[GENERAL] session hang for same row update

[AI Rumman]

Could anyone please tell me why the session 2 is hanging?
I am using Postgresql 9.1.

show deadlock_timeout ;
 deadlock_timeout
------------------
 1s
(1 row)


  select * from t2;
 i | nam
---+-----
 2 | t4
 1 | t3

Session 1:
BEGIN
update t2 set nam = 't3' where i=2;
UPDATE 1

Session 2:
BEGIN
update t2 set nam = 't3' where i=2;
hanged