From: Tom Lane <tgl@sss.pgh.pa.us>
To: 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: Tue, 10 May 2005 09:53:18 -0400
Message-ID: <9871.1115733198@sss.pgh.pa.us> (raw)
In-Reply-To: <878y2nsj7i.fsf@stark.xeocode.com>
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>
Greg Stark <gsstark@mit.edu> writes:
> Tom Lane <tgl@sss.pgh.pa.us> writes:
>> I think that efficient implementation of this would require explicitly
>> storing the hash code for each index entry,
> It seems that means doubling the size of the hash index. That's a pretty big
> i/o to cpu tradeoff.
Hardly. The minimum possible size of a hash entry today is 8 bytes
header plus 4 bytes datum, plus there's a 4-byte line pointer to factor
in. So under the most pessimistic assumptions, storing the hash code
would add 25% to the size. (On MAXALIGN=8 hardware, it might cost you
nothing at all.)
> What if the hash index stored *only* the hash code? That could be useful for
> indexing large datatypes that would otherwise create large indexes.
Hmm, that could be a thought.
regards, tom lane
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To: pgsql-performance@postgresql.org
Cc: tgl@sss.pgh.pa.us, gsstark@mit.edu, neilc@samurai.com, decibel@decibel.org, petrilli@gmail.com, ying_lu@cs.concordia.ca, pgsql-general@postgresql.org
Subject: Re: [PERFORM] "Hash index" vs. "b-tree index" (PostgreSQL
In-Reply-To: <9871.1115733198@sss.pgh.pa.us>
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