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So far the hashtable stored representative tuples in the form of its input slot, with all columns in the hashtable that are not needed (i.e. not grouped upon or functionally dependent) set to NULL. Thats good for saving memory, but it turns out that having tuples full of NULL isn't free. slot_deform_tuple is faster if there's no NULL bitmap even if no NULLs are encountered, and skipping over leading NULLs isn't free. So compute a separate tuple descriptor that only contains the needed columns. As columns have already been moved in/out the slot for the hashtable that does not imply additional per-row overhead. Author: Andres Freund Reviewed-By: Heikki Linnakangas Discussion: https://postgr.es/m/20161103110721.h5i5t5saxfk5eeik@alap3.anarazel.de
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/.