@Override
            public void loadNext() throws IOException {
                if (spillReader != null) {
                    if (spillReader.hasNext()) {
                        spillReader.loadNext();
                        priorityQueue.add(spillReader);
                    }
                }
                spillReader = priorityQueue.remove();
            }
Add an UnsafeSorterIterator to this merger
indexr-query-opt/src/main/java/org/apache/spark/util/collection/unsafe/sort/UnsafeSorterSpillMerger.java