Selecting MappedSuperclass from the database (Hibernate)

Question

Problem

I have a @MappedSuperclass called Data as the parent of every Entity in my database. It contains common attributes like Id etc. I then have an

Answer 1

I seem to be able to do this (although using InheritanceType.JOINED) with hibernate 5.0.8, java 1.8.0_73 and Oracle 12c - either I am misunderstanding or maybe hibernate has changed..

I have the following hierarhcy:

@MappedSuperclass
@Inheritance(strategy=InheritanceType.JOINED)
CommonRoot
 |
 |  @MappedSuperclass
 +- Mapped
        |  @Entity(name="Concrete1")
        |  @Table(name="CON1")
        +- Concrete1
        |
        |  @Entity(name="Concrete2")
        |  @Table(name="CON2")
        +- Concrete2

And I can do the following HQL:

SELECT entityId FROM com.hibernatetest.Mapped ORDER BY entityId ASC

which gives these 2 SQL statements:

select concrete2x0_.entityId as col_0_0_ from CON2 concrete2x0_ order by concrete2x0_.entityId ASC
select concrete1x0_.entityId as col_0_0_ from CON1 concrete1x0_ order by concrete1x0_.entityId ASC

and the warning

WARN: HHH000180: FirstResult/maxResults specified on polymorphic query; applying in memory!

Not sure what they mean though, as this can be done with SQL as:

(select entityId from CON2
union all
select entityId from CON1)
order by entityId ASC

(And you can also add limit/rownum clauses to that if you desire, although that gets a bit clunky:

select * from (
(select * from (select entityId from CON2 order by entityId ASC) where rownum <= 10)
UNION ALL
(select * from (select entityId from CON1 order by entityId ASC) where rownum <= 10)
) where rownum <= 10 order by entityId ASC

Not sure why hibernate shouldn't be able to do this - might suggest it to them.)