Re: Understanding explain costs

Tom Lane wrote:
>
> David Link <dlink@soundscan.com> writes:
> >   tiger=# explain select * from bk_inv where isbn = 'foo';
> >   NOTICE:  QUERY PLAN:
>
> >   Index Scan using bk_inv_isbn_idx on bk_inv  (cost=0.00..53.13
> >   rows=13 width=48)
>
> > However, however using the store index costs are still rather high  -
> > why is that!!??
>
> >   tiger=# explain select * from bk_inv where store = 'foo';
> >   NOTICE:  QUERY PLAN:
>
> >   Index Scan using bk_inv_store_idx on bk_inv  (cost=0.00..53456.09
> >   rows=13488 width=48)
>
> Note the difference in estimated numbe.r of rows retrieved; that's where
> the cost difference comes from.  You may care to read
>
> http://www.ca.postgresql.org/users-lounge/docs/7.1/postgres/performance-tips.html
>
>                         regards, tom lane

Yes I read that Tom, thanks.  It seems to be the only thing out there on
performance tuning for Postgres that I have been able to find.

Thanks for the explaination.  That makes sense.  There are far fewer
unique isbns then there are vendors in my table.

--David