Modify SQL query generated behind Spring Data REST projections

I don't know why it's missing from the docs, but this spring sample (from spring) shows that you can use projections as the return type for a @Query. So you can do:

public interface ActionId {
    String getId(); 
}

@Query("select a.id as id from Action a where a.type = :type")
public List<ActionId> findByType(@Param("type") String type);

Now instead of having to use constructor expressions, you can more succinctly just select the columns you want, and return an object. I wish that projections could be applied to the domain object itself, so you could still return an "Action" with just the id field, but that doesn't look possible now-

ref:https://github.com/spring-projects/spring-data-examples/blob/master/jpa/example/src/main/java/example/springdata/jpa/projections/CustomerRepository.java

I think you can use "nested projections", let's say.

An example: resource A contains field bubi and resource B, which has field foo, bar, zed.

You should create a projection for B where you list just the fields you want:

@Projection(name="reduced", types = B.class)
public interface BReduced {
    String foo;
    //exclude bar, for instance
    int zed;
}

Then you use that projection in A's projection.

@Projection(name="reduced", types = A.class)
public interface AReduced {
    int bubi;
    BReduced b;
}

Have I undestood well, or were you talking about performance in SQL query?