Postgresql is a robust and ACID compliant relational database. The free version is fully functional and offers excellent performance. Also available from Platform as a Service providers like Heroku and Azure.
There is also a paid version that offers Enterprise functionality like high availability.
For the easiest way of turning a PostgreSQL database into a REST API, see: PostgREST.
Pro High performance
PostgreSQL performance increases with each release, this is also proven by many benchmark tests.
Notable performance features include:
As PostgreSQL only supports one storage engine, it has been able to integrate and optimise it and with the rest of the database. This has resulted in multiple benefits such as the ability to allow different transaction types to co-exist efficiently without the need to select storage engine types once for each table ahead of time.
On the fly data compression resulting in less IO required for reading.
Asynchronous + synchronous Replication.
PostgreSQL supports a asynchronous API for use by client applications. It is reported to increase performance by up to 40% and is not supported by MySQL.
Designed to scale very well with large numbers of cores at high concurrency levels.
Con pgAdmin 4
The newest version of pgAdmin, written in Python and jQuery with Bootstrap, using the Flask framework, is not an improvement, it's bloated and not very intuitive. Some alternatives can be found here
Sadly also in the installers for older versions like PostgreSQL 9.6 the original pgAdmin was replaced by this version.