(Ozgun from Citus Data)

It's Postgres' Extension APIs. They enable you to extend, cooperate with, or override any database submodule's behavior without forking the database: https://goo.gl/rr2EIm

As an example, let's say you leveraged planner hooks to parallelize PostgreSQL queries. When a new feature such as JSONB gets checked into Postgres, it becomes immediately available for parallel querying.

To my knowledge, no other database provides the means to extend the database to such an extent. These APIs became official in PG 9.0, and two example links from the PostgreSQL documentation are:

http://www.postgresql.org/docs/9.4/static/extend.html http://www.postgresql.org/docs/9.4/static/spi.html