@Explain(displayName = "Sampling", explainLevels = { Level.EXTENDED })
  public String getSamplingTypeString() {
    return samplingType == 1 ? "SAMPLING_ON_PREV_MR" :
        samplingType == 2 ? "SAMPLING_ON_START" : null;
  }
For each map side operator - stores the alias the operator is working on
behalf of in the operator runtime state. This is used by reduce sink
operator - but could be useful for debugging as well.
ql/src/java/org/apache/hadoop/hive/ql/plan/MapWork.java