PostgreSQL can be extended in many more ways than what we have discussed so far in the book. This includes the ability to add new operators, new index access methods, and create your own aggregates. You can access foreign data sources, such as other databases, files, and web services using PostgreSQL foreign data wrappers. These wrappers are provided as extensions and should be used with caution, as most of them are not officially supported.

Even though PostgreSQL is very extensible, you can't plug in a new storage engine or change the parser/planner and executor interfaces. These components are very tightly coupled with each other and are, therefore, highly optimized and mature.