@Override
    public List<Object> next() throws HiveException {

      WriteBuffers.ByteSegmentRef byteSegmentRef = hashMapResult.next();
      if (byteSegmentRef == null) {
        return null;
      } else {
        return unpack(byteSegmentRef);
      }

    }
Sometimes, when container is empty in multi-table mapjoin, we need to add a dummy row.
This container does not normally support adding rows; this is for the dummy row.
ql/src/java/org/apache/hadoop/hive/ql/exec/persistence/HybridHashTableContainer.java