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This is advantageous first because it allows us to hash the smaller table regardless of the outer-join type, and second because hash join can be more flexible than merge join in dealing with arbitrary join quals in a FULL join. For merge join all the join quals have to be mergejoinable, but hash join will work so long as there's at least one hashjoinable qual --- the others can be any condition. (This is true essentially because we don't keep per-inner-tuple match flags in merge join, while hash join can do so.) To do this, we need a has-it-been-matched flag for each tuple in the hashtable, not just one for the current outer tuple. The key idea that makes this practical is that we can store the match flag in the tuple's infomask, since there are lots of bits there that are of no interest for a MinimalTuple. So we aren't increasing the size of the hashtable at all for the feature. To write this without turning the hash code into even more of a pile of spaghetti than it already was, I rewrote ExecHashJoin in a state-machine style, similar to ExecMergeJoin. Other than that decision, it was pretty straightforward.
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/.