PostgreSQL

PostgreSQL is an RDBMS (relational database management system) that is both free and open-source.

It was created to allow users to design different workloads: from single machines to data warehouses or web services. When it was released in 1996, it was designed to run on UNIX-like platforms only. However, it can now run on Windows and macOS, among other systems. PostgreSQL has the following features: user-defined types, table inheritance, a sophisticated locking mechanism, multi-version concurrency control, and asynchronous replication.

Additionally, it is designed to be extensible, so it is possible to develop custom plugins to meet your requirements.