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: using a database management system comprising a computer for: creating a first hash-sketch synopsis for the first data stream and a second hash-sketch synopsis for the second data stream, each hash-sketch synopsis comprising a random hash-table and a 2-level hash sketch for each hash-bucket of said random hash-tables, said 2-level hash sketch comprising a first-level hash-table and a counter away for each hash-bucket of said first-level hash-table; pre-hashing said first and second data-streams into said first and second random hash tables, respectively; hashing individual buckets of said random hashing tables to the corresponding 2-level hash sketch for each of those buckets; 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.