| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | David Link <dlink(at)soundscan(dot)com> |
| Cc: | "pgsql-general(at)postgresql(dot)org" <pgsql-general(at)postgresql(dot)org> |
| Subject: | Re: Understanding explain costs |
| Date: | 2001-10-12 17:41:03 |
| Message-ID: | 27379.1002908463@sss.pgh.pa.us |
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| Lists: | pgsql-general |
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 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
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