Re: Making Vars outer-join aware

Here's a new edition of this patch series.

I shoved some preliminary refactoring into the 0001 patch,
notably splitting deconstruct_jointree into two passes.
0002-0009 cover the same ground as they did before, though
with some differences in detail.  0010-0012 are new work
mostly aimed at removing kluges we no longer need.

There are two big areas that I would still like to improve, but
I think we've run out of time to address them in the v16 cycle:

* It'd be nice to apply the regular EquivalenceClass deduction
mechanisms to outer-join equalities, instead of the
reconsider_outer_join_clauses kluge.  I've made several stabs at that
without much success.  I think that the "join domain" framework added
in 0012 is likely to provide a workable foundation, but some more
effort is needed.

* I really want to get rid of RestrictInfo.is_pushed_down and
RINFO_IS_PUSHED_DOWN(), because those seem exceedingly awkward
and squishy.  I've not gotten far with finding a better
replacement there, either.

Despite the work being unfinished, I feel that this has moved us a
long way towards outer-join handling being less of a jury-rigged
affair.

                        regards, tom lane