1. A method comprising: identifying a comparison vector x in a set of vectors, the comparison vector x having non-zero features; determining an estimated similarity contribution of a particular feature of the comparison vector x to a similarity between the comparison vector x and each vector in the set of vectors, generating an index that includes the particular feature based on a comparison of the estimated similarity contribution of the particular feature with a similarity threshold; and identifying another vector in the set of vectors that is similar to the vector x using the index.