Hello
I try to look on one slow query with correlated subquery:
create table xx(a int primary key);
create table yy(a int);
insert into xx select generate_series(1,1000000);
insert into yy select (random()*1000000)::int from generate_series(1,100000);
create index on yy(a);
Query A
select a, (select true from yy where xx.a = yy.a limit 1) from xx
limit 10 offset 0;
postgres=> explain select a, (select true from yy where xx.a = yy.a
limit 1) from xx; QUERY PLAN
--------------------------------------------------------------------------------------Seq Scan on xx
(cost=0.00..4392325.00rows=1000000 width=4) SubPlan 1 -> Limit (cost=0.00..4.38 rows=1 width=0) ->
IndexOnly Scan using yy_a_idx on yy (cost=0.00..4.38
rows=1 width=0) Index Cond: (a = xx.a)
(5 rows)
plan for this query is expected
But when I rewrote query I got strange plan (query B):
postgres=> explain select a, exists(select 1 from yy where xx.a =
yy.a) from xx limit 10 offset 0;
QUERY PLAN
--------------------------------------------------------------------------------------Limit (cost=0.00..43.92 rows=10
width=4) -> Seq Scan on xx (cost=0.00..4392325.00 rows=1000000 width=4) SubPlan 1 -> Index Only Scan
usingyy_a_idx on yy (cost=0.00..4.38
rows=1 width=0) Index Cond: (a = xx.a) SubPlan 2 -> Seq Scan on yy (cost=0.00..1443.00
rows=100000width=4)
(7 rows)
Why there are a SubPlan 2?
But query B is two time faster than query A
public | xx | table | pavel | 35 MB |public | yy | table
|pavel | 3576 kB |
regards
Pavel
