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We should account for the per-group hashtable entry overhead when considering whether to use a hash aggregate to implement DISTINCT. The comparable logic in choose_hashed_grouping() gets this right, but I think I omitted it here in the mistaken belief that there would be no overhead if there were no aggregate functions to be evaluated. This can result in more than 2X underestimate of the hash table size, if the tuples being aggregated aren't very wide. Per report from Tomas Vondra. This bug is of long standing, but per discussion we'll only back-patch into 9.3. Changing the estimation behavior in stable branches seems to carry too much risk of destabilizing plan choices for already-tuned applications. |
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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/.