public void setColumnExprMap(Map<String, ExprNodeDesc> colExprMap) {
    if (this.getConf() != null) {
      this.getConf().setColumnExprMap(colExprMap);
    }
  }
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