pgsql: Fix sorting a child rel by an expression containing a PlaceHolde

Fix sorting a child rel by an expression containing a PlaceHolderVar

When building sort keys for a child relation, such as the per-child
sort columns of a MergeAppend over partitionwise child joins,
prepare_sort_from_pathkeys() may need to compute an EquivalenceClass
member from the child's targetlist. find_computable_ec_member()
accepts a member if all the Vars and quasi-Vars it contains appear in
that targetlist. It ignores child members that don't belong to the
given relids, but it can still choose a parent member. Usually that
is harmless, since a parent member's Vars never appear in a child's
targetlist. PlaceHolderVars are different: equal() ignores their
phexpr and phrels, so a parent PlaceHolderVar matches its translated
copy in the child's targetlist. As a result, an untranslated parent
expression could be added to the child plan's targetlist, leading to
"variable not found in subplan target lists" errors from setrefs.c.

To fix, when relids is given, also ignore parent members that don't
belong to it. This changes nothing for a non-child relation, because
any member computable from its targetlist belongs to it anyway. When
relids is NULL, only child members are filtered, as before.

Also update the comments for prepare_sort_from_pathkeys() to reflect
that 'relids' can identify a non-child relation being sorted, which
has long been the case.

Back-patch to all supported branches.

Author: Richard Guo <guofenglinux(at)gmail(dot)com>
Reviewed-by: wenhui qiu <qiuwenhuifx(at)gmail(dot)com>
Discussion: https://postgr.es/m/CAMbWs48bLLEGy6YC+n01w6=YR=28cj_kpoKcAgOHQR7jbtLb6w@mail.gmail.com
Backpatch-through: 14

Branch
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REL_19_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/f366c2ce1453bb3b92241be5dca72defc361c65b

Modified Files
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src/backend/optimizer/path/equivclass.c | 11 +++++--
src/backend/optimizer/plan/createplan.c | 6 ++--
src/test/regress/expected/partition_join.out | 44 ++++++++++++++++++++++++++++
src/test/regress/sql/partition_join.sql | 11 +++++++
4 files changed, 67 insertions(+), 5 deletions(-)