am Wed, dem 23.04.2008, um 9:23:07 +0200 mailte Hans Ekbrand folgendes: > I cannot understand why the following two queries differ so much in execution > time (almost ten times)
wild guess: different execution plans. Can you show us the plans? (EXPLAIN ANALYSE SELECT ...) > > Query A (two queries) > > select distinct moment.mid from moment,timecard where parent = 45 and (pid=17 > and timecard.mid = moment.mid) order by moment.mid; > select distinct moment.mid from moment,timecard where parent = 45 and (pbar = > 0) order by moment.mid; > > Query B (combining the two with OR) > > select distinct moment.mid from moment,timecard where parent = 45 and > ((pid=17 and timecard.mid = moment.mid) or (pbar = 0)) order by moment.mid; > > [ snip ] > > I should say that this is on postgresql 7.4.16 (debian stable). Uhh. Why not a recent version? We have 8.3.0... > > Can query B be rewritten so that it would execute faster? Quick and dirty: use both selects (query A) combined with UNION. I guess, with a recent version the planner can use a bitmap index scan to perform Query B faster. Andreas -- Andreas Kretschmer Kontakt: Heynitz: 035242/47150, D1: 0160/7141639 (mehr: -> Header) GnuPG-ID: 0x3FFF606C, privat 0x7F4584DA http://wwwkeys.de.pgp.net -- Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
