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Re: [PERFORM] "Hash index" vs. "b-tree index" (PostgreSQL

From: "Jim C(dot) Nasby" <decibel(at)decibel(dot)org>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Neil Conway <neilc(at)samurai(dot)com>,Christopher Petrilli <petrilli(at)gmail(dot)com>,Ying Lu <ying_lu(at)cs(dot)concordia(dot)ca>, pgsql-general(at)postgresql(dot)org,pgsql-performance(at)postgresql(dot)org
Subject: Re: [PERFORM] "Hash index" vs. "b-tree index" (PostgreSQL
Date: 2005-05-10 15:39:27
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Lists: pgsql-generalpgsql-performance
On Tue, May 10, 2005 at 12:10:57AM -0400, Tom Lane wrote:
> be responsive to your search.)  (This also brings up the thought that
> it might be interesting to support hash buckets smaller than a page ...
> but I don't know how to make that work in an adaptive fashion.)

IIRC, other databases that support hash indexes also allow you to define
the bucket size, so it might be a good start to allow for that. DBA's
usually have a pretty good idea of what a table will look like in
production, so if there's clear documentation on the effect of bucket
size a good DBA should be able to make a good decision.

What's the challange to making it adaptive, comming up with an algorithm
that gives you the optimal bucket size (which I would think there's
research on...) or allowing the index to accommodate different bucket
sizes existing in the index at once? (Presumably you don't want to
re-write the entire index every time it looks like a different bucket
size would help.)
Jim C. Nasby, Database Consultant               decibel(at)decibel(dot)org 
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