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When building multi-column MCV lists, we compute base frequency for each item, i.e. a product of per-column frequencies for values from the item. As a value may be in multiple groups, the code was scanning the whole array of groups while adding items to the MCV list. This works fine as long as the number of distinct groups is small, but it's easy to trigger trigger O(N^2) behavior, especially after increasing statistics target. This commit precomputes frequencies for values in all columns, so that when computing the base frequency it's enough to make a simple bsearch lookup in the array. Backpatch to 12, where multi-column MCV lists were introduced. Discussion: https://postgr.es/m/20190618205920.qtlzcu73whfpfqne@development |
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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: https://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 https://www.postgresql.org/download/. For more information look at our web site located at https://www.postgresql.org/.