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When adjusting the estimate for the number of distinct values from a rel in a grouped query to take into account the selectivity of the rel's restrictions, use a formula that is less likely to produce under-estimates. The old formula simply multiplied the number of distinct values in the rel by the restriction selectivity, which would be correct if the restrictions were fully correlated with the grouping expressions, but can produce significant under-estimates in cases where they are not well correlated. The new formula is based on the random selection probability, and so assumes that the restrictions are not correlated with the grouping expressions. This is guaranteed to produce larger estimates, and of course risks over-estimating in cases where the restrictions are correlated, but that has less severe consequences than under-estimating, which might lead to a HashAgg that consumes an excessive amount of memory. This could possibly be improved upon in the future by identifying correlated restrictions and using a hybrid of the old and new formulae. Author: Tomas Vondra, with some hacking be me Reviewed-by: Mark Dilger, Alexander Korotkov, Dean Rasheed and Tom Lane Discussion: http://www.postgresql.org/message-id/flat/56CD0381.5060502@2ndquadrant.com
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/.