Creating index on foreign key column?
It is highly recommended to create an index on Employee.dept_id.

It should make a significant improvement for such SELECT statements. Even more important the index becomes on updates. You may lock entire table (forcing other sessions to wait) until you finish your transaction. 

See more information here: https://asktom.oracle.com/pls/asktom/f?p=100:11:0::::P11_QUESTION_ID:292016138754