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::doubleprecision)
(2 rows)
--
Michael Fuhr
