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When either inputs of an INTERSECT [ALL] operator are proven not to return any results (a dummy rel), then mark the entire INTERSECT operation as dummy. Likewise, if an EXCEPT [ALL] operation's left input is proven empty, then mark the entire operation as dummy. With EXCEPT ALL, we can easily handle the right input being dummy as we can return the left input without any processing. That can lead to significant performance gains during query execution. We can't easily handle dummy right inputs for EXCEPT (without ALL), as that would require deduplication of the left input. Wiring up those Paths is likely more complex than it's worth as the gains during execution aren't that great, so let's leave that one to be handled by the normal Path generation code. 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/.