Result set fetching

Question

I'm pretty sure this is basic stuff, and I should probably know this already, but here goes:
when you execute a query through EntityManager, are results eagerly fetched, or are they being loaded as the result set is iterated through?
I was wondering if when I iterate through only the first 5 results, is the whole result set still being populated?
I've been learning this stuff for a while but, to make matters worse, I was required to learn some LINQ in the mean time, so similar concepts got messed up in my mind... thus this question.
Thanks for the time!

Answer 1

It would depend on the implementation of the JDBC driver what it does, but generally you're going to get eager fetching of at least the first level records.
Linked records (non-primitive fields fetched from other tables) may be eagerly or lazily fetched depending on the configuration set. by default however those are eagerly fetched (and that's what JPA means when it talks about eager fetching).
If your dependency tree is deep and the child records are needed relatively rarely (for example you're not going to display them initially after a fetch, but only when explicitly drilling down), lazy fetching of those can improve performance. If they are required to be fetched as the norm, lazy fetching there can cost performance, so it's a design decision that requires some thought.