strange plan - PostgreSQL 9.2

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..4392325.00 rows=1000000 width=4)
SubPlan 1
-> Limit (cost=0.00..4.38 rows=1 width=0)
-> Index Only 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 using yy_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=100000 width=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