Received: from maia.hub.org (maia-5.hub.org [200.46.204.29]) by mail.postgresql.org (Postfix) with ESMTP id 281801337B5C for ; Sun, 14 Nov 2010 20:33:03 -0400 (AST) Received: from mail.postgresql.org ([200.46.204.86]) by maia.hub.org (mx1.hub.org [200.46.204.29]) (amavisd-maia, port 10024) with ESMTP id 87975-03 for ; Mon, 15 Nov 2010 00:32:57 +0000 (UTC) X-Greylist: from auto-whitelisted by SQLgrey-1.7.6 Received: from hrndva-omtalb.mail.rr.com (hrndva-omtalb.mail.rr.com [71.74.56.122]) by mail.postgresql.org (Postfix) with ESMTP id B45391337B51 for ; Sun, 14 Nov 2010 20:32:55 -0400 (AST) X-Authority-Analysis: v=1.1 cv=kXGwZUU/u1JTMRv8Axk4W0omja+vfTT+sGlOkodD8F8= c=1 sm=0 a=76oTRRP_GOwA:10 a=JRZeEW2aRuwA:10 a=IkcTkHD0fZMA:10 a=RAii6je2cdnAH6ATCOpc6g==:17 a=_Ixf_7qHAAAA:8 a=jDqrWuhYI93VHuq89aUA:9 a=J77VVXWS8S3c4klmNygA:7 a=x6DXnoDmLvQG-5sWdwDJiCtPNU8A:4 a=QEXdDO2ut3YA: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] ([74.73.87.196:60603] helo=[192.168.1.100]) by hrndva-oedge02.mail.rr.com (envelope-from ) (ecelerity 2.2.3.46 r()) with ESMTP id CB/39-14897-6BF70EC4; Mon, 15 Nov 2010 00:32:55 +0000 Message-ID: <4CE07FB6.2000404@vmsinfo.com> Date: Sun, 14 Nov 2010 19:32:54 -0500 From: Mladen Gogala Reply-To: mladen.gogala@vmsinfo.com Organization: VMS User-Agent: Thunderbird 2.0.0.24 (X11/20101029) MIME-Version: 1.0 To: Marti Raudsepp CC: Craig Ringer , Kyriacos Kyriacou , "pgsql-performance@postgresql.org" Subject: Re: MVCC performance issue References: <8BCBF9DB739F034B87FE7C7D30EAE55C240D4BC8@hqex2k.francoudi.com> <4CDE27D7.7070706@postnewspapers.com.au> In-Reply-To: Content-Type: text/plain; charset=UTF-8; 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: 201011/241 X-Sequence-Number: 41265 Marti Raudsepp wrote: > > > Another advantage of Oracle's approach seems that they need much less > tuple-level overhead. IMO the 23-byte tuple overhead is a much bigger > drawback in Postgres than table fragmentation. > > Regards, > Marti > > Oracle, however, does have a problem with "ORA-1555 Snapshot too old", precisely because of their implementation of MVCC. In other words, if your query is running long and Oracle is not able to reconstruct the old rows from the UNDO segments, you're out of luck and your query will die. The greatest burden of the Postgres implementation is the fact that there is no row id, so that the table header and the indexes need to be updated much more frequently than is the case with Oracle. -- Mladen Gogala Sr. Oracle DBA 1500 Broadway New York, NY 10036 (212) 329-5251 www.vmsinfo.com