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; wherein, within the query, the operands to the particular operator include a left-hand source and a right-hand source; generating a plurality of query plans for executing said query, wherein each of the query plans includes a plan portion for executing, in parallel, the operation associated with said particular operator; generating cost estimates for the query plans in the set of one or more query plans; and selecting from said plurality of query plans a particular query plan to execute based, at least in part, on the cost estimates.