When considering portability between databases, another important decision is selecting the identifier generation strategy you want to use. Originally Hibernate provided the native generator for this purpose, which was intended to select between a sequence, identity, or table strategy depending on the capability of the underlying database. However, an insidious implication of this approach comes about when targeting some databases which support identity generation and some which do not. identity generation relies on the SQL definition of an IDENTITY (or auto-increment) column to manage the identifier value; it is what is known as a post-insert generation strategy because the insert must actually happen before we can know the identifier value. Because Hibernate relies on this identifier value to uniquely reference entities within a persistence context it must then issue the insert immediately when the users requests the entity be associated with the session (like via save() e.g.) regardless of current transactional semantics. The underlying issue is that the semanctics of the application itself changes in these cases.
Hibernate has been improved so that the insert is delayed in cases where that is feasible.
Starting with version 3.2.3, Hibernate comes with a set of enhanced
identifier generators targeting portability in a much different way.
There are specifically 2 bundled enhanced
The idea behind these generators is to port the actual semantics of the identifer value generation to the different databases. For example, the
org.hibernate.id.enhanced.SequenceStyleGenerator mimics the behavior of a sequence on databases which do not support sequences by using a table.