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Re: more problems with count(*) on large table

From: "A(dot) Kretschmer" <andreas(dot)kretschmer(at)schollglas(dot)com>
To: pgsql-general(at)postgresql(dot)org
Subject: Re: more problems with count(*) on large table
Date: 2007-09-28 17:54:42
Message-ID: 20070928175442.GA21469@a-kretschmer.de (view raw or flat)
Thread:
Lists: pgsql-general
am  Fri, dem 28.09.2007, um 12:50:34 -0400 mailte Alvaro Herrera folgendes:
> A. Kretschmer wrote:
> > am  Fri, dem 28.09.2007, um 11:56:46 -0400 mailte Mike Charnoky folgendes:
> > > Hi,
> > > 
> > > I am still having problems performing a count(*) on a large table.  This
> > > 
> > > Now, certain count(*) queries are failing to complete for certain time
> > > ranges (I killed the query after about 24 hours).  The table is indexed
> > > on a timestamp field.  Here is one query that hangs:
> > 
> > Again: an index can't help! Because of MVCC: 'select count(*)' without
> > WHERE-condition forces an seq. table-scan. 
> 
> But he does have a WHERE condition.  THe problem is, probably, that the
> condition is not selective enough so the planner chooses to do a
> seqscan.

Yes, i'm sorry, i havn't recognize this. Maybe wrong statistics. As
Andrew suggested, a 'ALTER TABLE...SET STATISTICS' can help.


Andreas
-- 
Andreas Kretschmer
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