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The header comment for get_cheapest_group_keys_order() claimed that the output arguments were set to a newly allocated list which may be freed by the calling function, however, this was not always true as the function would simply leave these arguments untouched in some cases. This tripped me up when working on 1349d2790 as I mistakenly assumed I could perform a list_concat with the output parameters. That turned out bad due to list_concat modifying the original input lists. In passing, make it more clear that the number of distinct values is important to reduce tiebreaks during sorts. Also, explain what the n_preordered parameter means. Backpatch-through: 15, where get_cheapest_group_keys_order was introduced.
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/.