Ingres 9.2 Migration Guide - Actian

Sort EnhancementsDMF Sort EnhancementsThe first set of DMF sort enhancements also involve fine-tuning of the sortalgorithms, which should result in a 5 to 10 percent reduction in CPU time oftypical sorts. As with the QEF sort, duplicate rows are detected and discardedsooner in duplicates removal sorts. This should result in smaller disk work filesand faster overall sort performance.Prior to Release 2.5, the entire result of a DMF sort was spooled to an internaltemporary table before the sorted rows were returned to the caller. In IngresII 2.5, the temporary table has been eliminated and the rows are returneddirectly from the sort structures to the caller. This has the same effect as theearly return of sorted rows described above for the QEF sort. That is, the firstrows 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 additionalthreads. The sort is split up and its rows delivered to the sub-threads forsorting. The sorted subsets of the rows are then delivered back to the parentthread executing the query, where they are merged to form a single sortedstream of rows.On multi-CPU machines, this results in a significant reduction in the elapsedtime required to sort (between 25 to 50 percent in testing). Even single CPUmachines benefit somewhat, because sort I/O and sort computation can beoverlapped. An added benefit to the parallel sort technique is that it isencapsulated within the DMF sort. This sort is used for the execution of querieswith sorting requirements (such as for order by, group by, and distinctrequests, or for implementing certain join algorithms). However, it is also usedto sort rows for index creation or update in modify, create index, and copyoperations. All users of the DMF sort derive the performance benefit of theparallel sort.Parallel Sort TechniquesThe “parallel sort” technique outlined in DMF Sort Enhancements (seepage 209) is used to sort rows for parallel index creation, greatly reducing thetime taken for index creation in multi-CPU environments.Features Introduced in Ingres II 2.5 209