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In order to estimate the cache hit ratio of a Memoize node, one of the inputs we require is the estimated number of times the Memoize node will be rescanned. The higher this number, the large the cache hit ratio is likely to become. Unfortunately, the value being passed as the number of "calls" to the Memoize was incorrectly using the Nested Loop's outer_path->parent->rows instead of outer_path->rows. This failed to account for the fact that the outer_path might be parameterized by some upper-level Nested Loop. This problem could lead to Memoize plans appearing more favorable than they might actually be. It could also lead to extended executor startup times when work_mem values were large due to the planner setting overly large MemoizePath->est_entries resulting in the Memoize hash table being initially made much larger than might be required. Fix this simply by passing outer_path->rows rather than outer_path->parent->rows. Also, adjust the expected regression test output for a plan change. Reported-by: Pavel Stehule Author: David Rowley Discussion: https://postgr.es/m/CAFj8pRAMp%3DQsMi6sPQJ4W3hczoFJRvyXHJV3AZAZaMyTVM312Q%40mail.gmail.com Backpatch-through: 14, where Memoize was introduced |
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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/.