MTS Sketch for Accurate Estimation of Set-Expression Cardinalities from Small Samples.

A computer implemented method of estimating a cardinality of a stream, comprising: receiving a query for estimating a cardinality of a stream comprising a plurality of elements, obtaining a sample comprising a group of the plurality of elements randomly sampled from the respective stream, computing a first and second data structures for the sample used to compute an estimated sample cardinality of the sample and a ratio indicative of a proportion between the estimated sample cardinality and the estimated cardinality of the stream and computing the estimated cardinality of the stream by applying the ratio to the estimated sample cardinality. Where the first data structure comprises a plurality of maximal hash values computed for the sample using a plurality of hash functions and the second data structure comprises a fixed- size subset of the elements having a minimal hash value among the elements of the group.