pgsql: Fix constant propagation through FULL JOIN clauses

Fix constant propagation through FULL JOIN clauses

Given a FULL JOIN clause "LEFTVAR = RIGHTVAR" and an EquivalenceClass
containing the expression COALESCE(LEFTVAR, RIGHTVAR) and a constant,
reconsider_full_join_clause() derives "LEFTVAR = CONSTANT" and "RIGHTVAR
= CONSTANT" and discards the join clause and the COALESCE condition.

It however assumed LEFTVAR comes from the join's lefthand input and
RIGHTVAR from its righthand input, which holds for FULL JOIN USING but
not necessarily for a user-written ON clause.

With the operands reversed, the derived equalities got the wrong
JoinDomains. That could cause an assertion failure or, in non-assert
builds, incorrect results, due to a derived qual being applied to the
wrong side of the join. With both operands from the same input, or one
referencing both inputs, nothing can be deduced, yet bogus equalities
were derived anyway. Both could produce wrong results.

Fix by determining which input each operand comes from, and giving up
unless they come from different inputs. A FULL JOIN whose only condition
is a clause of the latter kind now fails with "FULL JOIN is only
supported with merge-joinable or hash-joinable join conditions", as it
would without the COALESCE condition.

The JoinDomain problem was introduced in v16. The other one (bogus
equalities derived from a qual with both operands from the same input,
or one referencing both inputs) exists in all supported branches, but
before the introduction of outer-join-aware Vars (v16) only an operand
referencing both inputs can trigger it.

Backpatch to all supported versions. The 15 branch needs only the part
addressing the second issue.

Author: Tomas Vondra <tomas(at)vondra(dot)me>
Discussion: https://postgr.es/m/92808d26-1b4a-48fe-82f0-597da3ff471f@vondra.me
Backpatch-through: 15

Branch
------
REL_17_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/d35a59f1b9bcb239bdd6f1378fb996dac251ef7f

Modified Files
--------------
src/backend/optimizer/path/equivclass.c | 36 ++++--
src/backend/optimizer/plan/initsplan.c | 7 +-
src/test/regress/expected/join.out | 211 ++++++++++++++++++++++++++++++++
src/test/regress/sql/join.sql | 83 +++++++++++++
4 files changed, 326 insertions(+), 11 deletions(-)