presto-main/src/main/java/io/prestosql/sql/planner/plan/GroupIdNode.java/GroupIdNode/getInputSymbols
class GroupIdNode:    public Set<Symbol> getInputSymbols()
    {
        return ImmutableSet.<Symbol>builder()
                .addAll(aggregationArguments)
                .addAll(groupingSets.stream()
                        .map(set -> set.stream()
                                .map(groupingColumns::get).collect(Collectors.toList()))
                        .flatMap(Collection::stream)
                        .collect(toSet()))
                .build();
    }