Re: multivariate statistics v11

Hi,

attached is v11 of the patch - this is mostly a cleanup of v10, removing
redundant code, adding missing comments, removing obsolete FIXME/TODOs
and so on. Overall this shaves ~20kB from the patch (not a primary
objective, though).

The one thing this (hopefully) fixes is handling of varRelid. Apparently
I got that a slightly wrong in the previous versions.

One thing I'm not quite sure about is schema of the new system catalog.
The existing catalog pg_statistic uses generic design with stakindN,
stanumbersN and stavaluesN columns, while the new catalog uses dedicated
columns for each type of stats (MCV, histogram, ...). Not sure whether
it's desirable to switch to the pg_statistic approach or not.

There are a few things I plan to look into next:

* possibly more cleanups in clausesel.c (I'm wondering if some pieces
should be moved to utils/mvstats/*.c)

* a few FIXMEs in the infrastructure (e.g. deriving a name when not
specified in CREATE STATISTICS)

* move the ndistinct coefficients after functional dependencies in
the patch series (but only use them for GROUP BY for now)

* extend the functional dependencies to handle multiple columns on
the left side (condition), i.e. dependencies like (a,b) -> c

* address a few remaining FIXMEs in MCV/histograms building

regards

--
Tomas Vondra http://www.2ndQuadrant.com
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