Abstract: The present invention provides systems and methods for performing efficient k-NN approximate similarity search on a database of objects. The invention is based on the definition of an index data structure that enables to have fast searches and very good scalability with respect to the database size. Such index makes efficient use of both the main and secondary memory of the computer, taking advantage of the specific properties of both kinds of memories. A prefix tree is built on all the sequences assigned to the database objects by a sequence generation function. The prefix tree is stored in the main memory. The information required to identify each database object and to compute the similarity between database objects and query objects are stored in a data storage kept in the secondary memory. Given a query object and the request for the k nearest neighbors, the search functionality of the invention uses the prefix tree to quickly identify a set of candidate objects.