@Override
    public void visitMembers(ObjectVisitor visitor) {
        super.visitMembers(visitor);
        visitor.visit("id", id);
        visitor.visit("valid", valid);
        visitor.visit("all", all);
        visitor.visit("topN", topN);
        visitor.visit("firstLevel", firstLevel);
        visitor.visit("lastLevel", lastLevel);
        visitor.visit("groupingLevels", groupingLevels);
        visitor.visit("root", root);
    }
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