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Now that we've nailed down the principle that NullTest with !argisrow is fully equivalent to SQL's IS [NOT] DISTINCT FROM NULL, let's teach the parser about it. This produces a slightly more compact parse tree and is much more amenable to optimization than a DistinctExpr, since the planner knows a good deal about NullTest and next to nothing about DistinctExpr. I'm not sure that there are all that many queries in the wild that could be improved by this, but at least one source of such cases is the patch just made to postgres_fdw to emit IS [NOT] DISTINCT FROM NULL when IS [NOT] NULL isn't semantically correct. No back-patch, since to the extent that this does affect planning results, it might be considered undesirable plan destabilization.
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/.