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A WHERE clause applied to the output of a subquery with DISTINCT should theoretically be applied only once per distinct row; but if we push it into the subquery then it will be evaluated at each row before duplicate elimination occurs. If the qual is volatile this can give rise to observably wrong results, so don't do that. While at it, refactor a little bit to allow subquery_is_pushdown_safe to report more than one kind of restrictive condition without indefinitely expanding its argument list. Although this is a bug fix, it seems unwise to back-patch it into released branches, since it might de-optimize plans for queries that aren't giving any trouble in practice. So apply to 9.4 but not further back.
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/.