On Wed, Aug 30, 2017 at 3:01 PM, Robert Haas <robertmh...@gmail.com> wrote: > 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.
That's not hard. On my laptop: $ pgbench -i -s 100 ... postgres=# set work_mem = '2MB'; SET postgres=# show replacement_sort_tuples ; replacement_sort_tuples ───────────────────────── 150000 (1 row) (30784) /postgres=# select count(distinct aid) from pgbench_accounts ; count ──────────── 10,000,000 (1 row) Time: 4157.267 ms (00:04.157) (30784) /postgres=# set replacement_sort_tuples = 0; SET (30784) /postgres=# select count(distinct aid) from pgbench_accounts ; count ──────────── 10,000,000 (1 row) Time: 3343.789 ms (00:03.344) This is significantly faster, in a way that's clearly reproducible and consistent, despite the fact that we need about 10 merge passes without replacement selection, and only have enough memory for 7 tapes. I think that I could find a case that makes replacement selection look much worse, if I tried. -- Peter Geoghegan -- Sent via pgsql-hackers mailing list (email@example.com) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers