16. The method of claim 2 wherein said first and second data-streams are update streams having data elements inserted and deleted.
2. The method of claim 1 wherein said maintaining said step of first and second hash-sketch synopsis comprises the steps of: randomly hashing said data element from the data-element domain to said random hash-bucket; and logarithmically hashing said data-element from said random hash-bucket to said first-level hash-bucket, and wherein the probability of logarithmically hashing said data-elements to said first-level hash-bucket decreases exponentially with the index of said first-level hash-bucket.