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How select root of subclass without LEFT OUTHER JOIN

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Jan33

Post subject: How select root of subclass without LEFT OUTHER JOIN

Posted: Fri Jun 08, 2012 7:05 am

Newbie

Joined: Fri Jun 08, 2012 6:49 am
Posts: 2

Hi, I have a performance trouble (in real have 15 subclass). I have more subclass with InheritanceType.JOINED (common root table).

Code:

@Entity
@Inheritance(strategy = InheritanceType.JOINED)
public class FooEntity {
   @Id private int id;
}

@Entity
@PrimaryKeyJoinColumn(name = "id")
public class FooChild extends FooEntity { }

Now I need a query that selects the ID:

Code:

Criteria criteria = hibernate.getSessionFactory().getCurrentSession().createCriteria(FooEntity.class);
ProjectionList properties = Projections.projectionList();
properties.add(Projections.property("id"));
criteria.setProjection(properties);
criteria.list();

But I get this complex select:

Code:

    select
        this_.id as y0_ 
    from
        FooEntity this_ 
    left outer join
        FooChild this_1_ 
            on this_.id=this_1_.id

All entities are still in the table FooEntity. Why Hibernate add the "left outer join"? Just a simple select:

Code:

select this_.id as y0_ from FooEntity this_

How do I remove the join?

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reynolr

Post subject: Re: How select root of subclass without LEFT OUTHER JOIN

Posted: Fri Jun 08, 2012 2:27 pm

Beginner

Joined: Mon Jun 04, 2012 12:31 pm
Posts: 20

FooChild and FooEntity are two different entities.

FooChild is a child of FooEntity and another table will be created to hold the FooChild data.

Thus when hibernate constructs the SQL it has to do a join in order to provide the ability to select fields off of FooChild.

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Jan33

Post subject: Re: How select root of subclass without LEFT OUTHER JOIN

Posted: Mon Jun 11, 2012 5:16 am

Newbie

Joined: Fri Jun 08, 2012 6:49 am
Posts: 2

reynolr wrote:

Thus when hibernate constructs the SQL it has to do a join in order to provide the ability to select fields off of FooChild.

Thank you for your reply. I understand why (in general) the Hibernate makes the JOIN. But I need only common column! I do not FooChild nor FooEntity. I need only ID from the Foo table.

So far the only solution: to use named query.

Code:

@Entity
@Inheritance(strategy = InheritanceType.JOINED)
@NamedQueries({ @NamedQuery(name = "allFooIds", query = "select id from FooEntity") })
public class FooEntity {
   @Id private int id;
}

It's important to specify the column. Use only "from FooEntity" leads to LEFT JOIN with FooChild!
Used this query is simple:

Code:

List<Integer> fooIds = hibernate.getSessionFactory().getCurrentSession().getNamedQuery("allFooIds");

Using HQL has the advantage that Hibernate monitors and performs the change table and column names.
But why Hibernate does not same change in Criteria API, it is a mystery to me.

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