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From: Greg Stark <gsstark@mit.edu>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Greg Stark <gsstark@mit.edu>
Cc: Neil Conway <neilc@samurai.com>
Cc: Jim C. Nasby <decibel@decibel.org>
Cc: Christopher Petrilli <petrilli@gmail.com>
Cc: Ying Lu <ying_lu@cs.concordia.ca>
Cc: pgsql-general@postgresql.org
Cc: pgsql-performance@postgresql.org
Subject: Re: [PERFORM] "Hash index" vs. "b-tree index" (PostgreSQL
Date: 10 May 2005 19:56:15 -0400
Message-ID: <87ll6mr5y8.fsf@stark.xeocode.com> (raw)
In-Reply-To: <25690.1115754605@sss.pgh.pa.us>
References: <427F6853.90308@cs.concordia.ca>
	<427F757A.8000407@samurai.com>
	<59d991c405050908271b55e673@mail.gmail.com>
	<427F8321.5030702@samurai.com>
	<20050509163423.GH35026@decibel.org>
	<427F9211.2090008@samurai.com>
	<20050509165133.GJ35026@decibel.org>
	<427FFCD3.1010803@samurai.com>
	<4407.1115698257@sss.pgh.pa.us>
	<428037A2.4060304@samurai.com>
	<6686.1115700888@sss.pgh.pa.us>
	<878y2nsj7i.fsf@stark.xeocode.com>
	<9871.1115733198@sss.pgh.pa.us>
	<87wtq7q8zk.fsf@stark.xeocode.com>
	<25690.1115754605@sss.pgh.pa.us>


Tom Lane <tgl@sss.pgh.pa.us> writes:

> No, not at all, because searching such an index will require a tree
> descent, thus negating the one true advantage of hash indexes.  

The hash index still has to do a tree descent, it just has a larger branching
factor than the btree index.

btree indexes could have a special case hack to optionally use a large
branching factor for the root node, effectively turning them into hash
indexes. That would be useful for cases where you know the values will be very
evenly distributed and won't need to scan ranges, ie, when you're indexing a
hash function.

-- 
greg

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