@VisibleForTesting
  void initializeAsRoot(JobConf hconf, MapWork mapWork) throws Exception {
    setConf(mapWork);
    setChildren(hconf);
    passExecContext(new ExecMapperContext(hconf));
    initializeMapOperator(hconf);
  }
Initializes this map op as the root of the tree. It sets JobConf &
MapRedWork and starts initialization of the operator tree rooted at this
op.

@param hconf
@param mapWork
@throws HiveException
ql/src/java/org/apache/hadoop/hive/ql/exec/MapOperator.java