Received: from maia.hub.org (maia-2.hub.org [200.46.204.251]) by mail.postgresql.org (Postfix) with ESMTP id DB0231337B83 for ; Thu, 13 Jan 2011 18:26:31 -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 26798-03 for ; Thu, 13 Jan 2011 22:26:25 +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 8DA201337B82 for ; Thu, 13 Jan 2011 18:26:24 -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 p0DMQMjx011565; Thu, 13 Jan 2011 17:26:22 -0500 (EST) To: Robert Haas cc: Jon Nelson , pgsql-performance@postgresql.org Subject: Re: queries with lots of UNIONed relations In-reply-to: References: <19302.1294938802@sss.pgh.pa.us> <4879.1294949144@sss.pgh.pa.us> Comments: In-reply-to Robert Haas message dated "Thu, 13 Jan 2011 16:44:30 -0500" Date: Thu, 13 Jan 2011 17:26:22 -0500 Message-ID: <11563.1294957582@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: 201101/84 X-Sequence-Number: 41871 Robert Haas writes: > On Thu, Jan 13, 2011 at 3:12 PM, Jon Nelson wrote: >> I still think that having UNION do de-duplication of each contributory >> relation is a beneficial thing to consider -- especially if postgresql >> thinks the uniqueness is not very high. > This might be worth a TODO. I don't believe there is any case where hashing each individual relation is a win compared to hashing them all together. If the optimizer were smart enough to be considering the situation as a whole, it would always do the latter. regards, tom lane