@Override
  public void processGroup(int tag) throws HiveException {
    processGroupCalled[tag] = true;
    boolean shouldProceed = true;
    for (int i = 0; i < numParents; i++) {
      if (!processGroupCalled[i]) {
        shouldProceed = false;
        break;
      }
    }
    if (shouldProceed) {
      Operator<? extends OperatorDesc> child = childOperatorsArray[0];
      int childTag = childOperatorsTag[0];
      child.flush();
      child.endGroup();
      child.processGroup(childTag);
    }
  }
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