pg_stat_statements

PostgreSQL 9.2.10 Documentation
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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 `<itagaki.takahiro@oss.ntt.co.jp>`. Query normalization added by Peter Geoghegan `<peter@2ndquadrant.com>`.

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