I'm using bsddb in a Python script as basically a drop in replacement for a dictionary that's too large to fit into memory. Looking at the documentation for the Python wrapper, I can use either bsddb.hashopen, bsddb.btopen, or bsddb.rnopen, but I don't know what the difference between these formats is. I assumed that this link to some page on oracle.com from that documentation page would give more details, but it's a 404.
BTREE is supposed to stay fully in memory. In you case you need bsddb.hashopen.
Checkout the oracle documentation for more information.