How to avoid empty child tables when using InheritanceType.JOINED?

Question

I have one JPA superclass and dozens of subclasses. Most of those subclasses do not add specific properties.

When using InheritanceType.JOINED JPA assumes a sub-table for each subclass, resulting in a large number of obsolete database tables, making database administration and queries harder.

My superclass currently looks like this:

@Entity
@Inheritance(strategy = InheritanceType.JOINED)
@DiscriminatorColumn(name = "type")
public abstract class Employee {
    @Id public int id;
    public String firstName;
    public String lastName;
    public Date birthday;
}

Some subclasses have specific fields:

@Entity
public class ProjectManager extends Employee {
    public String project;
}

Most subclasses, however, are empty like that:

@Entity
public class Director extends Employee {
}

@Entity
public class FacilityManager extends Employee {
}

These database tables make sense:

employee(id, first_name, last_name, birthday)
project_manager(id, project)

How can I avoid these obsolete tables?

director(id)
facility_manager(id)

If I just do not create them, JPA-validation fails:

Caused by: javax.persistence.PersistenceException: [PersistenceUnit: default] Unable to build Hibernate SessionFactory
    at org.hibernate.jpa.boot.internal.EntityManagerFactoryBuilderImpl.persistenceException(EntityManagerFactoryBuilderImpl.java:954) ~[hibernate-entitymanager-5.0.12.Final.jar:5.0.12.Final]
    ...
Caused by: org.hibernate.tool.schema.spi.SchemaManagementException: Schema-validation: missing table [director]
    at org.hibernate.tool.schema.internal.SchemaValidatorImpl.validateTable(SchemaValidatorImpl.java:67) ~[hibernate-core-5.0.12.Final.jar:5.0.12.Final]
    ...

Answer 1

There is a solution but only when using EclipseLink. Hibernate does not support this:

Mixed Inheritance

JPA requires the inheritance strategy to only be defined in the root class. If you want to use a mixture of SINGLE_TABLE and JOINED this can be achieved through using JOINED inheritance in the root class, and specifying the Table on the subclass to be the same as the parent.

Example: Using JOINED with mixed inheritance

@Entity
// This subclass does not define its own table, but shares its parent's ACCOUNT table.
@Table(name="ACCOUNT")
@DiscriminatorValue("3")
public class StandardAccount extends Account {
   ...
}

Source: https://wiki.eclipse.org/EclipseLink/UserGuide/JPA/Basic_JPA_Development/Entities/Inheritance#Mixed_Inheritance