22.4. Identifier generation

<div class="paragraph">

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.

</div>
<div class="paragraph">

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.

</div>
<div class="paragraph">

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 that the entity be associated with the session (e.g. like via `save()` or `persist()`) , regardless of current transactional semantics.

</div>
<div class="admonitionblock note">
<table>
<tr>
<td class="icon">

</td>
<td class="content">
<div class="paragraph">

Hibernate was changed slightly, once the implications of this were better understood, so now the insert could be delayed in cases where this is feasible.

</div>
<div class="paragraph">

The underlying issue is that the actual semantics of the application itself changes in these cases.

</div>
</td>
</tr>
</table>
</div>
<div class="paragraph">

Starting with version 3.2.3, Hibernate comes with a set of [enhanced](http://in.relation.to/2082.lace) identifier generators targeting portability in a much different way.

</div>
<div class="admonitionblock note">
<table>
<tr>
<td class="icon">

</td>
<td class="content">
<div class="paragraph">

There are specifically 2 bundled _enhanced_generators:

</div>
<div class="ulist">

• org.hibernate.id.enhanced.SequenceStyleGenerator

• org.hibernate.id.enhanced.TableGenerator </div> </td> </tr> </table> </div>

  The idea behind these generators is to port the actual semantics of the identifier 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.

  </div> </div>