Received: from localhost (unknown [200.46.204.183]) by developer.postgresql.org (Postfix) with ESMTP id 5264E2E0043 for ; Mon, 19 May 2008 12:54:05 -0300 (ADT) Received: from developer.postgresql.org ([200.46.204.71]) by localhost (mx1.hub.org [200.46.204.183]) (amavisd-maia, port 10024) with ESMTP id 80750-06 for ; Mon, 19 May 2008 12:53:53 -0300 (ADT) 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 developer.postgresql.org (Postfix) with ESMTP id ABC8E2E0032 for ; Mon, 19 May 2008 12:53:53 -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 m4JFrn6T005900; Mon, 19 May 2008 11:53:49 -0400 (EDT) To: Greg Smith cc: Matthew Wakeling , pgsql-performance@postgresql.org Subject: Re: I/O on select count(*) In-reply-to: References: <920841.71690.qm@web44808.mail.sp1.yahoo.com> <482B1D5F.EE98.0025.0@wicourts.gov> <20080515020553.GN9838@alvh.no-ip.org> <20080515155232.GC8221@alvh.no-ip.org> <20080516201528.GJ13061@alvh.no-ip.org> <18382.1210992302@sss.pgh.pa.us> Comments: In-reply-to Greg Smith message dated "Mon, 19 May 2008 11:47:38 -0400" Date: Mon, 19 May 2008 11:53:49 -0400 Message-ID: <5899.1211212429@sss.pgh.pa.us> From: Tom Lane X-Virus-Scanned: Maia Mailguard 1.0.1 X-Archive-Number: 200805/269 X-Sequence-Number: 30237 Greg Smith writes: > The real question in my mind is why this turns into a bottleneck before > the similar task of cleaning the 16MB XLOG segment does. Because we do the latter off-line, or at least try to. regards, tom lane