pgsql: Fix generation of EC join conditions at the wrong plan level.

Fix generation of EC join conditions at the wrong plan level.

get_baserel_parampathinfo previously assumed without checking that
the results of generate_join_implied_equalities "necessarily satisfy
join_clause_is_movable_into". This turns out to be wrong in the
presence of outer joins, because the generated clauses could include
Vars that mustn't be evaluated below a relevant outer join. That
led to applying clauses at the wrong plan level and possibly getting
incorrect query results. We must check each clause's nullable_relids,
and really the right thing to do is test join_clause_is_movable_into.

However, trying to fix it that way exposes an oversight in
equivclass.c: it wasn't careful about marking join clauses for
appendrel children with the correct clause_relids. That caused the
modified get_baserel_parampathinfo code to reject some clauses it
still needs to accept. (See parallel commit for HEAD/v16 for more
commentary about that.)

Per bug #18429 from Benoît Ryder. This misbehavior existed for
a long time before commit 2489d76c4, so patch v12-v15 this way.

Discussion: https://postgr.es/m/18429-8982d4a348cc86c6@postgresql.org

Branch
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REL_15_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/5aacfa64e5dae1a26a5ad5192877683695de2e77

Modified Files
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src/backend/optimizer/path/equivclass.c | 15 +++++++++++++++
src/backend/optimizer/util/relnode.c | 25 ++++++++++++++++++-------
src/test/regress/expected/join.out | 31 +++++++++++++++++++++++++++++++
src/test/regress/sql/join.sql | 19 +++++++++++++++++++
4 files changed, 83 insertions(+), 7 deletions(-)