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..4392325.00rows=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