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This was not something that required consideration before MERGE was invented; but MERGE builds a join tree that left-joins to the result relation, meaning that remove_useless_joins will consider removing it. That should generally be stopped by the query's use of output variables from the result relation. However, if the result relation is inherited (e.g. a partitioned table) then we don't add any row identity variables to the query until expand_inherited_rtentry, which happens after join removal. This was exposed as of commit 3c569049b, which made it possible to deduce that a partitioned table could contain at most one row matching a join key, enabling removal of the not-yet-expanded result relation. Ooops. To fix, let's just teach join_is_removable that the query result rel is never removable. It's a cheap enough test in any case, and it'll save some cycles that we'd otherwise expend in proving that it's not removable, even in the cases we got right. Back-patch to v15 where MERGE was added. Although I think the case cannot be reached in v15, this seems like cheap insurance. Per investigation of a report from Alexander Lakhin. Discussion: https://postgr.es/m/36bee393-b351-16ac-93b2-d46d83637e45@gmail.com
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/.