protected static Operator<?> getSingleParent(Operator<?> operator) throws SemanticException {
    return getSingleParent(operator, false);
  }
@param operator the input operator
@param throwException if throw a exception when the input operator has multiple parents
@return the single parent or null when the input operator has multiple parents and
        throwException is false;
@throws SemanticException
ql/src/java/org/apache/hadoop/hive/ql/optimizer/correlation/CorrelationUtilities.java