1. A method comprising performing a machine-executed operation involving instructions, wherein the machine-executed operation is at least one of: A) sending said instructions over transmission media; B) receiving said instructions over transmission media; C) storing said instructions onto a machine-readable storage medium; and D) executing the instructions; wherein said instructions are instructions which, when executed by one or more processors, cause the one or more processors to perform the steps of: receiving a query that includes a particular operator, wherein the particular operator is one of MINUS and INTERSECT; generating a set of one or more transformed queries that produce the same result as said query but do not include said particular operator; generating cost estimates for the transformed queries in said set of transformed queries; and selecting from said set of transformed queries a particular transformed query to execute based, at least in part, on the cost estimates.