Re: Extended Statistics set/restore/clear functions.

Patch 0001 for ndistinct was missing a documentation update, we have
one query in perform.sgml that looks at stxdndistinct.  Patch 0003 is
looking OK here as well.

For dependencies, the format switches from a single json object
with key/vals like that:
"3 => 4": 1.000000
To a JSON array made of elements like that:
{"degree": 1.000000, "attributes": [3],"dependency": 4},

For ndistincts, we move from a JSON blob with key/vals like that:
"3, 4": 11
To a JSON array made of the following elements:
{"ndistinct": 11, "attributes": [3,4]}

Using a keyword within each element would force a stronger validation
when these get imported back, which is a good thing.  I like that.

Before going in-depth into the input functions to cross-check the
amount of validation we should do, have folks any comments about the
proposed format?  That's the key point this patch set depends on, and
I'd rather not spend more time the whole thing if somebody would like
a different format.  This is the format that Tomas has mentioned at
the top of the thread.  Note: as noted upthread, pg_dump would be in
charge of transferring the data of the old format to the new format at
the end.

While looking at 0002 and 0004 (which have a couple of issues
actually), I have been wondering about moving into a new file the four
data-type functions (in, out, send and receive) and the new input
functions that rely on a new JSON lexer and parser logic into for both
ndistinct and dependencies.  The new set of headers added at the top
of mvdistinct.c and dependencies.c for the new code points that a
separation may be better in the long-term, because the new code relies
on parts of the backend that the existing code does not care about,
and these files become larger than the relation and attribute stats
files.  I would be tempted to name these new files pg_dependencies.c
and pg_ndistinct.c, mapping with their catalog types.  With this
separation, it looks like the "core" parts in charge of the
calculations with ndistinct and dependencies can be kept on its own.
What do you think?

A second comment is for 0005.  The routines of attributes.c are
applied to the new clear and restore functions.  Shouldn't these be in
stats_utils.c at the end?  That's where the "common" functions used by
the stats manipulation logic are.