@Override
  public void startGroup() throws HiveException{
    for (int i = 0; i < numParents; i++) {
      processGroupCalled[i] = false;
    }
    super.startGroup();
  }
Calls initialize on each of the children with outputObjetInspector as the
output row format.
ql/src/java/org/apache/hadoop/hive/ql/exec/MuxOperator.java