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Previously, this function estimated the selectivity as 1 minus eqjoinsel() for the negator equality operator, regardless of join type (I think there was an expectation that eqjoinsel would handle the join type). But actually this is completely wrong for semijoin cases: the fraction of the LHS that has a non-matching row is not one minus the fraction of the LHS that has a matching row. In reality a semijoin with <> will nearly always succeed: it can only fail when the RHS is empty, or it contains a single distinct value that is equal to the particular LHS value, or the LHS value is null. The only one of those things we should have much confidence in estimating is the fraction of LHS values that are null, so let's just take the selectivity as 1 minus outer nullfrac. Per coding convention, antijoin should be estimated the same as semijoin. Arguably this is a bug fix, but in view of the lack of field complaints and the risk of destabilizing plans, no back-patch. Thomas Munro, reviewed by Ashutosh Bapat Discussion: https://postgr.es/m/CAEepm=270ze2hVxWkJw-5eKzc3AB4C9KpH3L2kih75R5pdSogg@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. 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/.