pgsql: Use the join collation when unique-ifying a semijoin's RHS

Use the join collation when unique-ifying a semijoin's RHS

A semijoin whose RHS is unique-ified groups the RHS on the expressions in
SpecialJoinInfo.semi_rhs_exprs. Those were recorded with whatever collation
the RHS expression itself exposes, which need not be the collation the join
compares with. Neither SortGroupClause nor the pathkey machinery carries a
collation of its own, so both Unique-over-Sort and HashAggregate then grouped
by the wrong equality: values the join considers equal survived, and the
following inner join emitted the outer row once per survivor.

With a non-deterministic collation on one side, "SELECT count(*) FROM t WHERE
c IN (SELECT c0 FROM t2)" therefore counted more rows than the same predicate
reports for the rows of t.

Label each RHS expression with the operator's input collation, the same
treatment process_equivalence() gives to equivalence class members. Every
consumer of semi_rhs_exprs reads the collation off the expression, so this
fixes the sort-based and hash-based paths together; in the branches where
create_unique_path() also passes these expressions to
relation_has_unique_index_for(), it likewise stops a unique index built with a
different collation from being taken as proof that unique-ification can be
skipped. The same goes for a subquery's DISTINCT computed under a different
collation, since translate_sub_tlist() punts on the relabeled expressions.

Reported-by: Suyang Zhong <syzhong16(at)gmail(dot)com>
Author: Andrey Rachitskiy <pl0h0yp1(at)gmail(dot)com>
Reviewed-by: Tender Wang <tndrwang(at)gmail(dot)com>
Reviewed-by: Richard Guo <guofenglinux(at)gmail(dot)com>
Reviewed-by: Alexander Korotkov <aekorotkov(at)gmail(dot)com>
Discussion: https://postgr.es/m/19633-647cd4c73a84b085%40postgresql.org
Backpatch-through: 14

Branch
------
REL_14_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/b8e0b45abb61ec42de9cb702041902d690629f6d

Modified Files
--------------
src/backend/optimizer/plan/initsplan.c | 11 +++++-
src/test/regress/expected/collate.icu.utf8.out | 54 ++++++++++++++++++++++++++
src/test/regress/sql/collate.icu.utf8.sql | 27 +++++++++++++
3 files changed, 91 insertions(+), 1 deletion(-)