Locality sensitive hashing (LSH) To support range and kNN queries Idea of randomized algorithms   Algorithms that make random choices   Monte Carlo: Probabilistic but bounded error results in bounded time accuracy improves with each run   Las Vegas: Correct and deterministic result but varying time Goal in LSH: To find a hashing function that is approximately distance-preserving (within some tolerance) A hash function is -sensitive if for any ,,   If , then   If , then Can also be viewed as a dimensionality reduction technique