PostgreSQL follows the SQL standard very closely. SQL itself does not force any particular type of model to be used, so PostgreSQL can easily be used for many types of models at the same time, in the same database.
With PostgreSQL acting as a relational database, we can utilize any level of denormalization, from the full third normal form (3NF), to the more normalized Star Schema models. PostgreSQL extends the relational model to provide arrays, row types, and range types.
A document-centric database is also possible using PostgreSQL's text, XML, and binary JSON (JSONB) data types, supported by indexes optimized for documents and by full text search capabilities.
Key/value stores are supported using the hstore extension.