what standard say ...

vac=> \d test

Table = test
+----------------------------------+----------------------------------+-------+
| Field | Type | Length|
+----------------------------------+----------------------------------+-------+
| x | int4 | 4 |
| y | int4 | 4 |
+----------------------------------+----------------------------------+-------+
vac=> select count(*) from test where exists (select t1.y from test t1 where t1.y = x);
^
Is this correlated subquery or not ?
(Note, that I don't use x with t1. prefix here)
With current parser this works as un-correlated subquery...
Is this Ok and I have to re-write query as

vac=> select count(*) from test t2 where exists
^^
(select t1.y from test t1 where t1.y = t2.x);
^^^
to get correlated one ?

Vadim