I wrote:
> > I have a field whose distribution of frequencies of values is
> > roughly geometric, rather than flat.
> My problem is frequent
> > over-estimation of rows when restricting by this field with
> > values not known at plan time.
> Is there any facility already in PostgreSQL to help me here?
>
> Hopefully an index type that I don't know about yet?
> (Geometric distributions are similar to those found in word
> count distributions).
>
> If not... is there any merit in this idea:
>
> During the analyze process, the geometric mean of sampled
> rows was calculated, and if determined to be significantly
> different from the arithmetic mean, stored in a new stats
> column. When estimating the number of rows that will be
> returned by queries of the form shown above, if there is a
> geometric mean stored, use it instead of the arithmetic mean.
I came up with another (much easier) means of adjusting the planners estimation of how many rows will be returned:
Increase the number of distinct values in the statistics.
For example:
update pg_statistic set stadistinct=2691 where starelid=29323 and staattnum=2;
I can then pick a number of distinct values such that the effective arithmetic mean is equal to what I calculated the
geometricmean to be.
Stephen Denne.
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