Occasionally, deleting collection elements one by one can be extremely inefficient. Hibernate isn't completely stupid, so it knows not to do that in the case of an newly-empty collection (if you called
list.clear(), for example). In this case, Hibernate will issue a single
DELETE and we are done!
Suppose we add a single element to a collection of size twenty and then remove two elements. Hibernate will issue one
INSERT statement and two
DELETE statements (unless the collection is a bag). This is certainly desirable.
However, suppose that we remove eighteen elements, leaving two and then add thee new elements. There are two possible ways to proceed
Hibernate isn't smart enough to know that the second option is probably quicker in this case. (And it would probably be undesirable for Hibernate to be that smart; such behaviour might confuse database triggers, etc.)
Fortunately, you can force this behaviour (ie. the second strategy) at any time by discarding (ie. dereferencing) the original collection and returning a newly instantiated collection with all the current elements. This can be very useful and powerful from time to time.
Of course, one-shot-delete does not apply to collections mapped