JPAQuery executes Projections 50% faster than QuerydslPredicateExecutor

[membersound]

I detected a strange performance issue that I'd like to share.
My goal is to find all rows based on a querydsl Predicate, and return them as a stream of Projections.

Now I created this function once in a Repository interface, and once manually using JPAQuery.
Surprisingly. the JPAQuery only takes 50% of the time, even though the resulting query should be equal.

public interface MyEntityRepository extends JpaRepository<MyEntity, Long>, QuerydslPredicateExecutor<MyEntity> {

}

Usage:

try (Stream<MyEntityView> stream = dao.findBy(predicate, q -> q.as(MyEntityView.class).stream())) {
    stream.forEach(row -> {});
}

Whereas MyEntityView is a projection interface with only some fields of the real entity.

Now the "manual" query, whereas MyEntityView is a real class with a constructor in this case:

try(Stream<FlightCacheGdsView2> stream = new JPAQuery<>(em)
				.select(Projections.constructor(MyEntityView.class,
						QMyEntity.myEntity.field1, QMyEntity.myEntity.field2, QMyEntity.myEntity.field5))
		                .from(QMyEntity.myEntity)
				.where(predicate)
				.stream()) {
    stream.forEach(row -> {});
}

So maybe anybody has an explaination for this performance gain then using JPAQuery manually? I don't think this happens just by chance. Maybe it's a bug?

At the end, both approaches are calling AbstractJPAQuery.stream(), so I wonder why there is a difference at all.

Update: what I just found the reason: the findBy() method creates a query that selects all fields from the database. select myEntity from MyEntity myEntity where....

Whereas the JPAQuery only selects the desired fields for the projection (select field1, field2, field5 from myEntity...).

So probably this is a bug in cases where a Projection interface is passed to the FetchableFluentQuery function?

[mp911de]

That should be addressed via #2327. Previously, we've loaded full entities and then applied projections.