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For a partial path, the cardinality estimate needs to reflect the number of rows we think each worker will see, rather than the total number of rows; otherwise, costing will go wrong. The previous coding got this completely wrong for parallel joins. Unfortunately, this change may destabilize plans for users of 9.6 who have enabled parallel query, but since 9.6 is still fairly new I'm hoping expectations won't be too settled yet. Also, this is really a brown-paper-bag bug, so leaving it unfixed for the entire lifetime of 9.6 seems unwise. Related reports (whose import I initially failed to recognize) by Tomas Vondra and Tom Lane. Discussion: http://postgr.es/m/CA+TgmoaDxZ5z5Kw_oCQoymNxNoVaTCXzPaODcOuao=CzK8dMZw@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: http://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 http://www.postgresql.org/download/. For more information look at our web site located at http://www.postgresql.org/.