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btcostestimate() makes an estimate of the number of index tuples that will be visited based on knowledge of which index clauses can actually bound the scan within nbtree. However, it forgot to account for partial indexes in this calculation, with the result that the cost of the index scan could be significantly overestimated for a partial index. Fix that by merging the predicate with the abbreviated indexclause list, in the same way as we do with the full list to estimate how many heap tuples will be visited. Also, slightly increase the "fudge factor" that's meant to give preference to smaller indexes over larger ones. While this is applied to all indexes, it's most important for partial indexes since it can be the only factor that makes a partial index look cheaper than a similar full index. Experimentation shows that the existing value is so small as to easily get swamped by noise such as page-boundary-roundoff behavior. I'm tempted to kick it up more than this, but will refrain for now. Per report from Ruben Blanco. These are long-standing issues, but given the lack of prior complaints I'm not going to risk changing planner behavior in back branches by back-patching. |
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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. Changes between all PostgreSQL releases are recorded in the file HISTORY. 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/.