Berkeley DB

Opening Multiple Databases in a Single File

Applications may create multiple databases within a single physical file. This is useful when the databases are both numerous and reasonably small, when you want to avoid creating a large number of underlying files, or when it is desirable to include secondary index databases in the same file as the primary index database. Multiple databases are an administrative convenience, and using them is unlikely to affect database performance. To open or create a file that will include more than a single database, specify a database name when calling the DB→open method.

Physical files do not need to be comprised of a single type of database, and databases in a file may be of any type (Btree, Hash, or Recno) ...