public static GroupingSetDescriptor singleGroupingSet(List<Symbol> groupingKeys) {
    Set<Integer> globalGroupingSets;
    if (groupingKeys.isEmpty()) {
      globalGroupingSets = ImmutableSet.of(0);
    } else {
      globalGroupingSets = ImmutableSet.of();
    }

    return new GroupingSetDescriptor(groupingKeys, 1, globalGroupingSets);
  }
@return whether this node should produce default output in case of no input pages. For example
    for query:
    <p>SELECT count(*) FROM nation WHERE nationkey < 0
    <p>A default output of "0" is expected to be produced by FINAL aggregation operator.
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/queryengine/plan/relational/planner/node/AggregationNode.java