Received: from maia.hub.org (maia-2.hub.org [200.46.204.251]) by mail.postgresql.org (Postfix) with ESMTP id 7E3281337999 for ; Mon, 11 Oct 2010 00:14:52 -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 11573-02 for ; Mon, 11 Oct 2010 03:14:45 +0000 (UTC) X-Greylist: from auto-whitelisted by SQLgrey-1.7.6 Received: from hrndva-omtalb.mail.rr.com (hrndva-omtalb.mail.rr.com [71.74.56.125]) by mail.postgresql.org (Postfix) with ESMTP id E4B3B1336705 for ; Mon, 11 Oct 2010 00:14:44 -0300 (ADT) X-Authority-Analysis: v=1.1 cv=QISHQ+x4xS+apxgDHgAvNvlUjyrXCzYsHvuXcbPYiKg= c=1 sm=0 a=_6Iw8BWMfOIA:10 a=8nJEP1OIZ-IA:10 a=RAii6je2cdnAH6ATCOpc6g==:17 a=_Ixf_7qHAAAA:8 a=sffTjGwXGClKJeyCMJ4A:9 a=ZceONC7dkKACybLBJROMK_yYBssA:4 a=wPNLvfGTeEIA:10 a=ryAo9hy1-OkA:10 a=6vsjEBoLwlAA:10 a=0tfO7MgyNwYA:10 a=RAii6je2cdnAH6ATCOpc6g==:117 X-Cloudmark-Score: 0 X-Originating-IP: 74.73.87.196 Received: from [74.73.87.196] ([74.73.87.196:51260] helo=[192.168.1.103]) by hrndva-oedge04.mail.rr.com (envelope-from ) (ecelerity 2.2.3.46 r()) with ESMTP id 99/EC-11377-32182BC4; Mon, 11 Oct 2010 03:14:43 +0000 Message-ID: <4CB28123.5080300@vmsinfo.com> Date: Sun, 10 Oct 2010 23:14:43 -0400 From: Mladen Gogala User-Agent: Mozilla/5.0 (Windows; U; Windows NT 6.0; en-US; rv:1.9.2.9) Gecko/20100915 Lightning/1.0b2 Thunderbird/3.1.4 MIME-Version: 1.0 To: Joshua Tolley CC: Craig Ringer , Neil Whelchel , "pgsql-performance@postgresql.org" Subject: Re: Slow count(*) again... References: <201010091626.18636.neil.whelchel@gmail.com> <4CB11CC7.8070300@vmsinfo.com> <4CB167CA.9070109@postnewspapers.com.au> <4CB1F46E.80205@vmsinfo.com> <4CB2410C.6010603@postnewspapers.com.au> <4cb25a0e.2a978e0a.7797.33d6@mx.google.com> In-Reply-To: <4cb25a0e.2a978e0a.7797.33d6@mx.google.com> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit X-Virus-Scanned: Maia Mailguard 1.0.1 X-Spam-Status: No, hits=-1.9 tagged_above=-10 required=5 tests=BAYES_00=-1.9, RCVD_IN_DNSWL_NONE=-0.0001 X-Spam-Level: X-Archive-Number: 201010/105 X-Sequence-Number: 40524 On 10/10/2010 8:27 PM, Joshua Tolley wrote: > It was asserted that reading bigger chunks would help performance; a response > suggested that, at least in Linux, setting readahead on a device would > essentially do the same thing. Or that's what I got from the thread, anyway. > I'm interested to know how similar performance might be between the large > block size case and the large readahead case. Comments, anyone? > Craig maybe right, the fact that Oracle is doing direct I/O probably does account for the difference. The fact is, however, that the question about slow sequential scan appears with some regularity on PostgreSQL forums. My guess that a larger chunk would be helpful may not be correct, but I do believe that there is a problem with a too slow sequential scan. Bigger chunks are a very traditional solution which may not work but the problem is still there. -- Mladen Gogala Sr. Oracle DBA 1500 Broadway New York, NY 10036 (212) 329-5251 www.vmsinfo.com