On Fri, Oct 27, 2017 at 12:49 AM, David Rowley <david.row...@2ndquadrant.com> wrote: > On 27 October 2017 at 01:44, Ashutosh Bapat > <ashutosh.ba...@enterprisedb.com> wrote: >> I think Antonin has a point. The join processing may deem some base >> relations dummy (See populate_joinrel_with_paths()). So an Append >> relation which had multiple child alive at the end of >> set_append_rel_size() might ultimately have only one child after >> partition-wise join has worked on it. > > hmm, I see. partition-wise join is able to remove eliminate partitions > on the other side of the join that can't be matched to: > > # set enable_partition_wise_join=on; > SET > # explain select count(*) from ab ab1 inner join ab ab2 on ab1.a=ab2.a > where ab1.a between 1 and 2 and ab2.a between 100000000 and 100000001; > QUERY PLAN > ------------------------------------------------ > Aggregate (cost=0.00..0.01 rows=1 width=8) > -> Result (cost=0.00..0.00 rows=0 width=0) > One-Time Filter: false > (3 rows) > > # \d+ ab > Table "public.ab" > Column | Type | Collation | Nullable | Default | Storage | Stats > target | Description > --------+---------+-----------+----------+---------+---------+--------------+------------- > a | integer | | | | plain | | > b | integer | | | | plain | | > Partition key: RANGE (a) > Partitions: ab_p1 FOR VALUES FROM (1) TO (100000000), > ab_p2 FOR VALUES FROM (100000000) TO (200000000) >
IIUC, ab1_p2 and ab2_p1 are marked dummy by constraint exclusion. Because of partition-wise join when the corresponding partitions are joined, their inner joins are marked dummy resulting in an empty join. This isn't the case of a join processing marking one of the joining relations dummy, that I was referring to. But I think it's relevant here since partition-wise join might create single child joins this way. -- Best Wishes, Ashutosh Bapat EnterpriseDB Corporation The Postgres Database Company -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
