PostgreSQL pg_stat_statement | SQL Monitoring

        What is pg_stat_statement : -

The pg_stat_statements module provides a means for tracking execution statistics of all SQL statements executed by a server.

It provides below information for each individual SQL.

OID of user who executed the statement

OID of database in which the statement was executed

Internal hash code, computed from the statement's parse tree

Text of a representative statement

Number of times executed

Total time spent in the statement, in milliseconds

Total number of rows retrieved or affected by the statement

Total number of shared block cache hits by the statement

Total number of shared blocks read by the statement

Total number of shared blocks dirtied by the statement

Total number of shared blocks written by the statement

Total number of local block cache hits by the statement

Total number of local blocks read by the statement

Total number of local blocks dirtied by the statement

Total number of local blocks written by the statement

Total number of temp blocks read by the statement

Total number of temp blocks written by the statement

Total time the statement spent reading blocks, in milliseconds (if track_io_timing is enabled, otherwise zero)

Total time the statement spent writing blocks, in milliseconds (if track_io_timing is enabled, otherwise zero)

        Installation of pg_stat_statement :-

Step 1

create extension pg_stat_statements ;

Step 2

Add below entries into postgresql.conf

shared_preload_libraries = 'pg_stat_statements'

pg_stat_statements.max = 10000

pg_stat_statements.track = all

Then Restart PostgreSQL service.

        Configuration Parameters: -

A) 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.

B) 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.

C) 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.

D) 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.

        Functions : -

A) 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.

B) 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.

        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 database ID, user ID and query ID (up to the maximum number of distinct statements that the module can track).

        Reference Queries -  

1)

Select * from pg_stat_statements ;

2)

SELECT  substring(query, 1, 50) AS short_query,

round(total_time::numeric, 2) AS total_time,

calls,

round(mean_time::numeric, 2) AS mean,

round((100 * total_time /

sum(total_time::numeric) OVER ())::numeric, 2) AS percentage_cpu

FROM    pg_stat_statements

ORDER BY total_time DESC

LIMIT 20;

3)

select userid,query,calls,total_time from pg_stat_statements;

4)

select * from pg_stat_statements(true);