03d40e4b52
This adjusts UNION planning so that the planner produces more optimal plans when one or more of the UNION's subqueries have been proven to be empty (a dummy rel). If any of the inputs are empty, then that input can be removed from the Append / MergeAppend. Previously, a const-false "Result" node would appear to represent this. Removing empty inputs has a few extra benefits when only 1 union child remains as it means the Append or MergeAppend can be removed in setrefs.c, making the plan slightly faster to execute. Also, we can provide better n_distinct estimates by looking at the sole remaining input rel's statistics. Author: David Rowley <dgrowleyml@gmail.com> Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us> Discussion: https://postgr.es/m/CAApHDvri53PPF76c3M94_QNWbJfXjyCnjXuj_2=LYM-0m8WZtw@mail.gmail.com
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/.