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When evaluating a query with a multi-column GROUP BY clause, we can minimize sort operations or avoid them if we synchronize the order of GROUP BY clauses with the ORDER BY sort clause or sort order, which comes from the underlying query tree. Grouping does not imply any ordering, so we can compare the keys in arbitrary order, and a Hash Agg leverages this. But for Group Agg, we simply compared keys in the order specified in the query. This commit explores alternative ordering of the keys, trying to find a cheaper one. The ordering of group keys may interact with other parts of the query, some of which may not be known while planning the grouping. For example, there may be an explicit ORDER BY clause or some other ordering-dependent operation higher up in the query, and using the same ordering may allow using either incremental sort or even eliminating the sort entirely. The patch always keeps the ordering specified in the query, assuming the user might have additional insights. This introduces a new GUC enable_group_by_reordering so that the optimization may be disabled if needed. Discussion: https://postgr.es/m/7c79e6a5-8597-74e8-0671-1c39d124c9d6%40sigaev.ru Author: Andrei Lepikhov, Teodor Sigaev Reviewed-by: Tomas Vondra, Claudio Freire, Gavin Flower, Dmitry Dolgov Reviewed-by: Robert Haas, Pavel Borisov, David Rowley, Zhihong Yu Reviewed-by: Tom Lane, Alexander Korotkov, Richard Guo, Alena Rybakina
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. Copyright and license information can be found in the file COPYRIGHT. General documentation about this version of PostgreSQL can be found at: https://www.postgresql.org/docs/devel/ In particular, information about building PostgreSQL from the source code can be found at: https://www.postgresql.org/docs/devel/installation.html The latest version of this software, and related software, may be obtained at https://www.postgresql.org/download/. For more information look at our web site located at https://www.postgresql.org/.