@Explain(displayName = "grouping sets")
  @Signature
  public List<Long> getListGroupingSets() {
    if (groupingSetsPresent) {
      return listGroupingSets;
    }
    return null;
  }
Checks if this grouping is like distinct, which means that all non-distinct grouping
columns behave like they were distinct - for example min and max operators.
ql/src/java/org/apache/hadoop/hive/ql/plan/GroupByDesc.java