pgsql: Fix nestloop parameter assignment for PHVs evaluated above outer

Fix nestloop parameter assignment for PHVs evaluated above outer joins

A NestLoop can supply a PlaceHolderVar as a parameter if the PHV can
be evaluated from the lefthand rels plus the required-outer rels of
the join. A PHV's ph_eval_at can include outer-join relids, but
required-outer sets need not: create_lateral_join_info() keeps
outer-join relids out of the lateral dependencies it derives from
ph_lateral. So when an outer join within ph_eval_at had been computed
by an enclosing nestloop's outer side, the test in
identify_current_nestloop_params() failed and no plan node claimed the
parameter. This resulted in "failed to assign all NestLoopParams to
plan nodes", or, if an unrelated upper NestLoop happened to cover
ph_eval_at and claimed the parameter, "plan should not reference
subplan's variable".

To fix, have create_nestloop_plan() also count the outer joins that
have already been computed by the outer rels in scope.

Back-patch to v18, where commit a16ef313f made it possible to compute
a PHV that is used as a nestloop parameter in the outer side of the
NestLoop. v16 and v17 can fail the same way, since ph_eval_at has
included outer-join relids since v16, but they lack that machinery.
Fixing them would mean adding join-order restrictions to
have_dangerous_phv() that could change plans that work today, which
does not seem worth the risk for a failure this unlikely.

Author: Richard Guo <guofenglinux(at)gmail(dot)com>
Discussion: https://postgr.es/m/CAMbWs4-fcAULTpWthZMHGOFK1J+Hz=Dvr1TO-M20cC317rOGOA@mail.gmail.com
Backpatch-through: 18

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/c1c930376854dda25ce07a74d34d1a16f91cd3aa

Modified Files
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src/backend/optimizer/plan/createplan.c | 9 ++++++
src/test/regress/expected/join.out | 53 +++++++++++++++++++++++++++++++++
src/test/regress/sql/join.sql | 18 +++++++++++
3 files changed, 80 insertions(+)