Re: Understanding explain costs

Tom Lane wrote:
>
> David Link <dlink(at)soundscan(dot)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