X-Original-To: pgsql-performance-postgresql.org@localhost.postgresql.org Received: from localhost (mx1.hub.org [200.46.208.251]) by postgresql.org (Postfix) with ESMTP id 4272211F6095 for ; Tue, 25 Apr 2006 13:33:50 -0300 (ADT) Received: from postgresql.org ([200.46.204.71]) by localhost (mx1.hub.org [200.46.208.251]) (amavisd-new, port 10024) with ESMTP id 43972-10 for ; Tue, 25 Apr 2006 13:33:39 -0300 (ADT) X-Greylist: from auto-whitelisted by SQLgrey- Received: from fafnir.androme.com (fafnir.androme.com [62.58.96.158]) by postgresql.org (Postfix) with ESMTP id 7987011F6093 for ; Tue, 25 Apr 2006 13:33:38 -0300 (ADT) Received: by fafnir.androme.com (Postfix, from userid 1003) id BC5ADB92A65; Tue, 25 Apr 2006 18:33:37 +0200 (CEST) Message-ID: <444E4F5D.2070808@andromeiberica.com> Date: Tue, 25 Apr 2006 18:33:33 +0200 From: Arnau Reply-To: arnaulist@andromeiberica.com User-Agent: Mozilla Thunderbird 1.0.6 (Windows/20050716) X-Accept-Language: en-us, en MIME-Version: 1.0 To: pgsql-performance@postgresql.org Subject: Re: Query on postgresql 7.4.2 not using index References: <444E240D.3050000@andromeiberica.com> <3c1395330604250634r6f215ceame157f3dd5f5f4fa3@mail.gmail.com> <444E28ED.3090900@andromeiberica.com> <19483.1145978579@sss.pgh.pa.us> <444E44A2.6030703@andromeiberica.com> <1145980463.23538.218.camel@state.g2switchworks.com> In-Reply-To: <1145980463.23538.218.camel@state.g2switchworks.com> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit X-Virus-Scanned: Maia Mailguard 1.0.1 X-Archive-Number: 200604/553 X-Sequence-Number: 18637 >>I have done the same tests on 8.1.0. >> >> >>espsm_moviltelevision=# EXPLAIN ANALYZE SELECT agenda_user_group_id FROM >>agenda_users_groups WHERE group_id = 9; >> QUERY PLAN >>---------------------------------------------------------------------------------------------------------------------------------------------- >> Bitmap Heap Scan on agenda_users_groups (cost=2722.26..30341.78 >>rows=400361 width=8) (actual time=145.533..680.839 rows=367026 loops=1) >> Recheck Cond: (group_id = 9::numeric) >> -> Bitmap Index Scan on i_agnusrsgrs_groupid (cost=0.00..2722.26 >>rows=400361 width=0) (actual time=142.958..142.958 rows=367026 loops=1) >> Index Cond: (group_id = 9::numeric) >> Total runtime: 1004.966 ms >>(5 rows) > > > How big are these individual records? I'm guessing a fairly good size, > since an index scan is winning. How I could know the size on an individual record? > > >>espsm_moviltelevision=# EXPLAIN ANALYZE SELECT agenda_user_group_id FROM >>agenda_users_groups WHERE group_id::int8 = 9; >> QUERY PLAN >>------------------------------------------------------------------------------------------------------------------------------- >> Seq Scan on agenda_users_groups (cost=0.00..60947.43 rows=12777 >>width=8) (actual time=457.963..2244.928 rows=367026 loops=1) >> Filter: ((group_id)::bigint = 9) >> Total runtime: 2571.496 ms >>(3 rows) > > > OK. Stop and think about what you're telling postgresql to do here. > > You're telling it to cast the field group_id to int8, then compare it to > 9. How can it cast the group_id to int8 without fetching it? That's > right, you're ensuring a seq scan. You need to put the int8 cast on the > other side of that equality comparison, like: > > where group_id = 9::int8 I just did what Chris Smith asked me to do :), here I paste the results I get when I change the cast. espsm_moviltelevision=# EXPLAIN ANALYZE SELECT agenda_user_group_id FROM agenda_users_groups WHERE group_id = 9::int8; QUERY PLAN ---------------------------------------------------------------------------------------------------------------------------------------------- Bitmap Heap Scan on agenda_users_groups (cost=2722.33..30343.06 rows=400379 width=8) (actual time=147.723..714.473 rows=367026 loops=1) Recheck Cond: (group_id = 9::numeric) -> Bitmap Index Scan on i_agnusrsgrs_groupid (cost=0.00..2722.33 rows=400379 width=0) (actual time=145.015..145.015 rows=367026 loops=1) Index Cond: (group_id = 9::numeric) Total runtime: 1038.537 ms (5 rows) espsm_moviltelevision=# EXPLAIN ANALYZE SELECT agenda_user_group_id FROM agenda_users_groups WHERE group_id = '9'::int8; QUERY PLAN ---------------------------------------------------------------------------------------------------------------------------------------------- Bitmap Heap Scan on agenda_users_groups (cost=2722.33..30343.06 rows=400379 width=8) (actual time=153.858..1192.838 rows=367026 loops=1) Recheck Cond: (group_id = 9::numeric) -> Bitmap Index Scan on i_agnusrsgrs_groupid (cost=0.00..2722.33 rows=400379 width=0) (actual time=151.298..151.298 rows=367026 loops=1) Index Cond: (group_id = 9::numeric) Total runtime: 1527.039 ms (5 rows) Thanks -- Arnau