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The DB->open function is the standard interface for opening a database, and takes five arguments:
The subdatabase name allows applications to have subdatabases, i.e., multiple logical databases inside of a single physical file. This is useful when the logical databases are both numerous and reasonably small, in order to avoid creating a large number of underlying files. Databases created as subdatabases are otherwise identical to any other database.
It is not an error to open a database file that contains subdatabases without specifying a subdatabase name, however it must be opened read-only. The handle that is returned from such a call is a handle on a database whose key values are the names of the subdatabases stored in the database file and whose data values are opaque objects. No keys or data values can be changed or stored through this database handle.
It is an error to attempt to open a subdatabase in a database file that was not initially created using a subdatabase name, i.e., a database file must be initially specified as holding subdatabases for a subdatabase to be created within it.
There are a few flags that you can set to customize open:
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