1. A method of making a feature specifier that represents a feature, a feature being a set of topology elements in a topology data model that is defined in a relational database management system, the topology data model including a topology element table in the relational database management system having rows representing topology elements and the method comprising the steps performed in the relational database management system of: receiving a specification of the set of topology elements; for each topology element in the specified set, if the topology element does not already have a row in the topology element table, making a row for the topology element in the topology element table; and making a row in a relations table in the relational database management system which relates the topology element's row to the feature specifier; and when rows have been made in the relations table for all of the topology elements in the set, providing the feature specifier.