Ability to choose which database to connect to in Redis, apart from 0 being the default.

Select the Redis logical database having the specified zero-based numeric index. New connections always use the database 0.

Selectable Redis databases are a form of namespacing: all databases are still persisted in the same RDB / AOF file. However different databases can have keys with the same name, and commands like FLUSHDB, SWAPDB or RANDOMKEY work on specific databases.