public Map<String, ExprNodeDesc> getColumnExprMap() {
    if (this.getConf() == null) {
      return null;
    }
    return this.getConf().getColumnExprMap();
  }
Returns a map of output column name to input expression map Note that
currently it returns only key columns for ReduceSink and GroupBy operators.

@return null if the operator doesn't change columns
ql/src/java/org/apache/hadoop/hive/ql/exec/Operator.java