public static Window from(Collection<Target> targets, Query query) {
            if (targets.size() == 1) // preserve requested top-level offsets
                return Window.from(query);
            else // request from offset 0 to enable correct upstream blending into a single top-level hit list
                return new Window(query.getHits() + query.getOffset(), 0);
        }
A target handler where the target generation logic is delegated to the application provided target selector
container-search/src/main/java/com/yahoo/search/federation/FederationSearcher.java