Re: **SPAM** Faster count(*)?

On Tue, Aug 09, 2005 at 09:29:13PM -0600, Michael Fuhr wrote:
> On Tue, Aug 09, 2005 at 10:49:14PM -0400, Tom Lane wrote:
> > Current best practice is to run the explain and parse out the "rows"
> > figure using a perl (or axe-of-choice) regexp, though we could be
> > persuaded to supply a simpler API if there's enough demand for it.
>
> Somebody else requested a row-count-estimate function a couple of
> weeks ago:
>
> http://archives.postgresql.org/pgsql-admin/2005-07/msg00256.php

Here's a simple example that parses EXPLAIN output. It should work
in 8.0.2 and later:

CREATE FUNCTION count_estimate(query text) RETURNS integer AS $$
DECLARE
rec record;
rows integer;
BEGIN
FOR rec IN EXECUTE 'EXPLAIN ' || query LOOP
rows := substring(rec."QUERY PLAN" FROM ' rows=([[:digit:]]+)');
EXIT WHEN rows IS NOT NULL;
END LOOP;

RETURN rows;
END;
$$ LANGUAGE plpgsql VOLATILE STRICT;

CREATE TABLE foo (r double precision);
INSERT INTO foo SELECT random() FROM generate_series(1, 1000);
ANALYZE foo;

SELECT count_estimate('SELECT * FROM foo WHERE r < 0.1');
count_estimate
----------------
97
(1 row)

EXPLAIN SELECT * FROM foo WHERE r < 0.1;
QUERY PLAN
-----------------------------------------------------
Seq Scan on foo (cost=0.00..17.50 rows=97 width=8)
Filter: (r < 0.1::double precision)
(2 rows)

--
Michael Fuhr