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IS NULL condition is rendered redundant by detection of an antijoin. If we know that a join is an antijoin, then *any* Var coming out of its righthand side must be NULL, not only the joining column(s). Also, it's still gonna be null after being passed up through higher joins, whether they're outer joins or not. I was misled by a faulty analogy to reduce_outer_joins() in the original coding. But consider select * from a left join b on a.x = b.y where b.y is null and b.z is null; The first IS NULL condition justifies deciding that the join is an antijoin (if the = is strict) and then the second one is just plain redundant. |
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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. Changes between all PostgreSQL releases are recorded in the file HISTORY. 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/.