@Explain(displayName = "quick start", displayOnlyOnTrue = true, explainLevels = {Explain.Level.EXTENDED })
  public final boolean isQuickStart() {
    return !isSlowStart();
  }
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