JPQL / SQL riddle

gistfile1.txt

JPQL: select p from Person p 
      left outer join p.address address 
      order by address.city
SQL:  select person0_ from Person person0_ 
      left outer join Address address1_ on person0_.id=address1_.person_id 
      order by address1_.city

JPQL: select p from Person p 
      left outer join p.address address 
      order by p.address.city
SQL:  select person0_ from Person person0_ 
      left outer join Address address1_ on person0_.id=address1_.person_id 
      cross join Address address2_ where person0_.id=address2_.person_id 
      order by address2_.city

The core difference here is that qualifying the sorting criteria to p.address.city 
instead of address.city results in an additional JOIN clause added which results in
persons not having an address being ruled out from the result set. Environment is
Hibernate 4.1.5.SP1, H2 1.3.168.

1. Should the two JPQL queries result in the SQL queries shown? In particular 
   shouldn't they actually generate the very same SQL?
2. Why does the second JPQL query add the additional JOIN which effectively rules 
   out persons without an address?
3. Is it a good idea that adding a sort criterion potentially adds an additional 
   join which influences the actual result set return (i.e. "the number of results 
   changes just because I added a sort criteria")

Nothing at all, it's invalid as addesses is not known at all. p.addresses would have been. Does the query actually make sense? Why would you want to outer join the tables (retaining null values) if you actually have a where clause that will definitely return false for these values?

Again, as soon as you manually add joins you do so for a reason and then it's up to you to define all related clauses correctly. But if you don't do (select p from Person p), adding an order byclause simply must not restrict the result set more than actually expressed in the query.

OK, I seem to have way too much time (I haven't), but well - it''s an interesting problem.

So, here's another addition.

1. my last comment over-emphasizes the concept of re-using an existing outer join. If applying the rule of "non-aliased order by path expressions implicitly add an outer join to the resulting SQL FROMclause, it would be clear what the query does and the actual selection in terms of rows would not be affected.
2. Navigation expressions are all about navigating an object graph, since JPA tries to hide the relational stuff and give us a "purely" object oriented view on our data, right? So when applying the rule from point 1, you would effectively say: this ORDER BY is not an object graph expression any more, because in an object graph we can only walk along transitions - and not non-transitions, which is what a left outer join would construct. So if ORDER BY a.b would result in an outer join, the path expression applied to the object graph would read [a | ].b, which feels utterly wrong to me in an object model.

Regarding your comment to the addresses-example:

Of course it should have read ORDER BY p.addresses. The example does make sense as a classic "map rows to columns" example, relying on an effective query planner and executer in the DBMS, causing least possible communication overhead between DB and JVM. More realistic would be: Generate result rows of form

firstname, lastname, home phone, business phone

given that phone numbers are attributes of an entity with 1:n relation to Persons and a discriminator column type

But again, the example not really good to make any case for my point in opposite to yours, as mentioned in my last comment - so forget about it :)