Received: from localhost (unknown [200.46.204.183]) by postgresql.org (Postfix) with ESMTP id 52BBB64FCE8 for ; Thu, 4 Sep 2008 02:35:29 -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 87147-02 for ; Thu, 4 Sep 2008 02:35:23 -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 5B84864FCD8 for ; Thu, 4 Sep 2008 02:35:23 -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 m845ZGam014103; Thu, 4 Sep 2008 01:35:16 -0400 (EDT) To: Simon Riggs cc: Xiao Meng , pgsql-patches@postgresql.org, "Jonah H. Harris" Subject: Re: hash index improving v3 In-reply-to: <1220494002.4371.813.camel@ebony.2ndQuadrant> References: <1220494002.4371.813.camel@ebony.2ndQuadrant> Comments: In-reply-to Simon Riggs message dated "Thu, 04 Sep 2008 03:06:42 +0100" Date: Thu, 04 Sep 2008 01:35:16 -0400 Message-ID: <14102.1220506516@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: 200809/37 X-Sequence-Number: 7052 Simon Riggs writes: > Right now it seems strange that the index is larger than a btree, yet > the performance tests show that 3 times as much I/O was used accessing > the btree. Well, in an ideal world a hash index probe is O(1) while a btree probe is O(log N), so that result is exactly what hash proponents would hope for. Whether it's real or not is another question, but it could be. regards, tom lane