Indexes play an important role in database performance optimization. An index is a special data structure that helps to make searches more effective. In db4o, as in many relational databases, indexes are based on B-Trees. You can define an index for any object field (including private ones).
Several indexes on different fields of the same class will result in better searching performance. Unlike relational databases db4o does not allow indexes over a set of fields. For more information see Indexing and Enable Field Indexes.