If your RAM exceeds the size of your tables and indexes, that data will be served from RAM in any modern relational database system. No special config usually necessary for the speed but you don't lose everything when the power goes out, unlike tmpfs/ramdrive option.
That would depend on the DB server settings. Such a config might be found on a dedicated database server, but I doubt such settings would make sense on a machine running e.g. an application server together with the database.
