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

From: Shridhar Daithankar <shridhar_daithankar(at)persistent(dot)co(dot)in>
To: Postgresql Performance <pgsql-performance(at)postgresql(dot)org>
Subject: Re: Select max(foo) and select count(*) optimization
Date: 2004-01-06 12:20:09
Message-ID: 200401061750.09530.shridhar_daithankar@persistent.co.in
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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..:-)

Shridhar

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