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Re: Hash index

From: "Jim C(dot) Nasby" <jnasby(at)pervasive(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Scott Marlowe <smarlowe(at)g2switchworks(dot)com>,RAJU kumar <raju_19db(at)rediffmail(dot)com>, pgsql-admin(at)postgresql(dot)org
Subject: Re: Hash index
Date: 2005-08-30 23:39:13
Message-ID: 20050830233913.GE77007@pervasive.com (view raw or flat)
Thread:
Lists: pgsql-admin
On Tue, Aug 30, 2005 at 04:20:03PM -0400, Tom Lane wrote:
> "Jim C. Nasby" <jnasby(at)pervasive(dot)com> writes:
> > So does that mean a hash index could (theoretically) improve the
> > performance of a hash join or hash aggregation?
> 
> I don't think so --- the whole point of a hash is to do your matching
> in-memory, rather than jumping all over the disk for it ...

True, but if you have an index on the value you're hashing you should be
able to find out very quickly if it exists or not, no? So if you were
doing a hash join between two tables and one of them had a hash index on
the appropriate field, couldn't you check the index first to see if
there was a match? Granted, you could probably do the same thing with a
B-tree index, but this might be faster in some scenarios.
-- 
Jim C. Nasby, Sr. Engineering Consultant      jnasby(at)pervasive(dot)com
Pervasive Software        http://pervasive.com        512-569-9461

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