9. A machine-executed method comprising the steps of: receiving a particular query that includes a MINUS operator; generating a plurality of transformed queries, wherein each query of said plurality of transformed queries, if executed, produces a same result as said query but does not include said MINUS operator; generating cost estimates for the transformed queries in said plurality of transformed queries; selecting from said plurality of transformed queries a particular transformed query to execute based, at least in part, on the cost estimates; wherein each of said steps of said method is performed by one or more computing devices; and wherein the step of generating a plurality of transformed queries includes generating a transformed query in which said particular operator is removed from said particular query, and in which said same result is produced with an anti-join operator.
9. A machine-executed method comprising the steps of: receiving a particular query that includes a MINUS operator; generating a plurality of transformed queries, wherein each query of said plurality of transformed queries, if executed, produces a same result as said query but does not include said MINUS operator; generating cost estimates for the transformed queries in said plurality of transformed queries; selecting from said plurality of transformed queries a particular transformed query to execute based, at least in part, on the cost estimates; wherein each of said steps of said method is performed by one or more computing devices; and wherein the step of generating a plurality of transformed queries includes generating a transformed query in which said particular operator is removed from said particular query, and in which said same result is produced with an anti-join operator. 12. The method of claim 9 wherein: the particular query specifies a left-hand source and a right-hand source for said MINUS operator; and the plurality of transformed queries includes a transformed query that is associated with an execution plan in which duplicate elimination is performed on the right-hand source before an anti-join is performed between the left-hand source and the right-hand source, and duplicate elimination is performed on a result set produced by said anti-join.