public boolean forward(Object row) throws HiveException {
      if (op.getDone()) {
        return false;
      }
      op.process(row, 0);
      return true;
    }
Map operator. This triggers overall map side processing. This is a little
different from regular operators in that it starts off by processing a
Writable data structure from a Table (instead of a Hive Object).
ql/src/java/org/apache/hadoop/hive/ql/exec/MapOperator.java