Similarity search

Similarity search is the most general term used for a range of mechanisms which share the principle of searching (typically very large) spaces of objects where the only available comparator is the similarity between any pair of objects. This is becoming increasingly important in an age of large information repositories where the objects contained do not possess any natural order, for example large collections of images, sounds and other sophisticated digital objects.

Nearest neighbor search and range queries are important subclasses of similarity search, and a number of solutions exist. Research in similarity search is dominated by the inherent problems of searching over complex objects. Such objects cause most known techniques to lose traction over large collections, due to a manifestation of the so-called curse of dimensionality, and there are still many unsolved problems. Unfortunately, in many cases where similarity search is necessary, the objects are inherently complex.

The most general approach to similarity search relies upon the mathematical notion of metric space, which allows the construction of efficient index structures in order to achieve scalability in the search domain.

Similarity search evolved independently in a number of different scientific and computing contexts, according to various needs. In 2008 a few leading researchers in the field felt strongly that the subject should be a research topic in its own right, to allow focus on the general issues applicable across the many diverse domains of its use. This resulted in the formation of the SISAP foundation, whose main activity is a series of annual international conferences on the generic topic.