From: Bruce Momjian <pgman@candle.pha.pa.us>
Message-Id: <200207191827.g6JIRw603603@candle.pha.pa.us>
Subject: Re: Two Index Questions
In-Reply-To: <200207191112.49885.josh@agliodbs.com>
To: josh@agliodbs.com
Date: Fri, 19 Jul 2002 14:27:58 -0400 (EDT)
Cc: Bruno Wolff III <bruno@wolff.to>, pgsql-sql@postgresql.org
MIME-Version: 1.0
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Josh Berkus wrote:
> Bruno, Tom,
> 
> > Typo. I meant to say columns. The issue is that max doesn't use an index,
> > but if there are a lot of different values of G for a given A, B and C,
> > it may be better to use an index then to search through the applicable
> > rows to find the maximum.
> 
> That's odd ... you're correct.   Tom, why doesn't MAX() use an index?   I 
> understand why indexes are generally useless for SUM(), AVG, and COUNT, but 
> it seems that MAX() and MIN() should *always* use an index.

Index access methods don't know about aggregates, and our type system
makes such linkage difficult.  The FAQ does have:
	
	    However, <SMALL>LIMIT</SMALL> combined with <SMALL>ORDER BY</SMALL>
	    often will use an index because only a small portion of the table
	    is returned.  In fact, though MAX() and MIN() don't use indexes,
	    it is possible to retrieve such values using an index with ORDER BY
	    and LIMIT:
	<PRE>
	    SELECT col
	    FROM tab
	    ORDER BY col [ DESC ]
	    LIMIT 1
	</PRE>

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