It could be that the constraint is not trusted. If you ever modified the constraint so it won't be checked and didn't changed it so it will be checked again or changed it so it will be active again but without the "with check" clause, then the constraint can not be trusted. In this case the optimizer won't take the constraint into consideration. You can check it with the objectproperty function. If this is the case, you should run alter statement that checks the constraint.

Adi