From: Tom Lane <tgl@sss.pgh.pa.us>
To: 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 00:54:48 -0400
Message-ID: <6686.1115700888@sss.pgh.pa.us> (raw)
In-Reply-To: <428037A2.4060304@samurai.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>
Neil Conway <neilc@samurai.com> writes:
> Tom Lane wrote:
>> On the other hand, once you reach the target index page, a hash index
>> has no better method than linear scan through all the page's index
>> entries to find the actually wanted key(s)
> I wonder if it would be possible to store the keys in a hash bucket in
> sorted order, provided that the necessary ordering is defined for the
> index keys -- considering the ubiquity of b+-trees in Postgres, the
> chances of an ordering being defined are pretty good.
I have a gut reaction against that: it makes hash indexes fundamentally
subservient to btrees. We shouldn't bring in concepts that are outside
the basic opclass abstraction.
However: what about storing the things in hashcode order? Ordering uint32s
doesn't seem like any big conceptual problem.
I think that efficient implementation of this would require explicitly
storing the hash code for each index entry, which we don't do now, but
it seems justifiable on multiple grounds --- besides this point, the
search could avoid doing the data-type-specific comparison if the hash
code isn't equal.
There is evidence in the code that indexes used to store more info than
what we now think of as a "standard" index tuple. I am not sure when
that went away or what it'd cost to bring it back, but it seems worth
looking into.
regards, tom lane
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To: pgsql-performance@postgresql.org
Cc: tgl@sss.pgh.pa.us, 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: <6686.1115700888@sss.pgh.pa.us>
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