Slow Select Statement -- Please Help

hello,
I have the following query. it returns result in less than 1 second.
select *
from employee e, address a
where e.id=a.emp_id
and e.id=1234

The problem is that it becomes extremely slow if i take the last line
out. So the query looks like:
select *
from employee e, address a
where e.id=a.emp_id

The above query is only supposed to return ~500 rows. but i still
haven't got the result back after 30 minutes.

Does anyone have any suggestions about troubleshooting this problem?

Thank you in advance!
Eddy