pgsql: Fix qual pushdown past grouping through simple CASE

Fix qual pushdown past grouping through simple CASE

Commit 44fb59fc6 taught the grouping-conflict walker to treat the arg
of a simple CASE as a direct operand of each WHEN comparison, but it
only checked the collation, on the assumption that the WHEN operator
is always the type-default "=" and thus matches the grouping eqop.
That assumption fails once the arg is relabeled to another type: the
WHEN then compares under that type's "=", which need not agree with
the grouping equality. For instance, with a DISTINCT over a citext
column, a qual such as "CASE t::text WHEN 'A' THEN ..." was pushed
below the Unique, although the equivalent "t::text = 'A'" is correctly
kept above it.

Instead of special-casing the arg, have the walker bind a Var arg
while walking the WHEN conditions and resolve each CaseTestExpr to it,
so that the arg is checked exactly as each WHEN uses it: with the
opfamily and collation checks of a direct operand when the WHEN is a
comparison, and as a non-operand reference otherwise. A non-Var arg
is walked once as a non-operand, as before. The CaseTestExpr in an
ArrayCoerceExpr's elemexpr and a JsonConstructorExpr's coercion stand
for something else and are left alone.

Reported-by: Tender Wang <tndrwang(at)gmail(dot)com>
Author: Richard Guo <guofenglinux(at)gmail(dot)com>
Reviewed-by: Tender Wang <tndrwang(at)gmail(dot)com>
Reviewed-by: Ewan Young <kdbase(dot)hack(at)gmail(dot)com>
Discussion: https://postgr.es/m/CAHewXNkvGTOgijRLjmudpg=wz0d-J8ChY2o7ksh3Be+Q_Bxwog@mail.gmail.com
Backpatch-through: 18

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/264ddc9a678c41fca3435f26d7c581fd2b034cf8

Modified Files
--------------
src/backend/optimizer/util/clauses.c | 127 +++++++++++++++++--------
src/test/regress/expected/collate.icu.utf8.out | 35 +++++++
src/test/regress/expected/subselect.out | 27 ++++++
src/test/regress/sql/collate.icu.utf8.sql | 12 +++
src/test/regress/sql/subselect.sql | 11 +++
5 files changed, 171 insertions(+), 41 deletions(-)