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competing alternatives for indexes to use in a bitmap scan. The former coding took estimated selectivity as an overriding factor, causing it to sometimes choose indexes that were much slower to scan than ones with a slightly worse selectivity. It was also too narrow-minded about which combinations of indexes to consider ANDing. The rewrite makes it pay more attention to index scan cost than selectivity; this seems sane since it's impossible to have very bad selectivity with low cost, whereas the reverse isn't true. Also, we now consider each index alone, as well as adding each index to an AND-group led by each prior index, for a total of about O(N^2) rather than O(N) combinations considered. This makes the results much less dependent on the exact order in which the indexes are considered. It's still a lot cheaper than an O(2^N) exhaustive search. A prefilter step eliminates all but the cheapest of those indexes using the same set of WHERE conditions, to keep the effective value of N down in scenarios where the DBA has created lots of partially-redundant indexes.
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 including some of the more common listed below: C++ - http://thaiopensource.org/development/libpqxx/ JDBC - http://jdbc.postgresql.org ODBC - http://odbc.postgresql.org Perl - http://search.cpan.org/~dbdpg/ PHP - http://www.php.net Python - http://www.initd.org/ Ruby - http://ruby.scripting.ca/postgres/ Other language binding are available from a variety of contributing parties. PostgreSQL also has a great number of procedural languages available, a short but not complete list is below: PL/pgSQL - included in PostgreSQL source distribution PL/Perl - included in PostgreSQL source distribution PL/PHP - http://projects.commandprompt.com/projects/public/plphp PL/Python - included in PostgreSQL source distribution PL/Java - http://gborg.postgresql.org/project/pljava/projdisplay.php PL/Tcl - included in PostgreSQL source distribution 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/.