I checked the behavior of SQL when we want to have values mapped immediately from an Entity to a DTO object, for example. If I gave an example in the constructor:
SELECT new Test(T) FROM Test T
I saw 2 queries in the logs. On the other hand, if I did:
SELECT new TEST(T.name, T.city) FROM TEST T
I saw only one query in the logs.
Why?
This is defined by the JPA specification. Anytime you use '.' over a relationship in JPA, you are performing an inner join (unless using it to define an outer join in a From clause). Since you are selecting t.city, you are forcing JPA to join to the City entity and include it in the results - so there is no need for it to issue yet another query to fetch that same data.
When you select just T, you have not specified how to fetch city at all, leaving the provider to do its thing and build you a complete Test entity (which you wrap in a constructor). This ends up with N+1 queries - but doesn't have to, as there are many other ways to fetch relationships like batching or even eager fetching that might be more appropriate.