I use joined inheritance mapping for my tables, and found to my dismay that
using the JPA 2.1 "treat" construct to downcast causes the usual left outer
join to a subclass table to become an inner join. This makes the construct
useless since the inner join makes polymorphic queries impossible: only the
treat-ed type will be retrieved. In the case where 2 or more downcasts are
made, absolutely no records can be retrieved.

Sample JPA 2.1 code:

CriteriaBuilder cb = entityManager.getCriteriaBuilder();
CriteriaQuery<Object[]> query = cb.createQuery(Object[].class);
Root<Pet> root = query.from(Pet.class);
query.multiselect(
root.get("id"),
root.get("name"),
cb.treat(root, Cat.class).get("felineProperty"),
cb.treat(root, Dog.class).get("canineProperty")
);

The resulting SQL looks like (mocked up):

select
...
from
Pet pet0_
left outer join
Bunny bunny0_
on pet0_.ID=bunny0_.ID
inner join
Cat cat0_
on pet0_.ID=cat0_.ID
inner join
Dog dog0_
on pet0_.ID=dog0_.ID