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We should never estimate the output of a semijoin to be more rows than we estimate for an inner join with the same input rels and join condition; it's obviously impossible for that to happen. However, given the relatively poor quality of our semijoin selectivity estimates --- particularly, but not only, in cases where we punt and return a default estimate --- we did often deliver such estimates. To improve matters, calculate both estimates inside eqjoinsel() and take the smaller one. The bulk of this patch is just mechanical refactoring to avoid repetitive information lookup when we call both eqjoinsel_semi and eqjoinsel_inner. The actual new behavior is just selec = Min(selec, inner_rel->rows * selec_inner); which looks a bit odd but is correct because of our different definitions for inner and semi join selectivity. There is one ensuing plan change in the regression tests, but it looks reasonable enough (and checking the actual row counts shows that the estimate moved closer to reality, not further away). Per bug #15160 from Alexey Ermakov. Although this is arguably a bug fix, I won't risk destabilizing plan choices in stable branches by back-patching. Tom Lane, reviewed by Melanie Plageman Discussion: https://postgr.es/m/152395805004.19366.3107109716821067806@wrigleys.postgresql.org |
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README
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/.