From: A. Kretschmer <andreas.kretschmer@schollglas.com>
To: pgsql-performance@postgresql.org
Subject: Re: mysterious difference in speed when combining two queries with OR
Date: Wed, 23 Apr 2008 09:58:10 +0200
Message-ID: <20080423075810.GB8401@a-kretschmer.de> (raw)
In-Reply-To: <20080423072303.GF11886@amin>
References: <20080423072303.GF11886@amin>
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
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Andreas Kretschmer
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To: pgsql-performance@postgresql.org
Cc: andreas.kretschmer@schollglas.com
Subject: Re: mysterious difference in speed when combining two queries with OR
In-Reply-To: <20080423075810.GB8401@a-kretschmer.de>
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