| From: | Zeugswetter Andreas OSB sIT <Andreas(dot)Zeugswetter(at)s-itsolutions(dot)at> |
|---|---|
| To: | Nathan Boley <npboley(at)gmail(dot)com>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
| Cc: | PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: Proposal - improve eqsel estimates by including histogram bucket numdistinct statistics |
| Date: | 2008-06-10 09:01:02 |
| Message-ID: | 6DAFE8F5425AB84DB3FCA4537D829A561BBF8B9246@M0164.s-mxs.net |
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| Lists: | pgsql-hackers |
> Obviously we run into problems when
> a) we have a poor estimate for ndistinct - but then we have
> worse problems
> b) our length measure doesn't correspond well with ndistinct
> in an interval
One more problem with low ndistinct values is that the condition might very well
hit no rows at all. But Idea 1 will largely overestimate the number of hits.
e.g. char(2) field has a histogram bin for 'a1' - 'b1' ndistinct is 2 because actual
values in the bin are 'a1' and 'a2'. A query for 'a3' now has a bogus estimate of nrowsperbin / 2.
I think for low ndistinct values we will want to know the exact
value + counts and not a bin. So I think we will want additional stats rows
that represent "value 'a1' stats".
Andreas
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