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add_path_precheck was doing exact comparisons of path costs, but it really needs to do them fuzzily to be sure it won't reject paths that could survive add_path's comparisons. (This can only matter if the initial cost estimate is very close to the final one, but that turns out to often be true.) Also, it should ignore startup cost for this purpose if and only if compare_path_costs_fuzzily would do so. The previous coding always ignored startup cost for parameterized paths, which is wrong as of commit 3f59be836c555fa6; it could result in improper early rejection of paths that we care about for SEMI/ANTI joins. It also always considered startup cost for unparameterized paths, which is just as wrong though the only effect is to waste planner cycles on paths that can't survive. Instead, it should consider startup cost only when directed to by the consider_startup/ consider_param_startup relation flags. Likewise, compare_path_costs_fuzzily should have symmetrical behavior for parameterized and unparameterized paths. In this case, the best answer seems to be that after establishing that total costs are fuzzily equal, we should compare startup costs whether or not the consider_xxx flags are on. That is what it's always done for unparameterized paths, so let's make the behavior for parameterized paths match. These issues were noted while developing the SEMI/ANTI join costing fix of commit 3f59be836c555fa6, but we chose not to back-patch these fixes, because they can cause changes in the planner's choices among nearly-same-cost plans. (There is in fact one minor change in plan choice within the core regression tests.) Destabilizing plan choices in back branches without very clear improvements is frowned on, so we'll just fix this in HEAD.
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/.