I have class A, and then classes B, C, D,... all of which extend class A.

I need only data that is in class A (say I need only ids). I make a simple hibernate query: from A where id = 100

Is there a way to make hibernate not check all the classes that extend class A to establish which concrete class does the resulting row belong to? Cause that results in a big query linking all the tables that extend class A and checking where the id is. And in this case I know I will only need the data from class A, so it would be nice, if I could prevent hibernate from trying to get anything else.


btw:
I found the same/similar question posted several times, but there is no satisfying answer:

http://forum.hibernate.org/viewtopic.php?t=935155
http://forum.hibernate.org/viewtopic.php?t=937721
http://forum.hibernate.org/viewtopic.php?t=930208

Hibernate version: Hibernate 2.1.7

Look under the select clause: http://www.hibernate.org/hib_docs/refer ... l-examples
- e.g.: select cat.name from eg.DomesticCat cat
You can do the same for your class A, and retreive only the attributes that are part of class A. That way hibernate will not link all the tables to establish which class this object is really of.

Wanted solution introduces conflict with Hibernate's principle - you would eventually have two objects of different class instance with same id - which one is right?

Probably the only solution is that you make a new class DefaultA which is a superclass of A (maybe A then becomes empty class). Then set DefaultA's polymorphism to 'explicit'. When trying to load only A's you will actually load DeafultA's. The rest of B, C, ... will have a normal expected polymorphic behaviour.

>>Explicit polymorphism is useful when two different classes are mapped to the same table (this allows a "lightweight" class that contains a subset of the table columns).

That will ofcourse requre new mapping for DeafultA (duplicated only A's).
And query then changes to 'from DefaultA where ...'.

Or you can run hbm2java on below mapping (get duplicated class DefaultA and A).

Here what your mapping should look like:
<hibernate-mapping>
<class
name="eg.DefaultA"
table="aaa"
dynamic-update="false"
dynamic-insert="false"
polymorphism="explicit"
>
<id
name="id"
column="aaa_id"
type="int"
unsaved-value="0"
>
<generator class="native">
</generator>
</id>
<property
name="ident"
type="java.lang.String"
update="true"
insert="true"
access="property"
column="ident"
length="127"
not-null="true"
/>
</class>
<class
name="eg.A"
table="aaa"
dynamic-update="false"
dynamic-insert="false"
>
<id
name="id"
column="aaa_id"
type="int"
unsaved-value="0"
>
<generator class="native">
</generator>
</id>
<property
name="ident"
type="java.lang.String"
update="true"
insert="true"
access="property"
column="ident"
length="127"
not-null="true"
/>
<joined-subclass
name="B"
table="bbb"
dynamic-update="false"
dynamic-insert="false"
>
<key
column="bbb_id"
/>
<property
name="protocolId"
type="java.lang.Integer"
update="true"
insert="true"
access="property"
column="protocol_id"
length="10"
/>
</joined-subclass>
</class>
</hibernate-mapping>