MySQL/MariaDB are the most popular choices as they are more popular among plugin developers and is supported in more plugins thank mongo. Also, I don't believe oracle has a database software? I think you are mistaking it for something else. Oracle is the company that currently develops java. EDIT: Look at below post about Oracle. Also, keep in mind, MariaDB and MySQL are the same thing. MariaDB is just better performance wise, like paper has more performance options than spigot. (Afaik). Plugins will work the same with mariadb as they would with mysql, even if the plugin says it only supports mysql, mariadb will work with it.
It's unlikely many use cases will cause bottlenecks with any of those database software. MySQL is a popular choice, and MariaDB is a fork of MySQL with more performance optimisations. PostgreSQL is a more advanced database. MongoDB is not a relational database, it is NoSQL and does not perform in the same way. Different database software and types may be more suitable for different use cases. However, as far as performance goes, neither of the solutions you mentioned will cause a bottleneck for a "big minecraft server".
if big means more than 1k players, i recommend NoSql, but warning with that, maybe you should take a lot of informations before starting using it
MySQL is not bad... It is made to work with more than any Minecraft server will exceed. No server will grow to a point where MySQL is the bottleneck. NoSQL is completely different to RDS. RDS is nicer to use for most cases.
Yea, if you are using MySQL you really don't have to worry about it. MySQL is used in much heavier traffic applications than Minecraft without problems. If you are very paranoid, use Oracle
