Locking vs. Exceptions

Hi,

The documentation says that function blocks with exceptions are far costlier
than without one.

So if I need to implement an INSTEAD OF trigger (after checking for unique
constraint violations) which way should I go ?

1. Get a table lock
2. Use 'Select ... For Update' (which could be used to lock only the desired
recordsets)
3. Use Exceptions

Any advice / experiences or even pointers would be helpful.

Thanks
Robins Tharakan