Hi when I was in talk with Silvio Moioli, I found strange hash join. Hash was created from bigger table.
https://www.postgresql.org/message-id/79dd683d-3296-1b21-ab4a-28fdc2d98807%40suse.de Now it looks so materialized CTE disallow hash create table bigger(a int); create table smaller(a int); insert into bigger select random()* 10000 from generate_series(1,100000); insert into smaller select i from generate_series(1,100000) g(i); analyze bigger, smaller; -- no problem explain analyze select * from bigger b join smaller s on b.a = s.a; postgres=# explain analyze select * from bigger b join smaller s on b.a = s.a; QUERY PLAN ---------------------------------------------------------------------------------------------------------------------------- Hash Join (cost=3084.00..7075.00 rows=100000 width=8) (actual time=32.937..87.276 rows=99994 loops=1) Hash Cond: (b.a = s.a) -> Seq Scan on bigger b (cost=0.00..1443.00 rows=100000 width=4) (actual time=0.028..8.546 rows=100000 loops=1) -> Hash (cost=1443.00..1443.00 rows=100000 width=4) (actual time=32.423..32.423 rows=100000 loops=1) Buckets: 131072 Batches: 2 Memory Usage: 2785kB -> Seq Scan on smaller s (cost=0.00..1443.00 rows=100000 width=4) (actual time=0.025..9.931 rows=100000 loops=1) Planning Time: 0.438 ms Execution Time: 91.193 ms (8 rows) but with materialized CTE postgres=# explain analyze with b as materialized (select * from bigger), s as materialized (select * from smaller) select * from b join s on b.a = s.a; QUERY PLAN ---------------------------------------------------------------------------------------------------------------------- Merge Join (cost=23495.64..773995.64 rows=50000000 width=8) (actual time=141.242..193.375 rows=99994 loops=1) Merge Cond: (b.a = s.a) CTE b -> Seq Scan on bigger (cost=0.00..1443.00 rows=100000 width=4) (actual time=0.026..11.083 rows=100000 loops=1) CTE s -> Seq Scan on smaller (cost=0.00..1443.00 rows=100000 width=4) (actual time=0.015..9.161 rows=100000 loops=1) -> Sort (cost=10304.82..10554.82 rows=100000 width=4) (actual time=78.775..90.953 rows=100000 loops=1) Sort Key: b.a Sort Method: external merge Disk: 1376kB -> CTE Scan on b (cost=0.00..2000.00 rows=100000 width=4) (actual time=0.033..39.274 rows=100000 loops=1) -> Sort (cost=10304.82..10554.82 rows=100000 width=4) (actual time=62.453..74.004 rows=99996 loops=1) Sort Key: s.a Sort Method: external sort Disk: 1768kB -> CTE Scan on s (cost=0.00..2000.00 rows=100000 width=4) (actual time=0.018..31.669 rows=100000 loops=1) Planning Time: 0.303 ms Execution Time: 199.919 ms (16 rows) It doesn't use hash join - the estimations are perfect, but plan is suboptimal Regards Pavel
