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as methods for implementing the DISTINCT step. This eliminates the former performance gap between DISTINCT and GROUP BY, and also makes it possible to do SELECT DISTINCT on datatypes that only support hashing not sorting. SELECT DISTINCT ON is still always implemented by sorting; it would take executor changes to support hashing that, and it's not clear it's worth the trouble. This is a release-note-worthy incompatibility from previous PG versions, since SELECT DISTINCT can no longer be counted on to deliver sorted output without explicitly saying ORDER BY. (Anyone who can't cope with that can consider turning off enable_hashagg.) Several regression test queries needed to have ORDER BY added to preserve stable output order. I fixed the ones that manifested here, but there might be some other cases that show up on other platforms.
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/.