presto-main/src/main/java/io/prestosql/sql/planner/plan/GroupIdNode.java/GroupIdNode/getOutputSymbols
class GroupIdNode:    @Override
    public List<Symbol> getOutputSymbols()
    {
        return ImmutableList.<Symbol>builder()
                .addAll(groupingSets.stream()
                        .flatMap(Collection::stream)
                        .collect(toSet()))
                .addAll(aggregationArguments)
                .add(groupIdSymbol)
                .build();
    }