[BalusC]

DB2 does not accept a correlated subquery as order expression: "The sort-key-expression cannot include a correlated scalar fullselect (SQLSTATE 42703)". So the ordering on a field of a @OneToMany or @ElementCollection from #61 fails on DB2 with all three providers. OptimusFaces CI shows this in testLazyWithOneToMany on DB2.

On DB2 only, when the page is ordered on a field of a @OneToMany or @ElementCollection which would otherwise be ordered via the correlated subquery, the page is now loaded in two queries:

1. An ID query: SELECT e.id ... LEFT JOIN e.collection c ... GROUP BY e.id ORDER BY MIN(CASE WHEN <criteria on the collection> THEN c.field ELSE NULLIF(c.field, c.field) END), e.id, with offset and limit. It applies the same restrictions as the entity query, and also the EXISTS subquery of the count query when a QueryBuilder is used.
2. The entity query, restricted to id IN (ids). The entities are then put in the order of the IDs.

Provider constraints:

• EclipseLink and OpenJPA do not register parameters which occur only in the ORDER BY clause. So the criteria on the collection also appear in a WHERE predicate which is always true: (criteria OR e.id IS NOT NULL).
• OpenJPA cannot render a NULL literal in a CASE, so NULLIF(field, field) is used instead.

Known limitations:

• For Page.ALL, id IN (ids) is not bounded. EclipseLink's PostponedFetchRoot already does the same.
• On OpenJPA, some criteria on a collection are weaker in a subquery. The count can therefore be too high, as it already was before.

Tests:

• New ITs check that each matching entity is on exactly one page. They cover entities which match the criteria on a collection without having one element which matches all of them, and ordering on an enum field and on an @ElementCollection.
• All OmniPersistence ITs pass on H2 with Hibernate, EclipseLink and OpenJPA, also with the ID query path forced on for all databases.
• OptimusFacesDB2IT passes on a local DB2 container with all three servers.

Follow-up to #61

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