RDBMS systems are not designed for storing keys, but are designed for storing data. An encrypted key blob is indistinguishable from 'data'. And, they have backup and failover systems, are fault-tolerant, support load-balancing, and are optimized for search and retrieval.
An RDBMS (i.e, in MySQL, SQL Server, Oracle, etc) is not provided by Utimaco as an option, as a back-end for its external keystores.
Depending on configuration, an RDBMS-based solution can store more than 1 billion individual keys.