Berkeley DB Reference Guide:
  Access Methods

Hash table size

When setting up the hash database, knowing the expected number of elements that will be stored in the hash table is useful. This value can be used by the Hash access method implementation to more accurately construct the necessary number of buckets that the database will eventually require.

The anticipated number of elements in the hash table can be specified by calling the DB->set_h_nelem method. If not specified, or set too low, hash tables will expand gracefully as keys are entered, although a slight performance degradation may be noticed. In order for the estimated number of elements to be a useful value to Berkeley DB, the DB->set_h_ffactor method must also be called to set the page fill factor.

Copyright (c) 1996,2008 Oracle. All rights reserved.