From: | Greg Stark <gsstark(at)mit(dot)edu> |
---|---|
To: | Josh Berkus <josh(at)agliodbs(dot)com> |
Cc: | pgsql-hackers(at)postgresql(dot)org |
Subject: | Re: Improving N-Distinct estimation by ANALYZE |
Date: | 2006-01-05 15:02:11 |
Message-ID: | 87irsy4t1o.fsf@stark.xeocode.com |
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Lists: | pgsql-hackers |
Josh Berkus <josh(at)agliodbs(dot)com> writes:
> > Only if your sample is random and independent. The existing mechanism tries
> > fairly hard to ensure that every record has an equal chance of being selected.
> > If you read the entire block and not appropriate samples then you'll introduce
> > systematic sampling errors. For example, if you read an entire block you'll be
> > biasing towards smaller records.
>
> Did you read any of the papers on block-based sampling? These sorts of issues
> are specifically addressed in the algorithms.
We *currently* use a block based sampling algorithm that addresses this issue
by taking care to select rows within the selected blocks in an unbiased way.
You were proposing reading *all* the records from the selected blocks, which
throws away that feature.
--
greg
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