On Wed, Nov 9, 2011 at 9:15 AM, Kevin Grittner <Kevin.Grittner@xxxxxxxxxxxx> wrote: > Jay Levitt <jay.levitt@xxxxxxxxx> wrote: > >> I don't get why the GROUP BY in this subquery forces it to scan >> the entire users table (seq scan here, index scan on a larger >> table) when there's only one row in users that can match: > >> explain analyze >> select questions.id >> from questions >> join ( >> select u.id >> from users as u >> group by u.id >> ) as s >> on s.id = questions.user_id >> where questions.id = 1; > >> Total runtime: 1.262 ms > > Are you sure there's a plan significantly faster than 1.3 ms? Well, this may not fit the OP's 'real' query, but the inner subquery is probably better written as a semi-join (WHERE EXISTS). merlin -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
