Haas: Parallel Query v2

Robert Haas describesthe many parallelism enhancements in the upcoming PostgreSQL 10release. "The Gather node introduced in PostgreSQL 9.6 gathersresults from all workers in an arbitrary order. That's fine if the datathat the workers were producing had no particular ordering anyway, but ifeach worker is producing sorted output, then it would be nice to gatherthose results in a way that preserves the sort order. This is what GatherMerge does. It can speed up queries where it's useful for the results ofthe parallel portion of the plan to have a particular sort order, and wherethe parallel portion of the plan produces enough rows that performing anordinary Gather followed by a Sort would be expensive."