presto-main/src/main/java/io/prestosql/sql/planner/plan/AggregationNode.java/AggregationNode/singleGroupingSet
class AggregationNode:    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);
    }