Re: Extended Statistics set/restore/clear functions.

Hmm.  Not sure yet which one is better here, TBH.  The "ok" pattern
feels a but better to me because a Datum == 0 could also be fine in
some cases, be they existing of future cases, so that's more flexible
by design.

When I review a patch and that I want to do a clean split, I tend to
apply the whole, then remove the parts that can be split out, reapply,
and then generate a diff of the previously-removed code.  It's just
easier to delete code.  For this patch set, it would mean removing the
MVC variables and related structures.  It is actually not that bad
based on how you have structured the restore function; there is no
strong dependency between each code path treating each stxkind.  This
is just thinking of possible strategy if this whole thing cannot make
it into the tree by the freeze deadline.  Some of it may be better
than nothing of it, and it is possible to add more parameters later
with the right basics in place.  For me, review and integration tends
to be easier as a incremental set of small-ish steps, but I think that
you are aware of my way of doing things :D