Heyy, is this regarding Appwrite? Appwrite currently uses MariaDB under the hood, but Postgres and MongoDB adapters are on the way 💪 For now your database will be using MariaDB.
If this is not regarding Appwrite, the database choice strongly depends on team experience and project needs. If you are not experienced with any database service, I would recommend MariaDB, if you want your data to have proper structure and relations. If you are more of a NoSQL guy, MongoDB would be my recommendation. These are technologies I know have a huge community around them, so you should be able to all of your problems with a quick google search.