Received: from maia.hub.org (maia-2.hub.org [200.46.204.251]) by mail.postgresql.org (Postfix) with ESMTP id 2A5031337999 for ; Fri, 29 Apr 2011 19:59:59 -0300 (ADT) 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 47183-08 for ; Fri, 29 Apr 2011 22:59:41 +0000 (UTC) X-Greylist: domain auto-whitelisted by SQLgrey-1.7.6 Received: from mail-bw0-f46.google.com (mail-bw0-f46.google.com [209.85.214.46]) by mail.postgresql.org (Postfix) with ESMTP id 2897D1337990 for ; Fri, 29 Apr 2011 19:59:40 -0300 (ADT) Received: by bwz15 with SMTP id 15so3447445bwz.19 for ; Fri, 29 Apr 2011 15:59:40 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=gamma; h=domainkey-signature:references:in-reply-to:mime-version :content-transfer-encoding:content-type:message-id:cc:x-mailer:from :subject:date:to; bh=Z3ZnbRpyHs8467lgrekqkIn+neCmNgckmHaW74r/3AM=; b=ShsyT57vSj5x5+62SUIKW7bCPROC0lwYeh58T+E4UH7jdZTg8gq/P63q6mUaRhvVxp 2d713QEkiw+ZRCwxOhYYGcF7k3UHyV6EriqpvBH0O3g6hjTr9lrqQH0tVoIZhuk9FJvp WyrLgsEEx0AlCvPGh3dAN707vlGVF6x6BKyIY= DomainKey-Signature: a=rsa-sha1; c=nofws; d=gmail.com; s=gamma; h=references:in-reply-to:mime-version:content-transfer-encoding :content-type:message-id:cc:x-mailer:from:subject:date:to; b=ZsIHrbViiX6Qt50PN7boXY4m4HbfPznPUh7mhoAhe2WhrHIRqiyWArrSBbDT0ALamH W60BO6lfLbq9d67WSxCjOjQzAqPhb6Q7jnRU/qhs38Hv4NSAYFF/KCEJwPN6oEM1hu2x WagCdnos/wcnErMRJxLHsPjAmZQl0iEW18/D4= Received: by 10.204.32.9 with SMTP id a9mr1157875bkd.182.1304117980415; Fri, 29 Apr 2011 15:59:40 -0700 (PDT) Received: from [192.168.2.143] (p5498AC0B.dip.t-dialin.net [84.152.172.11]) by mx.google.com with ESMTPS id x6sm1869039bkv.12.2011.04.29.15.59.38 (version=TLSv1/SSLv3 cipher=OTHER); Fri, 29 Apr 2011 15:59:38 -0700 (PDT) References: <20110412171855.GA14292@tux> <4DA496FA.3070908@fuzzy.cz> <8F22D592-23C1-4A3C-94A5-48363332ADD3@darkstatic.com> <4DA4BFA2.5060601@fuzzy.cz> <4DA4D40A.4010200@fuzzy.cz> <4DA56DA8020000250003C783@gw.wicourts.gov> <4DA6216E.9020907@fuzzy.cz> <4DA63125.5070106@fuzzy.cz> <4DBA75DC.6070506@mansionfamily.plus.com> In-Reply-To: <4DBA75DC.6070506@mansionfamily.plus.com> Mime-Version: 1.0 (iPhone Mail 8A293) Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset=us-ascii Message-Id: <61F862B4-6AB8-40BA-8B25-9C18988665B2@gmail.com> Cc: Claudio Freire , Tomas Vondra , "pgsql-performance@postgresql.org" X-Mailer: iPhone Mail (8A293) From: Robert Haas Subject: Re: Performance Date: Sat, 30 Apr 2011 01:00:23 +0200 To: James Mansion X-Virus-Scanned: Maia Mailguard 1.0.1 X-Spam-Status: No, hits=-1.897 tagged_above=-10 required=5 tests=BAYES_00=-1.9, FREEMAIL_FROM=0.001, MIME_QP_LONG_LINE=0.001, RFC_ABUSE_POST=0.001 X-Spam-Level: X-Archive-Number: 201104/428 X-Sequence-Number: 43490 On Apr 29, 2011, at 10:25 AM, James Mansion w= rote: > Robert Haas wrote: >> The server can and does measure hit rates for the PG buffer pool, but to m= y knowledge there is no clear-cut way for PG to know whether read() is satis= fied from the OS cache or a drive cache or the platter. >>=20 >> =20 > Does the server know which IO it thinks is sequential, and which it thinks= is random?=20 No. It models this in the optimizer, but the executor has no clue. And some= times we model I/O as partly random, partly sequential, as in the case of he= ap fetches on a clustered index. So the answer isn't even a Boolean. ...Robert=