Received: from maia.hub.org (maia-2.hub.org [200.46.204.251]) by mail.postgresql.org (Postfix) with ESMTP id EB1561337B6E for ; Thu, 13 Jan 2011 16:05:52 -0400 (AST) Received: from mail.postgresql.org ([200.46.204.86]) by maia.hub.org (mx1.hub.org [200.46.204.251]) (amavisd-maia, port 10024) with ESMTP id 67913-03 for ; Thu, 13 Jan 2011 20:05:46 +0000 (UTC) X-Greylist: from auto-whitelisted by SQLgrey-1.7.6 Received: from sss.pgh.pa.us (sss.pgh.pa.us [66.207.139.130]) by mail.postgresql.org (Postfix) with ESMTP id 8A19A1337B68 for ; Thu, 13 Jan 2011 16:05:45 -0400 (AST) Received: from sss2.sss.pgh.pa.us (tgl@localhost [127.0.0.1]) by sss.pgh.pa.us (8.14.2/8.14.2) with ESMTP id p0DK5iUx004880; Thu, 13 Jan 2011 15:05:44 -0500 (EST) To: Jon Nelson cc: pgsql-performance@postgresql.org Subject: Re: queries with lots of UNIONed relations In-reply-to: References: <19302.1294938802@sss.pgh.pa.us> Comments: In-reply-to Jon Nelson message dated "Thu, 13 Jan 2011 13:54:03 -0600" Date: Thu, 13 Jan 2011 15:05:44 -0500 Message-ID: <4879.1294949144@sss.pgh.pa.us> From: Tom Lane X-Virus-Scanned: Maia Mailguard 1.0.1 X-Spam-Status: No, hits=-1.91 tagged_above=-10 required=5 tests=BAYES_00=-1.9, T_RP_MATCHES_RCVD=-0.01 X-Spam-Level: X-Archive-Number: 201101/79 X-Sequence-Number: 41866 Jon Nelson writes: > Your comment regarding "each individual de-duplication looked like it > would fit in work_mem" doesn't really make sense, exactly. Maybe I'm > misunderstanding you. Yeah. What I was suggesting was to NOT add the DISTINCT's, but instead raise work_mem high enough so you get just one HashAggregation step at the top level. (Although I think this only works in 8.4 and up.) That should be faster than two levels of de-duplication. regards, tom lane