Received: from maia.hub.org (maia-2.hub.org [200.46.204.251]) by mail.postgresql.org (Postfix) with ESMTP id 61808B5DBC8 for ; Mon, 19 Sep 2011 09:28:17 -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 21948-01 for ; Mon, 19 Sep 2011 12:28:11 +0000 (UTC) X-Greylist: domain auto-whitelisted by SQLgrey-1.8.0-rc2 Received: from lo.gmane.org (lo.gmane.org [80.91.229.12]) by mail.postgresql.org (Postfix) with ESMTP id 912C0B5DBC4 for ; Mon, 19 Sep 2011 09:28:09 -0300 (ADT) Received: from list by lo.gmane.org with local (Exim 4.69) (envelope-from ) id 1R5cxD-0004XO-2n for pgsql-performance@postgresql.org; Mon, 19 Sep 2011 14:28:07 +0200 Received: from 217.110.94.67 ([217.110.94.67]) by main.gmane.org with esmtp (Gmexim 0.1 (Debian)) id 1AlnuQ-0007hv-00 for ; Mon, 19 Sep 2011 14:28:07 +0200 Received: from spam_eater by 217.110.94.67 with local (Gmexim 0.1 (Debian)) id 1AlnuQ-0007hv-00 for ; Mon, 19 Sep 2011 14:28:07 +0200 X-Injected-Via-Gmane: http://gmane.org/ To: pgsql-performance@postgresql.org From: Thomas Kellerer Subject: Re: Hash index use presently(?) discouraged since 2005: revive or bury it? Date: Mon, 19 Sep 2011 14:28:31 +0200 Lines: 24 Message-ID: References: Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Complaints-To: usenet@dough.gmane.org X-Gmane-NNTP-Posting-Host: 217.110.94.67 User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.8.1.23) Gecko/20090812 Thunderbird/2.0.0.23 Mnenhy/0.7.6.666 In-Reply-To: X-Virus-Scanned: Maia Mailguard 1.0.1 X-Spam-Status: No, hits=-1.229 tagged_above=-10 required=5 tests=BAYES_00=-1.9, FREEMAIL_FROM=0.001, RCVD_NUMERIC_HELO=1.164, RP_MATCHES_RCVD=-0.504, T_TO_NO_BRKTS_FREEMAIL=0.01 X-Spam-Level: X-Archive-Number: 201109/265 X-Sequence-Number: 45041 Robert Klemme, 19.09.2011 13:13: > On Sun, Sep 18, 2011 at 9:31 PM, Stefan Keller wrote: >> I'm simply referring to literature (like the intro Ramakrishnan& Gehrke). >> I just know that Oracle an Mysql actually do have them too and use it >> without those current implementation specific restrictions in >> Postgres. > > Where exactly do you take that from that Oracle has hash indexes? I > can't seem to find them: > http://download.oracle.com/docs/cd/E11882_01/server.112/e16508/indexiot.htm#sthref293 > > Are you mixing this up with hash partitioning? > http://download.oracle.com/docs/cd/E11882_01/server.112/e16508/schemaob.htm#sthref443 > > Or am I missing something? Maybe he was referring to a hash cluster: http://download.oracle.com/docs/cd/E11882_01/server.112/e17118/statements_5001.htm This is a storage option where you can store related rows (e.g. in a parent/child relationship) in the same phyiscal database block based on a hash value. That enables the databse to read parent and child rows with just a single IO. In the background Oracle probably has something like a hash index to support that. Thomas