public final void setReducerTraits(EnumSet<ReducerTraits> traits) {
    // we don't allow turning on auto parallel once it has been
    // explicitly turned off. That is to avoid scenarios where
    // auto parallelism could break assumptions about number of
    // reducers or hash function.

    boolean wasUnset = this.reduceTraits.remove(ReducerTraits.UNSET);

    if (this.reduceTraits.contains(ReducerTraits.FIXED)) {
      return;
    } else if (traits.contains(ReducerTraits.FIXED)) {
      this.reduceTraits.removeAll(EnumSet.of(
          ReducerTraits.AUTOPARALLEL,
          ReducerTraits.UNIFORM));
      this.reduceTraits.addAll(traits);
    } else {
      this.reduceTraits.addAll(traits);
    }
  }
Returns the null order in the key columns.

@return null, which means default for all key columns, or a String
        of the same length as key columns, that consists of only "a"
        (null first) and "z" (null last).
ql/src/java/org/apache/hadoop/hive/ql/plan/ReduceSinkDesc.java