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The planner generally expects that the estimated rowcount of any relation is at least one row, *unless* it has been proven empty by constraint exclusion or similar mechanisms, which is marked by installing a dummy path as the rel's cheapest path (cf. IS_DUMMY_REL). When I split up allpaths.c's processing of base rels into separate set_base_rel_sizes and set_base_rel_pathlists steps, the intention was that dummy rels would get marked as such during the "set size" step; this is what justifies an Assert in indxpath.c's get_loop_count that other relations should either be dummy or have positive rowcount. Unfortunately I didn't get that quite right for append relations: if all the child rels have been proven empty then set_append_rel_size would come up with a rowcount of zero, which is correct, but it didn't then do set_dummy_rel_pathlist. (We would have ended up with the right state after set_append_rel_pathlist, but that's too late, if we generate indexpaths for some other rel first.) In addition to fixing the actual bug, I installed an Assert enforcing this convention in set_rel_size; that then allows simplification of a couple of now-redundant tests for zero rowcount in set_append_rel_size. Also, to cover the possibility that third-party FDWs have been careless about not returning a zero rowcount estimate, apply clamp_row_est to whatever an FDW comes up with as the rows estimate. Per report from Andreas Seltenreich. Back-patch to 9.2. Earlier branches did not have the separation between set_base_rel_sizes and set_base_rel_pathlists steps, so there was no intermediate state where an appendrel would have had inconsistent rowcount and pathlist. It's possible that adding the Assert to set_rel_size would be a good idea in older branches too; but since they're not under development any more, it's likely not worth the trouble. |
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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: 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/.