Performing an Explain Plan on the same query, Optimizer shows a Full Table Scan where as Toad products show INDEX RANGE SCAN. Also the background queries show "ORA-12571: TNS:packet writer failure." error.
Use an Oracle client that is the same or higher version as the database used. Optimizer's Connection window allows to select a different Oracle Home for the connection. If no other clients are available, then a new client needs to be installed on the machine.
