presto-main/src/main/java/io/prestosql/operator/index/IndexLoader.java/EmptyLookupSource/getNextJoinPosition
class EmptyLookupSource:        @Override
        public long getNextJoinPosition(long currentJoinPosition, int probePosition, Page allProbeChannelsPage)
        {
            return IndexSnapshot.UNLOADED_INDEX_KEY;
        }