This is just a confusion of terms I think. I don't think "instance owning database partition" is the same thing at all, it sounds like something happening inside the database, not a partition outside of it; and a database won't be able to do much with a partition, but could store files inside its filesystem if you give it one. Usually, only root has access to the partitions; the filesystems inside them keep their own counsel so to speak.

A partition in OS terms is just a slice of disk space. It acts like a huge file of fixed size. A database wouldn't find this too useful unless you have something extremely specific in mind, nor would most other things, they'd rather create many files of smaller size and directories to order them; so usually a filesystem is created in these partitions.

Once the filesystem is created inside the partition, it can be mounted by the OS, after which it acts just like a directory. It has a user and group like any other directory and can be owned by whatever user you want irrespective of who has raw access to the partition.