On Sat, Jan 9, 2016 at 11:02 AM, Tomas Vondra
<tomas.vondra@2ndquadrant.com> wrote:
> So, this seems to bring reasonable speedup, as long as the selectivity is
> below 50%, and the data set is sufficiently large.
What about semijoins? Apparently they can use bloom filters
particularly effectively. Have you considered them as a special case?
Also, have you considered Hash join conditions with multiple
attributes as a special case? I'm thinking of cases like this:
regression=# set enable_mergejoin = off;
SET
regression=# explain analyze select * from tenk1 o join tenk2 t on
o.twenty = t.twenty and t.hundred = o.hundred; QUERY PLAN
──────────────────────────────────────────────────────────────────────Hash Join (cost=595.00..4103.00 rows=50000
width=488)(actual
time=12.086..1026.194 rows=1000000 loops=1) Hash Cond: ((o.twenty = t.twenty) AND (o.hundred = t.hundred)) -> Seq
Scanon tenk1 o (cost=0.00..458.00 rows=10000 width=244)
(actual time=0.017..4.212 rows=10000 loops=1) -> Hash (cost=445.00..445.00 rows=10000 width=244) (actual
time=12.023..12.023 rows=10000 loops=1) Buckets: 16384 Batches: 1 Memory Usage: 2824kB -> Seq Scan on
tenk2t (cost=0.00..445.00 rows=10000
width=244) (actual time=0.006..3.453 rows=10000 loops=1)Planning time: 0.567 msExecution time: 1116.094 ms
(8 rows)
(Note that while the optimizer has a slight preference for a merge
join in this case, the plan I show here is a bit faster on my
machine).
--
Peter Geoghegan
