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Normally, if a WHERE clause is implied by the predicate of a partial index, we drop that clause from the set of quals used with the index, since it's redundant to test it if we're scanning that index. However, if it's a hash index (or any !amoptionalkey index), this could result in dropping all available quals for the index's first key, preventing us from generating an indexscan. It's fair to question the practical usefulness of this case. Since hash only supports equality quals, the situation could only arise if the index's predicate is "WHERE indexkey = constant", implying that the index contains only one hash value, which would make hash a really poor choice of index type. However, perhaps there are other !amoptionalkey index AMs out there with which such cases are more plausible. To fix, just don't filter the candidate indexquals this way if the index is !amoptionalkey. That's a bit hokey because it may result in testing quals we didn't need to test, but to do it more accurately we'd have to redundantly identify which candidate quals are actually usable with the index, something we don't know at this early stage of planning. Doesn't seem worth the effort. Reported-by: Sergei Glukhov <s.glukhov@postgrespro.ru> Author: Tom Lane <tgl@sss.pgh.pa.us> Reviewed-by: David Rowley <dgrowleyml@gmail.com> Discussion: https://postgr.es/m/e200bf38-6b45-446a-83fd-48617211feff@postgrespro.ru Backpatch-through: 14
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/.