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Formerly we applied a functional dependency "a => b with dependency degree f" using the formula P(a,b) = P(a) * [f + (1-f)*P(b)] This leads to the possibility that the combined selectivity P(a,b) could exceed P(b), which is not ideal. The addition of support for IN and OR clauses (commits 8f321bd16c and ccaa3569f5) would seem to make this more likely, since the user-supplied values in such clauses are not necessarily compatible with the functional dependency. Mitigate this by using the formula P(a,b) = f * Min(P(a), P(b)) + (1-f) * P(a) * P(b) instead, which guarantees that the combined selectivity is less than each column's individual selectivity. Logically, this is modifies the part of the formula that accounts for dependent rows to handle cases where P(a) > P(b), whilst not changing the second term which accounts for independent rows. Additionally, this refactors the way that functional dependencies are applied, so now dependencies_clauselist_selectivity() estimates both the implying clauses and the implied clauses for each functional dependency (formerly only the implied clauses were estimated), and now all clauses for each attribute are taken into account (formerly only one clause for each implied attribute was estimated). This removes the previously built-in assumption that only equality clauses will be seen, which is no longer true, and opens up the possibility of applying functional dependencies to more general clauses. Patch by me, reviewed by Tomas Vondra. Discussion: https://postgr.es/m/CAEZATCXaNFZyOhR4XXAfkvj1tibRBEjje6ZbXwqWUB_tqbH%3Drw%40mail.gmail.com Discussion: https://postgr.es/m/20200318002946.6dvblukm3cfmgir2%40development
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/.