Count estimate - PostgreSQL wiki (original) (raw)

Snippets

count_estimate function

Works with PostgreSQL

Any version

Written in

sql

Depends on

Nothing

Authors: Erwin Brandstetter, Michael Fuhr
Comment by: Emanuel Calvo Franco

The basic SQL standard query to count the rows in a table is:

SELECT count(*) FROM table_name;

This can be rather slow because PostgreSQL has to check visibility for all rows, due to the MVCC model.

If you don't need an exact count, the current statistic from the catalog table pg_class might be good enough and is much faster to retrieve for big tables.

SELECT reltuples AS estimate FROM pg_class WHERE relname = 'table_name';

estimate

   100

Tables named "table_name" can live in multiple schemas of a database, in which case you get multiple rows for this query. To overcome ambiguity:

SELECT reltuples::bigint AS estimate FROM pg_class WHERE oid = 'schema_name.table_name'::regclass;

The cast to bigint formats the real number nicely, especially for big counts.

Quoting the manual for Postgres 13 on pg_class.reltuples:

Number of live rows in the table. This is only an estimate used by the planner. It is updated by VACUUM, ANALYZE, and a few DDL commands such as CREATE INDEX.

If you didn't ANALYZE recently (after last changes), the estimates will be off more or less.
If you are running the autovacuum daemon as is the default for modern PostgreSQL, ANALYZE is run automatically, too (except for temporary tables which need manual attention). So the estimates should be good unless you had major changes very recently.

For more sophisticated queries (other than counting all rows from a table), or if you cannot SELECT from the catalog table pg_class (which every user can by default), consider the following function, that takes advantage of JSON formatting added in Postgres 9.0. (Older versions can still benefit from plpgsql function by Michael Fuhr.)

⚠️ Do not pass an unsanitized `query` to this function, as it is subject to SQL injection.

CREATE OR REPLACE FUNCTION count_estimate( query text ) RETURNS integer LANGUAGE plpgsql AS DECLAREplanjsonb;BEGINEXECUTEFORMAT(′EXPLAIN(FORMATJSON)RETURNplan−>0−>′Plan′−>′PlanRows′;END;DECLARE plan jsonb; BEGIN EXECUTE FORMAT('EXPLAIN (FORMAT JSON) %s', query) INTO plan; RETURN plan->0->'Plan'->'Plan Rows'; END;DECLAREplanjsonb;BEGINEXECUTEFORMAT(′EXPLAIN(FORMATJSON)RETURNplan−>0−>′Plan′−>′PlanRows′;END;;

Demo:

CREATE TEMP TABLE tbl AS SELECT * FROM generate_series(1, 1000) AS t; ANALYZE tbl;

SELECT count_estimate('SELECT * FROM tbl WHERE t < 100');

count_estimate 

        100

EXPLAIN SELECT * FROM tbl WHERE t < 100;

                  QUERY PLAN

Seq Scan on tbl (cost=0.00..35.00 rows=100 width=4) Filter: (t < 100)

As you can see, it's an estimate - actual count would be 99.

Related web resources: