@Explain(displayName = "predicate", explainLevels = { Level.USER })
  public String getUserLevelExplainPredicateString() {
    return PlanUtils.getExprListString(Arrays.asList(predicate), true);
  }
sampleDesc is used to keep track of the sampling descriptor.
ql/src/java/org/apache/hadoop/hive/ql/plan/FilterDesc.java