pgsql: Invent "join domains" to replace the below_outer_join hack.

Invent "join domains" to replace the below_outer_join hack.

EquivalenceClasses are now understood as applying within a "join
domain", which is a set of inner-joined relations (possibly underneath
an outer join).  We no longer need to treat an EC from below an outer
join as a second-class citizen.

I have hopes of eventually being able to treat outer-join clauses via
EquivalenceClasses, by means of only applying deductions within the
EC's join domain.  There are still problems in the way of that, though,
so for now the reconsider_outer_join_clause logic is still here.

I haven't been able to get rid of RestrictInfo.is_pushed_down either,
but I wonder if that could be recast using JoinDomains.

I had to hack one test case in postgres_fdw.sql to make it still test
what it was meant to, because postgres_fdw is inconsistent about
how it deals with quals containing non-shippable expressions; see
https://postgr.es/m/1691374.1671659838@sss.pgh.pa.us.  That should
be improved, but I don't think it's within the scope of this patch
series.

Patch by me; thanks to Richard Guo for review.

Discussion: https://postgr.es/m/830269.1656693747@sss.pgh.pa.us

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/3bef56e11650a33f70adeb6dd442bc2b48bb9b72

Modified Files
--------------
contrib/postgres_fdw/expected/postgres_fdw.out |  14 +-
contrib/postgres_fdw/sql/postgres_fdw.sql      |   2 +-
src/backend/nodes/outfuncs.c                   |   1 -
src/backend/optimizer/path/equivclass.c        | 133 +++++++++------
src/backend/optimizer/path/joinpath.c          |  12 --
src/backend/optimizer/plan/createplan.c        |   5 +-
src/backend/optimizer/plan/initsplan.c         | 213 +++++++++++++------------
src/backend/optimizer/plan/planner.c           |  10 ++
src/backend/optimizer/prep/prepjointree.c      |   2 +
src/include/nodes/pathnodes.h                  |  67 ++++++--
src/include/optimizer/paths.h                  |   2 +-
src/include/optimizer/planmain.h               |   1 -
12 files changed, 269 insertions(+), 193 deletions(-)