Batch fetch optimization for lazy collections

Question

Hi,
I feel like this question must have been asked by somebody already but couldn't find any answers in the forum.
We're trying to evaluate migrating from Hibernate to KODO JDO. One feature we use extensively from Hibernate is the "batch fetch optimization for lazy collections".
For instance, there's a class User with collection "Set<String> permissions". In the DB, there're tables USER(id, name, etc) and USER_PERMISSIONS(user_id, permission) with one-to-many relationship b/w them.
Suppose the code is the following:
query = ....;
List<User> users = (List<User>) query.execute();
for (User user : users) {
println("User: " + user.getName());
println("Permissions: " + user.getPermissions());
I've setup EagerFetchMode to parallel. For field "permissions" I had to specify default-fetch-group="true" b/c I wanted this to happen lazily. When I look through the logs, I see that permissions SQL query is executed for each user.
With Hibernate we were to setup mapping the way that permissions are not fetched at first, but when they are requested for the first user (user.getPermissions()) they are automatically selected for several users in one query using SQL IN clause (similar to the parallel mode).
Is this possible to recreate the same in KODO JDO? (JPA?) If it is, this would greatly simplify migration. Please notice, that we can't use explicit fetch groups for this, b/c we don't know ahead which collection can be navigated (in the real life User class have many relationships).
thanks in advance,
Dimitry

Answer 1

Kodo doesn't have a direct analog for that behavior. The typical way to solve that problem is to keep the field out of the default fetch group, and then explicitly include the desired field in the current fetch group at runtime. In pure JDO, you can do this by creating a separate fetch group that includes the relationship field, and designating that the query should use that fetch group:
<pre>
query = ...;
query.getFetchPlan().addGroup("relationshipGroup");
List<User> users = (List<User>) query.execute();
</pre>
Kodo has JDO extensions that allow you to do this a bit more easily:
<pre>
query = ...;
((kodo.jdo.KodoFetchPlan) query.getFetchPlan()).addField("com.example.User.permissions");
List<User> users = (List<User>) query.execute();
</pre>
Finally, you can do this with Kodo's JPA extensions like so:
<pre>
import org.apache.openjpa.persistence.OpenJPAQuery;
query = (OpenJPAQuery) ...;
query.getFetchPlan().addField("com.example.User.permissions");
List<User> users = (List<User>) query.getResultList();
</pre>
Note that in all cases, you could also make this change to the current PersistenceManager / EntityManager's fetch plan instead, to make the change happen for the duration of the use of that manager. In that environment, the change would need to happen before the query was created.
(And no, I have no idea why the edit box has a 'pre' button but does not seem to do anything with the resulting tags.)
-Patrick