Re: Performance of count(*)

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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