Re: estimating # of distinct values

From: "Kevin Grittner" <Kevin(dot)Grittner(at)wicourts(dot)gov>
To: "Josh Berkus" <josh(at)agliodbs(dot)com>,"Tomas Vondra" <tv(at)fuzzy(dot)cz>
Cc: <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: estimating # of distinct values
Date: 2010-12-28 14:53:06
Message-ID: 4D19A5720200002500038C31@gw.wicourts.gov
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<tv(at)fuzzy(dot)cz> wrote:

> So even with 10% of the table, there's a 10% probability to get an
> estimate that's 7x overestimated or underestimated. With lower
> probability the interval is much wider.

Hmmm... Currently I generally feel I'm doing OK when the estimated
rows for a step are in the right order of magnitude -- a 7% error
would be a big improvement in most cases. Let's not lose track of
the fact that these estimates are useful even when they are not
dead-on accurate.

-Kevin

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