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Re: GSoC on WAL-logging hash indexes

From: Peter Geoghegan <pg(at)heroku(dot)com>
To: Robert Haas <robertmhaas(at)gmail(dot)com>
Cc: Tan Tran <tankimtran(at)gmail(dot)com>, Greg Stark <stark(at)mit(dot)edu>, pgsql-advocacy <pgsql-advocacy(at)postgresql(dot)org>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: GSoC on WAL-logging hash indexes
Date: 2014-04-30 07:26:20
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Lists: pgsql-advocacypgsql-hackerspgsql-students
On Mon, Mar 3, 2014 at 8:12 AM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>> As a GSoC student, I will implement WAL recovery of hash indexes using the
>> other index types' WAL code as a guide.

Frankly, I'm skeptical of the idea that hash indexes will ever really
be useful. I realize that that's a counter-intuitive conclusion, but
there are many things we could do to improve B-Tree CPU costs to make
them closer to those of hash indexes, without making them any less
flexible. I myself would much rather work on that, and intend to.

The O(1) cost seems attractive when you consider that that only
requires that we read one index page from disk to service any given
index scan, but in fact B-Trees almost always only require the same.
They are of course also much more flexible. The concurrency
characteristics B-Trees are a lot better understood. I sincerely
suggest that we forget about conventional hash table type indexes. I
fear they're a lost cause.

Peter Geoghegan

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