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From: Thomas Kellerer <spam_eater@gmx.net>
To: pgsql-performance@postgresql.org
Subject: Re: Hash index use presently(?) discouraged since 2005: revive or bury it?
Date: Mon, 19 Sep 2011 14:28:31 +0200
Message-ID: <j57cga$9f2$1@dough.gmane.org> (raw)
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Robert Klemme, 19.09.2011 13:13:
> On Sun, Sep 18, 2011 at 9:31 PM, Stefan Keller<sfkeller@gmail.com>  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




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