pgsql: Don't reprocess PHV copies within outer aggregates' arguments

Don't reprocess PHV copies within outer aggregates' arguments

Since commit e28cf96e9, the copies of a query level's PlaceHolderVars
that were pushed down into its subqueries are preprocessed by that
level before its SubLinks are turned into SubPlans, so such a copy may
already contain SubPlans by the time the subquery is planned. That
commit taught build_subplan not to process the SubLinks of a PHV that
is passed down as a SubPlan argument, but overlooked that such a copy
can also appear within the arguments of an outer-level Aggref,
GroupingFunc, or ReturningExpr. The SubLinks in those arguments are
still processed when build_subplan collects the node, and that
processing descended into the already-preprocessed PHV and tripped the
no-SubPlan assertion in process_sublinks_mutator.

To fix, don't descend into PHVs when processing the SubLinks in such a
node's arguments. Any PHV found there is a pushed-down copy that its
owning level has already preprocessed.

Back-patch to v16, as with commit e28cf96e9.

Author: Ayush Tiwari <ayushtiwari(dot)slg01(at)gmail(dot)com>
Reviewed-by: Richard Guo <guofenglinux(at)gmail(dot)com>
Discussion: https://postgr.es/m/CAJTYsWW8THbrnsmYnDqnUgg0PjGrqw2vNOWb8qx7eu191KPW3g@mail.gmail.com
Backpatch-through: 16

Branch
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REL_18_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/3bfc7c8491e6744fd0a1d98f7a98b01528eeae41

Modified Files
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src/backend/optimizer/plan/subselect.c | 18 +++++++++++++++---
src/test/regress/expected/join.out | 25 +++++++++++++++++++++++++
src/test/regress/sql/join.sql | 6 ++++++
3 files changed, 46 insertions(+), 3 deletions(-)