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query_is_distinct_for() treated a subquery's DISTINCT ON clause as proof that its output is unique over the DISTINCT ON columns, even if the targetlist contains set-returning functions. That's not true: when the query has an ORDER BY, the planner postpones evaluation of SRFs that are not DISTINCT ON or ORDER BY columns until after the Unique step, so the subquery can produce duplicates of the DISTINCT ON columns. Relying on this bogus uniqueness proof allowed join removal and unique-inner joins to produce wrong results. Plain DISTINCT is not affected, since all tlist columns are DISTINCT columns there, and so any SRFs get expanded before the Unique step. To fix, make query_supports_distinctness() and query_is_distinct_for() refuse to prove distinctness via DISTINCT ON if the targetlist contains any SRFs. This is more conservative than necessary, since the SRFs are only postponed when there is an ORDER BY and none of them appear in a sort/group column, but it doesn't seem worth the trouble to check that precisely. Author: Richard Guo <guofenglinux@gmail.com> Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us> Discussion: https://postgr.es/m/CAMbWs4-hfd1Pyy_zBejsVUSy-3dx16rz2hgUakkKnAg3qg2q=Q@mail.gmail.com 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/.