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This prevents failures in cases where we pull up a constant or var-free expression from a subquery and put it into a full join's qual. That can result in not recognizing the qual as containing a mergejoin-able or hashjoin-able condition. A PHV prevents the problem because it is still recognized as belonging to the side of the join the subquery is in. I'm not very sure about the net effect of this change on plan quality. In "typical" cases where the join keys are Vars, nothing changes. In an affected case, the PHV-wrapped expression is less likely to be seen as equal to PHV-less instances below the join, but more likely to be seen as equal to similar expressions above the join, so it may end up being a wash. In the one existing case where there's any visible change in a regression-test plan, it amounts to referencing a lower computation of a COALESCE result instead of recomputing it, which seems like a win. Given my uncertainty about that and the lack of field complaints, no back-patch, even though this is a very ancient problem. Discussion: https://postgr.es/m/32090.1539378124@sss.pgh.pa.us |
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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/.