Re: Understanding explain costs

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 number 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