Re: Group-count estimation statistics

From: Kris Jurka <books(at)ejurka(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Greg Stark <gsstark(at)mit(dot)edu>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: Group-count estimation statistics
Date: 2005-01-28 21:30:00
Message-ID: Pine.BSO.4.56.0501281628001.23832@leary.csoft.net
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On Fri, 28 Jan 2005, Tom Lane wrote:

> you don't GROUP BY unique combinations of columns over huge
> tables --- or at least, you shouldn't expect great performance if you do.

The proposed change biases towards a hash plan which has no provision for
spilling to disk. Slow is one thing, but excessive memory usage and
possibly failing is another thing.

Kris Jurka

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