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Re: Group-count estimation statistics

From: Stephen Frost <sfrost(at)snowman(dot)net>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: pgsql-hackers(at)postgresql(dot)org
Subject: Re: Group-count estimation statistics
Date: 2005-01-28 18:25:40
Message-ID: 20050128182540.GA10437@ns.snowman.net (view raw or flat)
Thread:
Lists: pgsql-hackers
* Tom Lane (tgl(at)sss(dot)pgh(dot)pa(dot)us) wrote:
> The only real solution, of course, is to acquire cross-column
> statistics, but I don't see that happening in the near future.

That'd be nice, but sounds like alot of work.

> As a short-term hack, I am thinking that the "clamp to size of table"
> part of the rule is overly pessimistic, and that we should consider
> something like "clamp to size of table / 10" instead.  The justification
> for this is the thought that you aren't going to bother grouping unless
> it actually reduces the data volume.  We have used similar rules in the

I definitely agree with this.

	Stephen

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