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.
7. The method of claim 2 wherein said set expression is a union of said first and second data-streams; and wherein said step of obtaining a set-expression singleton count over said first and second hash-sketch comprises the steps of: determining a smallest first-level hash-bucket index i at which the number of non-empty hash-buckets is less than or equal to a predetermined value; and generating a set-union cardinality estimate of said first and second data-streams using said index i and said number of none empty hash buckets.