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Back in commit 400e2c934457bef4bc3cc9a3e49b6289bd761bc0 I rewrote GEQO's gimme_tree function to improve its heuristic for modifying the given tour into a legal join order. In what can only be called a fit of hubris, I supposed that this new heuristic would *always* find a legal join order, and ripped out the old logic that allowed gimme_tree to sometimes fail. The folly of this is exposed by bug #12760, in which the "greedy" clumping behavior of merge_clump() can lead it into a dead end which could only be recovered from by un-clumping. We have no code for that and wouldn't know exactly what to do with it if we did. Rather than try to improve the heuristic rules still further, let's just recognize that it *is* a heuristic and probably must always have failure cases. So, put back the code removed in the previous commit to allow for failure (but comment it a bit better this time). It's possible that this code was actually fully correct at the time and has only been broken by the introduction of LATERAL. But having seen this example I no longer have much faith in that proposition, so back-patch to all supported branches.
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: http://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 http://www.postgresql.org/download/. For more information look at our web site located at http://www.postgresql.org/.