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In the dim past, the planner kept the fully-processed version of the query targetlist (the result of preprocess_targetlist) in grouping_planner's local variable "tlist", and only grudgingly passed it to individual other routines as needed. Later we discovered a need to still have it available after grouping_planner finishes, and invented the root->processed_tlist field for that purpose, but it wasn't used internally to grouping_planner; the tlist was still being passed around separately in the same places as before. Now comes a proposed patch to allow appendrel expansion to add entries to the processed tlist, well after preprocess_targetlist has finished its work. To avoid having to pass around the tlist explicitly, it's proposed to allow appendrel expansion to modify root->processed_tlist. That makes aliasing the tlist with assorted parameters and local variables really scary. It would accidentally work as long as the tlist is initially nonempty, because then the List header won't move around, but it's not exactly hard to think of ways for that to break. Aliased values are poor programming practice anyway. Hence, get rid of local variables and parameters that can be identified with root->processed_tlist, in favor of just using that field directly. And adjust comments to match. (Some of the new comments speak as though it's already possible for appendrel expansion to modify the tlist; that's not true yet, but will happen in a later patch.) Discussion: https://postgr.es/m/9d7c5112-cb99-6a47-d3be-cf1ee6862a1d@lab.ntt.co.jp
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/.