SQL & PL/SQL

Hi Brothers

What is the difference between below 2 queries. are they same?

select * from T1
LEFT JOIN
(
A1 JOIN A2
ON A1.oi_id = A2.pk_id
)
ON (T1.ext_id = A1.ext_org_id1)

select * from T1
LEFT JOIN ON
(select oi_id, pk_id from A1, A2 where A1.oi_id = A2.pk_id)A3
ON (T1.ext_id = A3.ext_org_id1)

First Query is running forever, where 2nd is very very fast.

Thanks for you support in advance.

Regards
Sunil Mohanty