@Override
        public boolean hasNext() {
            while (!current.hasNext() && !iterators.isEmpty()) {
                current = iterators.remove();
            }
            return current.hasNext();
        }
Chain multiple UnsafeSorterIterator together as single one.
indexr-query-opt/src/main/java/org/apache/spark/util/collection/unsafe/sort/UnsafeExternalSorter.java