1. A method of obtaining an estimate of a set-expression cardinality relating to at least a first and second data-stream, the method comprising the steps of: creating a first and a second hash-sketch synopsis comprising a first and a second random hash-table and a first-level hash table for each hash-bucket of said random hash-tables; maintaining said first and said second hash-sketch synopsis using one or more data elements from said first and second data-streams respectively; obtaining a set-expression singleton count over said first and second hash-sketch; and estimating said set-expression cardinality estimate using said set-expression singleton count.