Received: from maia.hub.org (maia-3.hub.org [200.46.204.243]) by mail.postgresql.org (Postfix) with ESMTP id 8BFF01337B57 for ; Fri, 1 Oct 2010 11:40:19 -0300 (ADT) Received: from mail.postgresql.org ([200.46.204.86]) by maia.hub.org (mx1.hub.org [200.46.204.243]) (amavisd-maia, port 10024) with ESMTP id 01829-03 for ; Fri, 1 Oct 2010 14:40:12 +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 4785B133729E for ; Fri, 1 Oct 2010 11:40:12 -0300 (ADT) 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 o91EeCSh012384; Fri, 1 Oct 2010 10:40:12 -0400 (EDT) To: "Kevin Grittner" cc: fabricio.silva@linkcom.com.br, "Craig Ringer" , pgsql-performance@postgresql.org Subject: Re: How does PG know if data is in memory? In-reply-to: <4CA59FCF02000025000362AE@gw.wicourts.gov> References: <4CA5D2F5.5070900@postnewspapers.com.au> <4CA59FCF02000025000362AE@gw.wicourts.gov> Comments: In-reply-to "Kevin Grittner" message dated "Fri, 01 Oct 2010 08:46:07 -0500" Date: Fri, 01 Oct 2010 10:40:11 -0400 Message-ID: <12383.1285944011@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: 201010/8 X-Sequence-Number: 40427 "Kevin Grittner" writes: > I agree with that, but I think there's an even more insidious issue > here. Biasing plans heavily toward using what is already in cache > could have a destabilizing effect on performance. Not to mention the destabilizing effect on the plans themselves. Behavior like that would make EXPLAIN nearly useless, because the plan you get would vary from moment to moment even when "nothing is changing". It's fairly clear that people don't actually want that. regards, tom lane