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If we flatten a subquery whose target list contains constants or expressions, when those output columns are used in GROUPING SET columns, the planner was capable of doing the wrong thing by merging a pulled-up expression into the surrounding expression during const-simplification. Then the late processing that attempts to match subexpressions to grouping sets would fail to match those subexpressions to grouping sets, with the effect that they'd not go to null when expected. To fix, wrap such subquery outputs in PlaceHolderVars, ensuring that they preserve their separate identity throughout the planner's expression processing. This is a bit of a band-aid, because the wrapper defeats const-simplification even in places where it would be safe to allow. But a nicer fix would likely be too invasive to back-patch, and the consequences of the missed optimizations probably aren't large in most cases. Back-patch to 9.5 where grouping sets were introduced. Heikki Linnakangas, with small mods and better test cases by me; additional review by Andrew Gierth Discussion: https://postgr.es/m/7dbdcf5c-b5a6-ef89-4958-da212fe10176@iki.fi
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/.