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get_foreign_key_join_selectivity() looks for join clauses that equate the two sides of the FK constraint. However, if we have a query like "WHERE fktab.a = pktab.a and fktab.a = 1", it won't find any such join clause, because equivclass.c replaces the given clauses with "fktab.a = 1 and pktab.a = 1", which can be enforced at the scan level, leaving nothing to be done for column "a" at the join level. We can fix that expectation without much trouble, but then a new problem arises: applying the foreign-key-based selectivity rule produces a rowcount underestimate, because we're effectively double-counting the selectivity of the "fktab.a = 1" clause. So we have to cancel that selectivity out of the estimate. To fix, refactor process_implied_equality() so that it can pass back the new RestrictInfo to its callers in equivclass.c, allowing the generated "fktab.a = 1" clause to be saved in the EquivalenceClass's ec_derives list. Then it's not much trouble to dig out the relevant RestrictInfo when we need to adjust an FK selectivity estimate. (While at it, we can also remove the expensive use of initialize_mergeclause_eclasses() to set up the new RestrictInfo's left_ec and right_ec pointers. The equivclass.c code can set those basically for free.) This seems like clearly a bug fix, but I'm hesitant to back-patch it, first because there's some API/ABI risk for extensions and second because we're usually loath to destabilize plan choices in stable branches. Per report from Sigrid Ehrenreich. Discussion: https://postgr.es/m/1019549.1603770457@sss.pgh.pa.us Discussion: https://postgr.es/m/AM6PR02MB5287A0ADD936C1FA80973E72AB190@AM6PR02MB5287.eurprd02.prod.outlook.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/.