------------------------------------------------+
| dbid `oid` (references [pg\_database](#catalog-pg-database).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 (if        |
| `pg_stat_statements.track_planning` is enabled, otherwise zero)       |
+-----------------------------------------------------------------------+
| max\_plan\_time `double precision`                                    |
|                                                                       |
| Maximum time spent planning the statement, in milliseconds (if        |
| `pg_stat_statements.track_planning` is enabled, otherwise zero)       |
+-----------------------------------------------------------------------+
| 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           |
+-----------------------------------------------------------------------+
| max\_exec\_time `double precision`                                    |
|                                                                       |
| Maximum time spent executing the statement, in milliseconds           |
+-----------------------------------------------------------------------+
| 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                  |
+-----------------------------------------------------------------------+
| blk\_read\_time `double precision`                                    |
|                                                                       |
| Total time the statement spent reading data file blocks, in           |
| milliseconds (if [???](#guc-track-io-timing) is enabled, otherwise    |
| zero)                                                                 |
+-----------------------------------------------------------------------+
| blk\_write\_time `double precision`                                   |
|                                                                       |
| Total time the statement spent writing data file blocks, in           |
| milliseconds (if [???](#guc-track-io-timing) is enabled, otherwise    |
| zero)                                                                 |
+-----------------------------------------------------------------------+
| temp\_blk\_read\_time `double precision`                              |
|                                                                       |
| Total time the statement spent reading temporary file blocks, in      |
| milliseconds (if [???](#guc-track-io-timing) is enabled, otherwise    |
| zero)                                                                 |
+-----------------------------------------------------------------------+
| temp\_blk\_write\_time `double precision`                             |
|                                                                       |
| Total time the statement spent writing temporary file blocks, in      |
| milliseconds (if [???](#guc-track-io-timing) is enabled, otherwise    |
| zero)                                                                 |
+-----------------------------------------------------------------------+
| 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 `bigint`                                        |
|                                                                       |
| 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 `bigint`                                          |
|                                                                       |
| 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 `bigint`                                      |
|                                                                       |
| Total time spent by the statement on optimizing, in milliseconds      |
+-----------------------------------------------------------------------+
| jit\_emission\_count `bigint`                                         |
|                                                                       |
| Number of times code has been emitted                                 |
+-----------------------------------------------------------------------+
| jit\_emission\_time `bigint`                                          |
|                                                                       |
| Total time spent by the statement on emitting code, in milliseconds   |
+-----------------------------------------------------------------------+

: pg\_stat\_statements Columns

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`, 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.

> **Note**
>
> The following details about constant replacement and queryid only
> apply when [???](#guc-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.

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 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.

The pg\_stat\_statements\_info View
-----------------------------------

pg\_stat\_statements\_info

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\_title](#pgstatstatementsinfo-columns).

+-----------------------------------------------------------------------+
| Column Type                                                           |
|                                                                       |
| Description                                                           |
+=======================================================================+
| 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.                                                           |
+-----------------------------------------------------------------------+

: pg\_stat\_statements\_info Columns

Functions
---------

`pg_stat_statements_reset(userid Oid, dbid Oid, queryid bigint) returns void` pg\_stat\_statements\_reset

:   `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. 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` pg\_stat\_statements function

:   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.

Configuration Parameters
------------------------

`pg_stat_statements.max` (`integer`) `pg_stat_statements.max` configuration parameter

:   `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` configuration parameter

:   `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` configuration parameter

:   `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.track_planning` (`boolean`) `pg_stat_statements.track_planning` configuration parameter

:   `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` configuration parameter

:   `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

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 ]---+--------------------------------------------------ZWSP------------------
    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 ]---+--------------------------------------------------ZWSP------------------
    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 ]---+--------------------------------------------------ZWSP------------------
    query           | copy pgbench_accounts from stdin
    calls           | 1
    total_exec_time | 291.865911
    rows            | 100000
    hit_percent     | 100.0000000000000000
    -[ RECORD 4 ]---+--------------------------------------------------ZWSP------------------
    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 ]---+--------------------------------------------------ZWSP------------------
    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 ]---+--------------------------------------------------ZWSP------------------
    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 ]---+--------------------------------------------------ZWSP------------------
    query           | copy pgbench_accounts from stdin
    calls           | 1
    total_exec_time | 291.865911
    rows            | 100000
    hit_percent     | 100.0000000000000000
    -[ RECORD 3 ]---+--------------------------------------------------ZWSP------------------
    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 ]---+--------------------------------------------------ZWSP------------------
    query           | alter table pgbench_accounts add primary key (aid)
    calls           | 1
    total_exec_time | 160.588563
    rows            | 0
    hit_percent     | 100.0000000000000000
    -[ RECORD 5 ]---+--------------------------------------------------ZWSP------------------
    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 ]---+--------------------------------------------------ZWSP---------------------------
    query           | SELECT pg_stat_statements_reset(0,0,0)
    calls           | 1
    total_exec_time | 0.189497
    rows            | 1
    hit_percent     | 
    -[ RECORD 2 ]---+--------------------------------------------------ZWSP---------------------------
    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     | 

Authors
-------

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