On Wed, Aug 30, 2017 at 4:18 PM, Peter Geoghegan <p...@bowt.ie> wrote: > On Wed, Aug 30, 2017 at 12:51 PM, Robert Haas <robertmh...@gmail.com> wrote: >> On Fri, Jul 14, 2017 at 6:20 PM, Peter Geoghegan <p...@bowt.ie> wrote: >>> With the additional enhancements made to Postgres 10, I doubt that >>> there are any remaining cases where it wins. >> >> The thing to do about that would be to come up with some cases where >> someone might plausibly think it would win and benchmark them to find >> out what happens. I find it really hard to believe that sorting a >> long presorted stream of tuples (or, say, 2-1-4-3-6-5-8-7-10-9 etc.) >> is ever going to be as fast with any other algorithm as it is with >> replacement selection. > > Replacement selection as implemented in Postgres is supposed to be > about the "single run, no merge" best case. This must use > TSS_SORTEDONTAPE processing, which is optimized for random access, > which is usually the wrong thing. > > In general, sorting is only one cost that is involved here, and is not > the predominant cost with presorted input.
That may all be true, but my point is that if it wins in some cases, we should keep it -- and proving it no longer wins in those cases will require running tests. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company -- Sent via pgsql-hackers mailing list (email@example.com) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers