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These estimators returned 1 minus the corresponding equality/match estimate, which is incorrect: we need to subtract off the fraction of nulls in the column, since those are neither equal nor not equal to the comparison value. The error only becomes obvious if the nullfrac is large, but it could be very bad in a mostly-nulls column, as reported in bug #14676 from Marko Tiikkaja. To fix the <> case, refactor eqsel() and neqsel() to call a common support routine, which can be made to account for nullfrac correctly. The pattern-match cases were already factored that way, and it was simply an oversight that patternsel() wasn't subtracting off nullfrac. neqjoinsel() has a similar problem, but since we're elsewhere discussing changing its behavior entirely, I left it alone for now. This is a very longstanding bug, but I'm hesitant to back-patch a fix for it. Given the lack of prior complaints, such cases must not come up often, so it's probably not worth the risk of destabilizing plans in stable branches. Discussion: https://postgr.es/m/20170529153847.4275.95416@wrigleys.postgresql.org |
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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/.