Tom Lane wrote:
> Martijn van Oosterhout <kleptog@svana.org> writes:
> > On Fri, Mar 07, 2008 at 07:25:25PM +0100, Peter Eisentraut wrote:
> >> What's the problem with setting it to ten million if I
> have ten million values
> >> in the table and I am prepared to spend the resources to
> maintain those
> >> statistics?
>
> > That it'll probably take 10 million seconds to calculate the plans
> > using it? I think Tom pointed there are a few places that are O(n^2)
> > the number entries...
>
> I'm not wedded to the number 1000 in particular --- obviously that's
> just a round number. But it would be good to see some
> performance tests
> with larger settings before deciding that we don't need a limit.
I recently encountered a situation where I would have liked to be able to try a larger limit (amongst other ideas for
improvingmy situation):
I have a field whose distribution of frequencies of values is roughly geometric, rather than flat.
Total rows = 36 million
relpages=504864
Distinct field values in use = 169
10 values account for 50% of the rows.
41 values account for 90% of the rows.
After setting statistics target to 1000 for that field, and analyzing the table, the statistics row for that field had
75most frequent values and a histogram with 76 entries in it. Estimating 151 values in total.
For this situation using a larger statistics target should result in more pages being read, and a more accurate record
ofstatistics. It shouldn't result in significantly more work for the planner.
It wouldn't solve my problem though, which is frequent over-estimation of rows when restricting by this field with
valuesnot known at plan time.
Regards,
Stephen Denne.
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