On Wed, Aug 30, 2017 at 6:14 PM, Peter Geoghegan <p...@bowt.ie> wrote: > 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.
Wow. Just to be clear, I am looking for the BEST case for replacement selection, not the worst case. But I would have expected that case to be a win for replacement selection, and it clearly isn't. I can reproduce your results here. -- 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