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

From: Alvaro Herrera <alvherre(at)commandprompt(dot)com>
To: "A(dot) Kretschmer" <andreas(dot)kretschmer(at)schollglas(dot)com>
Cc: pgsql-general(at)postgresql(dot)org
Subject: Re: more problems with count(*) on large table
Date: 2007-09-28 16:50:34
Message-ID: 20070928165034.GG18001@alvh.no-ip.org (view raw or flat)
Thread:
Lists: pgsql-general
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.

-- 
Alvaro Herrera                                http://www.CommandPrompt.com/
The PostgreSQL Company - Command Prompt, Inc.

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