On 24 June 2016 at 05:25, Tom Lane <t...@sss.pgh.pa.us> wrote: > Robert Haas <robertmh...@gmail.com> writes: >> Hmm, well I guess I would have to disagree with the idea that those >> other modes aren't useful. I seem to recall that David had some >> performance results showing that pushing partial aggregate steps below >> an Append node resulted in a noticeable speed-up even in the absence >> of any parallelism, presumably because it avoids whatever projection >> the Append might do, and also improves icache and dcache locality. > > I don't believe that for one second, because introducing another layer of > intermediate aggregation implies another projection step, plus all the > other overhead of a Plan node.
It's certainly not difficult to mock up a test to prove it is faster. create table t1 (a int not null); insert into t1 select generate_series(1,1000000); create table t2 (a int not null); insert into t2 select generate_series(1,1000000); select sum(c) from (select count(*) c from t1 union all select count(*) from t2) t; sum --------- 2000000 (1 row) Time: 82.038 ms select count(*) from (select * from t1 union all select * from t2) t; count --------- 2000000 (1 row) Time: 180.540 ms -- David Rowley http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Training & Services -- Sent via pgsql-hackers mailing list (firstname.lastname@example.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers