I hope this will treat as a "planner-bug". I think the problem is tricky but not hard to solve.
-------- Original-Nachricht -------- > Datum: Tue, 21 Apr 2009 13:53:01 +0200 > Von: Guillaume Smet <guillaume.s...@gmail.com> > An: vac...@quantentunnel.de > CC: pgsql-hackers@postgresql.org > Betreff: Re: [HACKERS] 8.4 semi-join slows down query performance (EXISTS) > On Tue, Apr 21, 2009 at 11:38 AM, <vac...@quantentunnel.de> wrote: > > I've queries returning tupels of A, with a (sub-)selected > constant-expression that indicates whether a referenced tupel exists in B or > not. For > this issue the EXISTS clause is used. > > > > In past (8.3) the planner resolves this into index-scans using existing > foreign-key indices -> fast query (1.5 seconds for comparison). Now (in > 8.4) the planner wants "semi-joins". Index-scans are not longer used and my > query needs 600 seconds to return. > > That worries me a bit for one of our applications too. We use EXISTS > in several places to trick the planner when the statistics are way off > (cross columns/cross tables) and I'm not sure making EXISTS more > clever will help us. > > -- > Guillaume > > -- > Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) > To make changes to your subscription: > http://www.postgresql.org/mailpref/pgsql-hackers -- Neu: GMX FreeDSL Komplettanschluss mit DSL 6.000 Flatrate + Telefonanschluss für nur 17,95 Euro/mtl.!* http://dslspecial.gmx.de/freedsl-surfflat/?ac=OM.AD.PD003K11308T4569a -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
