@Explain(displayName = "sampleDesc", explainLevels = { Level.EXTENDED })
  @Signature
  public String getSampleDescExpr() {
    return sampleDescr == null ? null : sampleDescr.toString();
  }
sampleDesc is used to keep track of the sampling descriptor.
ql/src/java/org/apache/hadoop/hive/ql/plan/FilterDesc.java