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Re: Performance of count(*)

From: ismo(dot)tuononen(at)solenovo(dot)fi
To: Albert Cervera Areny <albert(at)sedifa(dot)com>
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: Performance of count(*)
Date: 2007-03-22 11:30:35
Message-ID: Pine.LNX.4.64.0703221323590.1503@ismoli.solenovo.jns (view raw or flat)
Thread:
Lists: pgsql-performance
approximated count?????

why? who would need it? where you can use it?

calculating costs and desiding how to execute query needs 
approximated count, but it's totally worthless information for any user 
IMO.

Ismo

On Thu, 22 Mar 2007, Albert Cervera Areny wrote:

> As you can see, PostgreSQL needs to do a sequencial scan to count because its 
> MVCC nature and indices don't have transaction information. It's a known 
> drawback inherent to the way PostgreSQL works and which gives very good 
> results in other areas. It's been talked about adding some kind of 
> approximated count which wouldn't need a full table scan but I don't think 
> there's anything there right now.
> 
> A Dijous 22 Març 2007 11:53, Andreas Tille va escriure:
> > Hi,
> >
> > I just try to find out why a simple count(*) might last that long.
> > At first I tried explain, which rather quickly knows how many rows
> > to check, but the final count is two orders of magnitude slower.
> >
> > My MS_SQL server using colleague can't believe that.
> >
> > $ psql InfluenzaWeb -c 'explain SELECT count(*) from agiraw ;'
> >                                QUERY PLAN
> > -----------------------------------------------------------------------
> >   Aggregate  (cost=196969.77..196969.77 rows=1 width=0)
> >     ->  Seq Scan on agiraw  (cost=0.00..185197.41 rows=4708941 width=0)
> > (2 rows)
> >
> > real    0m0.066s
> > user    0m0.024s
> > sys     0m0.008s
> >
> > $ psql InfluenzaWeb -c 'SELECT count(*) from agiraw ;'
> >    count
> > ---------
> >   4708941
> > (1 row)
> >
> > real    0m4.474s
> > user    0m0.036s
> > sys     0m0.004s
> >
> >
> > Any explanation?
> >
> > Kind regards
> >
> >           Andreas.
> 
> -- 
> Albert Cervera Areny
> Dept. Informàtica Sedifa, S.L.
> 
> Av. Can Bordoll, 149
> 08202 - Sabadell (Barcelona)
> Tel. 93 715 51 11
> Fax. 93 715 51 12
> 
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