14. A computer-readable storage medium encoded with a computer program product for implementing aggregation combination using rollup depth lists for optimizing database query processing in a computer system, said computer-readable storage medium encoded with said computer program product including instructions executed by the computer system to cause the computer system to perform the steps of: identifying grouping sets and combining multiple grouping sets into multiple single rollups; each grouping set including a collection of elements; assigning rollup depth lists defining the levels of grouping to be performed for each of said multiple single rollups; each said rollup depth list including respective element counts for each element; wherein identifying grouping sets and combining multiple grouping sets into multiple single rollups and assigning rollup depth lists includes: identifying a list of source groups; each source group including a collection of elements; identifying one source group, reading the source group, initially assigning each source group element a count of zero and assigning a last element a count of one; storing the source group within a target group list; identifying one target group, processing the stored source group and the identified target group in the target group list; such that the source group elements are sorted to match element order of target group elements; and combining the processed stored source group with the processed identified target group into a single rollup, wherein the element counts of the rollup depth list for said single rollup are determined based on said combining; generating an access plan utilizing said multiple single rollups; and said corresponding rollup depth lists; providing an execution engine executing a database query with said access plan; and said execution engine using respective element counts of each rollup depth list to identify how many times to perform each level of aggregation of elements within each of said multiple single rollups, optimizing database query processing.
14. A computer-readable storage medium encoded with a computer program product for implementing aggregation combination using rollup depth lists for optimizing database query processing in a computer system, said computer-readable storage medium encoded with said computer program product including instructions executed by the computer system to cause the computer system to perform the steps of: identifying grouping sets and combining multiple grouping sets into multiple single rollups; each grouping set including a collection of elements; assigning rollup depth lists defining the levels of grouping to be performed for each of said multiple single rollups; each said rollup depth list including respective element counts for each element; wherein identifying grouping sets and combining multiple grouping sets into multiple single rollups and assigning rollup depth lists includes: identifying a list of source groups; each source group including a collection of elements; identifying one source group, reading the source group, initially assigning each source group element a count of zero and assigning a last element a count of one; storing the source group within a target group list; identifying one target group, processing the stored source group and the identified target group in the target group list; such that the source group elements are sorted to match element order of target group elements; and combining the processed stored source group with the processed identified target group into a single rollup, wherein the element counts of the rollup depth list for said single rollup are determined based on said combining; generating an access plan utilizing said multiple single rollups; and said corresponding rollup depth lists; providing an execution engine executing a database query with said access plan; and said execution engine using respective element counts of each rollup depth list to identify how many times to perform each level of aggregation of elements within each of said multiple single rollups, optimizing database query processing. 15. The computer-readable storage medium encoded with a computer program product for implementing aggregation combination using rollup depth lists for optimizing database query processing as recited in claim 14 further includes implementing cube aggregation conversion to rollup aggregation for optimizing database query processing.