By default, ISPConfig only supports MySQL databases. In addition, SQLite databases are supported as an extension of PHP — although SQLite is a different animal than MySQL entirely. It is possible to install PostgreSQL on the same server as ISPConfig, but the two products will not integrate on their own and you’ll have to build bridges between the two for things to work properly. In other words, PostgreSQL can be installed and used alongside ISPConfig, but only by experienced Linux sysadmins (or extremely persistent and capable Linux server hobbyists).