private Object deserializeValue(BytesWritable valueWritable, byte tag)
      throws HiveException {

    try {
      return inputValueSerDe.deserialize(valueWritable);
    } catch (SerDeException e) {
      throw new HiveException(
          "Hive Runtime Error: Unable to deserialize reduce input value (tag="
              + tag
              + ") from "
          + Utilities.formatBinaryString(valueWritable.getBytes(), 0, valueWritable.getLength())
          + " with properties " + valueTableDesc.getProperties(), e);
    }
  }
Process input from tez LogicalInput and write output - for a map plan
Just pump the records through the query plan.
ql/src/java/org/apache/hadoop/hive/ql/exec/tez/ReduceRecordSource.java