Table Inheritance: what is a leaf entity?

[stephanvierkant]

Interesting question I've found on StackOverflow: What is a leaf entity in the inheritance hierarchy and what are examples of?

From Doctrine's documentation:

There is a general performance consideration with Class Table Inheritance: If the target-entity of a many-to-one or one-to-one association is a CTI entity, it is preferable for performance reasons that it be a leaf entity in the inheritance hierarchy, (ie. have no subclasses).
Otherwise Doctrine CANNOT create proxy instances of this entity and will ALWAYS load the entity eagerly.

I don't understand this part of the documentation. What is exactly preferable and what kind of performance issues can one expect if non-leaf entities are used?

Let's say we've got these entites:

#[InheritanceType('SINGLE_TABLE')]
class AbstractEngine {}

class V6Engine extends AbstractEngine {}
class V8Engine extends AbstractEngine {}
class AudiV8Engine extends V8Engine {}

Can I have a Automobile entity with a relationship with AbstractEngine? Or should it be with a leaf entity, so only V6Engine or AudiV8Engine can be used?

[greg0ire]

Can I have a Automobile entity with a relationship with AbstractEngine?

Yes, it can, but there will be a performance hit you would not have if you were using a leaf entity: the cost in performance is the cost of hydrating the engine, from what I understand. When you instantiate an Automobile, the ORM cannot create a proxy instance for the engine, so it will:

• perform an SQL request to get the engine's data
• hydrate an instance of AbstractEngine with that data

If you are instead targeting a leaf class, the query + hydration would only happen when doing $automobile->engine()->name(). In the first case, it happens when doing new Automobile().

While there is a performance hit, it's probably pretty mild though, but only you can tell.

[beberlei]

The performance hit often comes from N+1 queries. Say you have a list of 20 automobiles, then it will trigger 20 queries for the AbstractEngine.