materialization blocks hash join
От | Pavel Stehule |
---|---|
Тема | materialization blocks hash join |
Дата | |
Msg-id | CAFj8pRDsvxD1rBdDgAnbp+LHRTj0shv8oX3cLzqrmarB-5zCHQ@mail.gmail.com обсуждение исходный текст |
Ответы |
Re: materialization blocks hash join
(Pavel Stehule <pavel.stehule@gmail.com>)
|
Список | pgsql-hackers |
Hi
when I was in talk with Silvio Moioli, I found strange hash join. Hash was created from bigger table.
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)
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)
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
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