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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@gmail.com> Author: Andrey Rachitskiy <pl0h0yp1@gmail.com> Reviewed-by: Tender Wang <tndrwang@gmail.com> Reviewed-by: Richard Guo <guofenglinux@gmail.com> Reviewed-by: Alexander Korotkov <aekorotkov@gmail.com> Discussion: https://postgr.es/m/19633-647cd4c73a84b085%40postgresql.org Backpatch-through: 14
PostgreSQL Database Management System ===================================== This directory contains the source code distribution of the PostgreSQL database management system. PostgreSQL is an advanced object-relational database management system that supports an extended subset of the SQL standard, including transactions, foreign keys, subqueries, triggers, user-defined types and functions. This distribution also contains C language bindings. PostgreSQL has many language interfaces, many of which are listed here: https://www.postgresql.org/download/ See the file INSTALL for instructions on how to build and install PostgreSQL. That file also lists supported operating systems and hardware platforms and contains information regarding any other software packages that are required to build or run the PostgreSQL system. Copyright and license information can be found in the file COPYRIGHT. A comprehensive documentation set is included in this distribution; it can be read as described in the installation instructions. The latest version of this software may be obtained at https://www.postgresql.org/download/. For more information look at our web site located at https://www.postgresql.org/.