Received: from localhost (maia-2.hub.org [200.46.204.187]) by postgresql.org (Postfix) with ESMTP id B22CD9FB93B for ; Wed, 11 Jul 2007 15:33:47 -0300 (ADT) Received: from postgresql.org ([200.46.204.71]) by localhost (mx1.hub.org [200.46.204.187]) (amavisd-maia, port 10024) with ESMTP id 96496-01 for ; Wed, 11 Jul 2007 15:33:34 -0300 (ADT) X-Greylist: from auto-whitelisted by SQLgrey-1.7.5 Received: from 137086.vserver.de (vs137086.vserver.de [62.75.137.86]) by postgresql.org (Postfix) with ESMTP id 0BD469FB929 for ; Wed, 11 Jul 2007 15:33:43 -0300 (ADT) Received: from a-kretschmer.de (p54B39BDE.dip0.t-ipconnect.de [84.179.155.222]) (authenticated bits=0) by 137086.vserver.de (8.12.8/8.12.8) with ESMTP id l6BIXajC001623 for ; Wed, 11 Jul 2007 20:33:36 +0200 Received: from kretschmer by a-kretschmer.de with local (Exim 4.61) (envelope-from ) id 1I8h3G-0000JJ-Ve for pgsql-performance@postgresql.org; Wed, 11 Jul 2007 20:36:39 +0200 Date: Wed, 11 Jul 2007 20:36:38 +0200 From: Andreas Kretschmer To: pgsql-performance@postgresql.org Subject: bitmap-index-scan slower than normal index scan Message-ID: <20070711183638.GA30872@KanotixBox> Mime-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit X-OS: Debian/GNU Linux - weil ich es mir Wert bin! X-GPG-Fingerprint: EE16 3C01 7B9C 10F7 2C8B 3B86 4DB3 D9EE 7F45 84DA X-Message-Flag: "Windows" is not the answer. "Windows" is the question and the answer is "no"! X-Lugdd: Gerd Kube X-Info: My name is root. Just root. And I am licensed to kill -9 User-Agent: mutt-ng 1.5.9i (Linux) X-Virus-Scanned: Maia Mailguard 1.0.1 X-Spam-Status: No, hits=0.022 tagged_above=0 required=5 tests=AWL=0.207, BAYES_40=-0.185 X-Spam-Level: X-Archive-Number: 200707/109 X-Sequence-Number: 25793 Hi, Okay, i know, not really a recent version: PostgreSQL 8.1.4 on i386-pc-linux-gnu, compiled by GCC cc (GCC) 3.3.5 (Debian 1:3.3.5-13) I have a fresh ANALYZED table with some indexes. scholl=*# set enable_bitmapscan=1; SET scholl=*# explain analyse select sum(flaeche) from bde_meldungen where maschine=1200 and ab = 347735; QUERY PLAN -------------------------------------------------------------------------------------------------------------------------------------------------- Aggregate (cost=1371.95..1371.96 rows=1 width=8) (actual time=163.788..163.790 rows=1 loops=1) -> Bitmap Heap Scan on bde_meldungen (cost=1217.69..1371.85 rows=39 width=8) (actual time=163.702..163.758 rows=2 loops=1) Recheck Cond: ((ab = 347735) AND (maschine = 1200)) -> BitmapAnd (cost=1217.69..1217.69 rows=39 width=0) (actual time=163.681..163.681 rows=0 loops=1) -> Bitmap Index Scan on idx_ab (cost=0.00..5.95 rows=558 width=0) (actual time=0.078..0.078 rows=109 loops=1) Index Cond: (ab = 347735) -> Bitmap Index Scan on idx_maschine (cost=0.00..1211.49 rows=148997 width=0) (actual time=163.459..163.459 rows=164760 loops=1) Index Cond: (maschine = 1200) Total runtime: 163.901 ms (9 rows) Okay, 163.901 ms with Bitmap Index Scan. And now i disable this and runs the same select: scholl=*# set enable_bitmapscan=0; SET scholl=*# explain analyse select sum(flaeche) from bde_meldungen where maschine=1200 and ab = 347735; QUERY PLAN -------------------------------------------------------------------------------------------------------------------------------- Aggregate (cost=2142.77..2142.78 rows=1 width=8) (actual time=0.229..0.231 rows=1 loops=1) -> Index Scan using idx_ab on bde_meldungen (cost=0.00..2142.67 rows=39 width=8) (actual time=0.046..0.209 rows=2 loops=1) Index Cond: (ab = 347735) Filter: (maschine = 1200) Total runtime: 0.326 ms (5 rows) Okay, i got a really different plan, but i expected _NOT_ a performance-boost like this. I expected the opposite. It's not a really problem, i just played with this. But i'm confused about this... Andreas -- Really, I'm not out to destroy Microsoft. That will just be a completely unintentional side effect. (Linus Torvalds) "If I was god, I would recompile penguin with --enable-fly." (unknow) Kaufbach, Saxony, Germany, Europe. N 51.05082°, E 13.56889°