Received: from localhost (unknown [200.46.204.183]) by postgresql.org (Postfix) with ESMTP id 4606064FCEF for ; Thu, 4 Sep 2008 22:46:02 -0300 (ADT) Received: from postgresql.org ([200.46.204.86]) by localhost (mx1.hub.org [200.46.204.183]) (amavisd-maia, port 10024) with ESMTP id 45832-09 for ; Thu, 4 Sep 2008 22:45:58 -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 postgresql.org (Postfix) with ESMTP id A7FF164FCE0 for ; Thu, 4 Sep 2008 22:45:58 -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 m851jrLx011526; Thu, 4 Sep 2008 21:45:53 -0400 (EDT) To: "Alex Hunsaker" cc: "Zdenek Kotala" , "Xiao Meng" , pgsql-patches@postgresql.org, "Jonah H. Harris" Subject: Re: hash index improving v3 In-reply-to: <10995.1220577198@sss.pgh.pa.us> References: <48BFDA8D.3080506@sun.com> <29987.1220560114@sss.pgh.pa.us> <7627.1220569888@sss.pgh.pa.us> <34d269d40809041742r6a0703dbgfbdd012f7f19de2e@mail.gmail.com> <10995.1220577198@sss.pgh.pa.us> Comments: In-reply-to Tom Lane message dated "Thu, 04 Sep 2008 21:13:18 -0400" Date: Thu, 04 Sep 2008 21:45:53 -0400 Message-ID: <11525.1220579153@sss.pgh.pa.us> From: Tom Lane X-Virus-Scanned: Maia Mailguard 1.0.1 X-Spam-Status: No, hits=0 tagged_above=0 required=5 tests=none X-Spam-Level: X-Archive-Number: 200809/45 X-Sequence-Number: 7060 I wrote: > You have the unique-versus-not dimension, On second thought, actually not. What we want to look at is the penalty for false matches due to *distinct* key values that happen to have the same hash codes. Your test case for all-the-same is using all the same key values, which means it'll hit the heap a lot, but none of those will be wasted trips. So what we need for testing is a few different key values that hash to the same code. Not sure about an easy way to find such. regards, tom lane