private boolean processRow(Object value) {
    try {
      if (mapOp.getDone()) {
        return false; // done
      } else {
        // Since there is no concept of a group, we don't invoke
        // startGroup/endGroup for a mapper
        mapOp.process((Writable) value);
      }
    } catch (Throwable e) {
      if (e instanceof OutOfMemoryError) {
        // Don't create a new object if we are already out of memory
        throw (OutOfMemoryError) e;
      } else {
        LOG.error("Failed to process row", e);
        closeReader();
        throw new RuntimeException(e);
      }
    }
    return true; // give me more
  }
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/MapRecordSource.java