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Re: Improving N-Distinct estimation by ANALYZE

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Simon Riggs <simon(at)2ndquadrant(dot)com>
Cc: pgsql-hackers(at)postgresql(dot)org
Subject: Re: Improving N-Distinct estimation by ANALYZE
Date: 2006-01-04 19:49:16
Message-ID: 6579.1136404156@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackers
Simon Riggs <simon(at)2ndquadrant(dot)com> writes:
> [ ... a large amount of analysis based on exactly one test case ... ]

I think you are putting too much emphasis on fixing one case and not
enough on considering what may happen in other cases ...

In general, estimating n-distinct from a sample is just plain a hard
problem, and it's probably foolish to suppose we'll ever be able to
do it robustly.  What we need is to minimize the impact when we get
it wrong.  So I agree with the comment that we need to finish the
unfinished project of making HashAggregate tables expansible, but
I'm dubious about the rest of this.

			regards, tom lane

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