@Override
    protected void onSerialize(Serializer buf) {
        buf.putLong(null, maxGroups);
        buf.putLong(null, precision);
        serializeOptional(buf, classify);
        if (v2) {
            serializeOptional(buf, filter);
        } else if (filter != null) {
            throw new IllegalStateException("Filter set on v1 GroupingLevel");
        }
        collect.serializeWithId(buf);
    }
<p>Tell if ordering will need results collected in children.</p>

@return If deeper resultcollection is needed.
searchlib/src/main/java/com/yahoo/searchlib/aggregation/GroupingLevel.java