public Object process(Object row) throws HiveException {
      // Evaluate the keys
      for (int i = 0; i < keyEval.length; i++) {
        outputKey[i] = keyEval[i].evaluate(row);
      }
      // Evaluate the value
      for (int i = 0; i < valueEval.length; i++) {
        outputValue[i] = valueEval[i].evaluate(row);
      }
      forwardedRow.clear();
      // JoinOperator assumes the key is backed by an list.
      // To be consistent, the value array is also converted
      // to a list.
      forwardedRow.add(Arrays.asList(outputKey));
      forwardedRow.add(Arrays.asList(outputValue));
      return forwardedRow;
    }
The evaluators for the value columns. Value columns are passed to reducer
in the "value".
ql/src/java/org/apache/hadoop/hive/ql/exec/MuxOperator.java