Re: Performance of count(*)

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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