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In the dim past we figured it was okay to ignore collations when combining UNION set-operation nodes into a single N-way UNION operation. I believe that was fine at the time, but it stopped being fine when we added nondeterministic collations: the semantics of distinct-ness are affected by those. v17 made it even less fine by allowing per-child sorting operations to be merged via MergeAppend, although I think we accidentally avoided any live bug from that. Add a check that collations match before deciding that two UNION nodes are equivalent. I also failed to resist the temptation to comment plan_union_children() a little better. Back-patch to all supported branches (v13 now), since they all have nondeterministic collations. Discussion: https://postgr.es/m/3605568.1731970579@sss.pgh.pa.us
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/.