| 摘要 |
Various techniques are described for processing database commands that include MINUS and/or INTERSECT operators. The queries containing the MINUS and/or INTERSECT operators are transformed to create a plurality of transformed queries. Each of the transformed queries produces the same result as the original query, but does not include the MINUS and/or INTERSECT operator. To achieve the same result set as the original query, the transformed queries employ equijoins, antijoins, and/or semijoins, and duplicate elimination operations. Costs are estimated for each of the various transformed queries. Based on the cost estimates, one of the transformed queries is selected as the query that is to be executed to perform the operations specified in the original query.
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