Allow for master table only models on joined subclass inheritance

Think of a joined subclass inheritance setup where abstract base class A has many concrete child classes C1 ... CN. For each child class a table necessarily has to created. Yet if there are many child classes not defining any additional fields you will get many "id only" child tables. This leads to unnecessary join and insert overhead on database operations as well as a bunch of quite senseless tables in your schema that need to be maintained.

While there are already tickets requesting support for mixed inheritance mapping (e.g. DDC-138) I want to propose another - obviously easy to implement - solution that addresses the "id only table" problem. The basic idea is to extend ClassMetadata by a flag "hasOwnTable" which is true by default and applicable for child classes of a joined subclass tree. Setting this flag to <false> would lead to...
1.) no child table creation for corresponding model
2.) no joins to this table while rendering SQL from DQL statements
3.) no INSERT, UPDATE and DELETE statements for this table in methods executeInserts(), update() and delete() on Doctrine\ORM\Persisters\JoinedSubclassPersister.

(3) can easily be implemented since the mentioned methods all loop on ClassMetadata::parentClasses. For those classes which set the flag "hasOwnTable" to false the operation will be skipped. On the other hand (2) doesn't seem to a big deal either. Extending SqlWalker::_generateClassTableInheritanceJoins() by means of a flag test seems to be enough. Of course setting the flag to <false> while defining additional fields on child class level must be rejected.

If you go for this feature I would be pleased to provide an implementation.