Hi all!
Maybe if this task is hard, you could do a simplier variant, "FROM Class1 a1 JOIN a1,a2 WITH a2 INSTANCE OF Class2"?
Doctrine 2.3 supports it, but in fact, it does not work. I can't use Class2 fields in query, and Doctrine ignores the INSTANCE OF operator when building a native queries.
I am working with system where I have many inherited classes with Class Table Inheritance. When I do a JOIN query, it generates native sql query more than 1KB(?!) length, and MySQL freezes for more than 7 minutes (?!) on it. It must join only tables for Class2, but it joins all my 20 tables inherited of my base class
I don't know where to send bugreport
It will be very good if I could manually select a joined class to make Doctrine do a better queries.
Sorry for my english, I am from Moscow.

Again I can pick this up if this improvement is agreed upon.

I can pick this up if it's agreed upon that this could indeed be improved.

The problem here is that joining with a JTI causes LEFT JOINS, which don't perform very well when it comes to sorting the results.

Just as a quick reference, here's where "something" should be changed to get this working:
https://github.com/doctrine/doctrine2/blob/master/lib/Doctrine/ORM/Query/SqlWalker.php#L316

When the field is part of the primary key, the field used for sorting results should be the one of the table of the entity itself, and not of the root of the CTI.