The Postgres database management system was originally designed as object-relational. We remind why PostgreSQL is an object-relational rather than just relational database management system, and briefly discuss what kind of object-oriented features it offers. We demonstrate how these features can be used to manage hierarchies, and how applications can benefit using them.
The discussion is illustrated with examples of SQL code based on the postgres_air database [6, https://github.com/hettie-d/postgres_air].
There are many ways to implement a hierarchy in a relational database. Techniques with names like adjacency lists and nested sets, not to mention some other RDBMS have built methods of