Abstract: A metric space search method can include building a tree data structure representing a database and providing the metric space. The tree can include one or more nodes each having a cluster of one or more data points. Each cluster can have a center data point. During the building of the tree, nodes on one level of the tree can be permitted to overlap by containing mutual data points so long as an overlapping portion does not exhaust a metric subspace on that level. The method can also include searching the tree, one level at a time in a breadth-first manner, to locate a number of nearest neighbors to a query point and generating a list of candidate nearest neighbors during the searching. The method can also include using the list of candidate nearest neighbors to determine whether a portion of the tree is to be searched, and pruning the tree if it is determined that the portion should not be searched.