Re: Why count(*) doest use index?

On 03/03/2011 05:29 AM, obamabarak(at)e1(dot)ru wrote:
> I use pgsql 9.0.3 and I know that postgresql tries to use the fields in
> indexes instead of the original table if it possible
>
> But when I run
>
> SELECT COUNT(id) FROM tab
>
> or
>
> SELECT COUNT(*) FROM tab
>
> where there "id" is PRIMARY KEY and there are other indexes there I get
> execution plan that doesnt use any indexes, but sequentab scanning the
> original table.
>
> "Aggregate (cost=38685.98..38685.99 rows=1 width=0)"
> " -> Seq Scan on tab (cost=0.00..36372.38 rows=925438 width=0)"
>
> Why is it so?

See here:
http://wiki.postgresql.org/wiki/FAQ#Why_is_.22SELECT_count.28.2A.29_FROM_bigtable.3B.22_slow.3F

>
> ---
>
> Paul
>

--
Adrian Klaver
adrian(dot)klaver(at)gmail(dot)com