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In constructs such as "x IN (1,2,3,4)" and "x <> ALL(ARRAY[1,2,3,4])", we formerly always used a general-purpose assumption that the probability of success is independent for each comparison of "x" to an array element. But in real-world usage of these constructs, that's a pretty poor assumption; it's much saner to assume that the array elements are distinct and so the match probabilities are disjoint. Apply that assumption if the operator appears to behave as equality (for ANY) or inequality (for ALL). But fall back to the normal independent-probabilities calculation if this yields an impossible result, ie probability > 1 or < 0. We could protect ourselves against bad estimates even more by explicitly checking for equal array elements, but that is expensive and doesn't seem worthwhile: doing it would amount to optimizing for poorly-written queries at the expense of well-written ones. Daniele Varrazzo and Tom Lane, after a suggestion by Ants Aasma |
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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/.