Re: RFC: planner statistics in 7.2

At 18:37 19/04/01 -0400, Tom Lane wrote:
>(2) Statistics should be computed on the basis of a random sample of the
>target table, rather than a complete scan. According to the literature
>I've looked at, sampling a few thousand tuples is sufficient to give good
>statistics even for extremely large tables; so it should be possible to
>run ANALYZE in a short amount of time regardless of the table size.

This sounds great; can the same be done for clustering. ie. pick a random
sample of index nodes, look at the record pointers and so determine how
well clustered the table is?

>A simple approach would be a SET
>variable or explicit parameter for ANALYZE. But I am inclined to think
>that it'd be better to create a persistent per-column state for this,
>set by say
> ALTER TABLE tab SET COLUMN col STATS COUNT n

Sounds fine - user-selectability at the column level seems a good idea.
Would there be any value in not making it part of a normal SQLxx statement,
and adding an 'ALTER STATISTICS' command? eg.

ALTER STATISTICS FOR tab[.column] COLLECT n
ALTER STATISTICS FOR tab SAMPLE m

etc.

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