@Override
    public Grouping clone() {
        Grouping obj = (Grouping)super.clone();
        obj.groupingLevels = new ArrayList<>();
        for (GroupingLevel level : groupingLevels) {
            obj.groupingLevels.add(level.clone());
        }
        obj.root = root.clone();
        return obj;
    }
Returns whether ordering will need results collected in children.
In that case we will probably just do a single pass.
searchlib/src/main/java/com/yahoo/searchlib/aggregation/Grouping.java