Time complexity: O(1)
Select the Dragonfly logical database having the specified zero-based numeric index. New connections always use the database 0.
Selectable databases are a form of namespacing: all databases are still persisted in the same snapshot files. However different databases can have keys with the same name, and commands like
RANDOMKEY work on specific databases.
In practical terms, Dragonfly databases should be used to separate different keys belonging to the same application (if needed), and not to use a single instance for multiple unrelated applications.
Since the currently selected database is a property of the connection, clients should track the currently selected database and re-select it on reconnection.