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Date: Fri, 19 Jul 2002 12:00:00 -0500
From: Bruno Wolff III <bruno@wolff.to>
To: Josh Berkus <josh@agliodbs.com>
Cc: pgsql-sql@postgresql.org
Subject: Re: Two Index Questions
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On Fri, Jul 19, 2002 at 09:33:59 -0700,
  Josh Berkus <josh@agliodbs.com> wrote:
> Folks (esp. Tom, Stephan, and Bruce):
> 
> I have two questions for my "Adventures in PostgreSQL" article reasearch:
> 
> Multi-Column Indexes and GROUP BY:
> Q:  If you group a table by multiple colums, e.g.
> 	SELECT t1.A, t1.B, t1.C, MAX(t1.G)
> 	FROM t1
> 	GROUP BY t1.A, t1.B, t1.C
>   Then would a multi-column index on A, B, C be faster than seperate indexes 
> on A, B and C?   I've run a few tests, but I don't have enough data in the 
> seperate tables to really get a feel for the difference.

If there are lots of G entries for fixed As, Bs and Cs, then another option
would be to have an index on all 4 tables and use a subquery with a limit 1
clause to get the row with the max G value for any A, B and C.