X-Original-To: pgsql-docs-postgresql.org@localhost.postgresql.org Received: from localhost (unknown [200.46.204.144]) by svr1.postgresql.org (Postfix) with ESMTP id B9B00534BF for ; Tue, 12 Apr 2005 01:42:19 -0300 (ADT) Received: from svr1.postgresql.org ([200.46.204.71]) by localhost (av.hub.org [200.46.204.144]) (amavisd-new, port 10024) with ESMTP id 09297-09 for ; Tue, 12 Apr 2005 04:42:15 +0000 (GMT) Received: from wolff.to (wolff.to [66.93.249.74]) by svr1.postgresql.org (Postfix) with SMTP id C35BB5344C for ; Tue, 12 Apr 2005 01:42:14 -0300 (ADT) Received: (qmail 308 invoked by uid 500); 12 Apr 2005 05:01:26 -0000 Date: Tue, 12 Apr 2005 00:01:26 -0500 From: Bruno Wolff III To: Michael Fuhr Cc: pgsql-docs@postgresql.org Subject: Re: Possible copy error in explain example Message-ID: <20050412050126.GA32694@wolff.to> Mail-Followup-To: Bruno Wolff III , Michael Fuhr , pgsql-docs@postgresql.org References: <20050412044001.GA32308@wolff.to> <20050412043421.GA60302@winnie.fuhr.org> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20050412043421.GA60302@winnie.fuhr.org> User-Agent: Mutt/1.5.6i X-Virus-Scanned: by amavisd-new at hub.org X-Spam-Status: No, hits=0.01 tagged_above=0 required=5 tests=AWL X-Spam-Level: X-Archive-Number: 200504/16 X-Sequence-Number: 2946 On Mon, Apr 11, 2005 at 22:34:21 -0600, Michael Fuhr wrote: > On Mon, Apr 11, 2005 at 11:40:01PM -0500, Bruno Wolff III wrote: > > I was looking at: > > http://candle.pha.pa.us/main/writings/pgsql/sgml/performance-tips.html > > > > And noticed in the first example that it is claimed the cost estimate > > is 233 disk block reads. However, "233" doesn't appear in the explain > > output shown. I think that "233" is supposed to match the "333" in > > the explain output, but I am not 100% sure. > > This part? > > This is about as straightforward as it gets. If you do > > SELECT * FROM pg_class WHERE relname = 'tenk1'; > > you will find out that tenk1 has 233 disk pages and 10000 rows. > So the cost is estimated at 233 page reads, defined as costing > 1.0 apiece, plus 10000 * cpu_tuple_cost which is currently 0.01 > (try SHOW cpu_tuple_cost). > > Doesn't that work out to > > (233 * 1.0) + (10000 * 0.01) = 233.0 + 100.0 = 333.0 > > or am I missing something? No, I was. I misread a comma as a period and when I scanned ahead to see if I could find something similar in other examples I missed the part where it added in the cpu cost. After I sent the message I looked some more and spotted what I missed. I didn't have a copy back yet of the original message, so my oops message wasn't threaded with the original.