Re: multivariate statistics (v25)

I tried patch 0002 today and again there are conflicts, so I rebased and
fixed the merge problems.  I also changed a number of minor things, all
AFAICS cosmetic in nature:

* moved src/backend/statistics/common.h to
  src/include/statistics/common.h, as previously commented.  I also took
  out postgres.h and most of the includes; instead, put all these into
  each .c source file.  That aligns with our established practice.
  I also removed two prototypes that should actually be in stats.h.
  I think statistics/common.h should be further renamed to
  statistics/stats_ext_internal.h, and statistics/stats.h to something
  different though I don't know what ATM.

* Moved src/include/utils/stats.h to src/include/statistics, clean it up
  a bit.

* Moved some structs from analyze.c into statistics/common.h, removing
  some duplication; have analyze.c include that file.

* renamed src/test/regress/sql/mv_ndistinct.sql to stats_ext.sql, to
  collect all ext.stats. related tests in a single file, instead of
  having a large number of them.  I also added one test that drops a
  column, per David Rowley's reported failure, but I didn't actually fix
  the problem nor add it to the expected file.  (I'll follow up with
  that tomorrow, if Tomas doesn't beat me to it).  Also, put the test in
  an earlier parallel test group, 'cause I see no reason to put it last.

* A bunch of stylistic changes.

The added tests pass (or they passed before I added the drop column
tests; not a surprise really that they pass, since I didn't touch
anything functionally), but they aren't terribly exhaustive at the stage
of the first patch in the series.

I didn't get around to addressing all of David Rowley's input.  Also I
didn't try to rebase the remaining patches in the series on top of this
one.

-- 
Álvaro Herrera                https://www.2ndQuadrant.com/
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