The first set of DMF sort enhancements also involve fine-tuning of the sort algorithms, which should result in a 5 to 10 percent reduction in CPU time of typical sorts. As with the QEF sort, duplicate rows are detected and discarded sooner in duplicates removal sorts. This should result in smaller disk work files and faster overall sort performance.

Prior to Release 2.5, the entire result of a DMF sort was spooled to an internal temporary table before the sorted rows were returned to the caller. In Ingres II 2.5, the temporary table has been eliminated and the rows are returned directly from the sort structures to the caller. This has the same effect as the early return of sorted rows described above for the QEF sort. That is, the first rows should be returned much sooner than they were in previous releases.

The final DMF sort enhancement is the introduction of a “parallel sort” technique. Sorts that exceed a user-configurable threshold spawn additional threads. The sort is split up and its rows delivered to the sub-threads for sorting. The sorted subsets of the rows are then delivered back to the parent thread executing the query, where they are merged to form a single sorted stream of rows.

On multi-CPU machines, this results in a significant reduction in the elapsed time required to sort (between 25 to 50 percent in testing). Even single CPU machines benefit somewhat, because sort I/O and sort computation can be overlapped. An added benefit to the parallel sort technique is that it is encapsulated within the DMF sort. This sort is used for the execution of queries with sorting requirements (such as for order by, group by, and distinct requests, or for implementing certain join algorithms). However, it is also used to sort rows for index creation or update in modify, create index, and copy operations. All users of the DMF sort derive the performance benefit of the parallel sort.