Which database Mendix does use?
According this document document Mendix uses HSSQL database and according to this document document Mendix uses PostgreSQL database .
For apps deployed to the Mendix Cloud, Mendix uses a PostgreSQL database for storing the data defined in the app domain model(s). If you are deploying to a different infrastructure, you will need to provide your own database.
See System Requirements for the list of supported databases.
see more about database support