private void addInputPaths(JobConf job, ColumnTruncateWork work) {
    FileInputFormat.addInputPath(job, work.getInputDir());
  }
start a new map-reduce job to do the truncation, almost the same as ExecDriver.
ql/src/java/org/apache/hadoop/hive/ql/io/rcfile/truncate/ColumnTruncateTask.java