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Re: Select max(foo) and select count(*) optimization

From: Robert Treat <xzilla(at)users(dot)sourceforge(dot)net>
To: Shridhar Daithankar <shridhar_daithankar(at)persistent(dot)co(dot)in>
Cc: Postgresql Performance <pgsql-performance(at)postgresql(dot)org>
Subject: Re: Select max(foo) and select count(*) optimization
Date: 2004-01-06 16:03:23
Message-ID: 1073405003.20751.2.camel@camel (view raw or flat)
Thread:
Lists: pgsql-performance
On Tue, 2004-01-06 at 07:20, Shridhar Daithankar wrote:
> On Tuesday 06 January 2004 17:48, D'Arcy J.M. Cain wrote:
> > On January 6, 2004 01:42 am, Shridhar Daithankar wrote:
> > cert=# select relpages,reltuples::bigint from pg_class where relname=
> > 'certificate';
> >  relpages | reltuples
> > ----------+-----------
> >    399070 |  24858736
> > (1 row)
> >
> > But:
> >
> > cert=# select count(*) from certificate;
> > [*Crunch* *Crunch* *Crunch*]
> >   count
> > ----------
> >  19684668
> > (1 row)
> >
> > Am I missing something?  Max certificate_id is 20569544 btw.
> 
> Do 'vacuum analyze certificate' and try..:-)
> 
> The numbers from pg_class are estimates updated by vacuum /analyze. Of course 
> you need to run vacuum frequent enough for that statistics to be updated all 
> the time or run autovacuum daemon..
> 
> Ran into same problem on my machine till I remembered about vacuum..:-)
> 

Actually you only need to run analyze to update the statistics.

Robert Treat
-- 
Build A Brighter Lamp :: Linux Apache {middleware} PostgreSQL


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Subject: Re: Select max(foo) and select count(*) optimization
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Subject: Inefficient SELECT with OFFSET and LIMIT

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