The Metadata Storage is an external dependency of Druid. Druid uses it to store various metadata about the system, but not to store the actual data. There are a number of tables used for various purposes.
Derby is the default metadata store for Druid, however, it is not suitable for production. MySQL and PostgreSQL are more production suitable metadata stores. In this post, I will show you how to configure druid to use PostgreSQL as the metadata storage.