According to http://www.sleepycat.com/docs/ref/am...bt_prefix.html
Berkeley DB is supposed to do prefix compression but it doesn't seem to
be working in my tests.

IIUC, adding a fixed prefix to all database keys is should increase the
database size by less than (prefix size)*(number of keys). The prefix
is supposed to be stored only once per index page. I tried it but found
no evidence of prefix compression. Running strings(1) on the database
shows the all keys with the prefix clearly visible.

Am I missing something here? I know that setting a custom comparison
function disables prefix compression but I am not using set_bt_compare.