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As demonstrated by David Johnston, the Memoize cache hit ratio calculation wasn't quite correct. This change only affects the estimated hit ratio when the estimated number of entries to cache is estimated not to fit inside the cache. For example, if we expect 2000 distinct cache key values and only expect to be able to cache 1000 of those at once due to memory constraints, with an estimate of 10000 calls, if we could store all entries then the hit ratio should be 80% to account for the first 2000 of the 10000 calls to be a cache miss due to the value not being cached yet. If we can only store 1000 entries for each of the 2000 distinct possible values at once then the 80% should be reduced by half to make the final estimate of 40%. Previously, the calculation would have produced an estimated hit ratio of 30%, which wasn't correct. Apply to master only so as not to destabilize plans in the back branches. Reported-by: David G. Johnston Discussion: https://postgr.es/m/CAKFQuwZEmcNk3YQo2Xj4EDUOdY6qakad31rOD1Vc4q1_s68-Ew@mail.gmail.com Discussion: https://postgr.es/m/CAApHDvrV44LwiF4W_qf_RpbGYWSgp1kF=cZr+kTRRaALUfmXqw@mail.gmail.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: 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/.