27. The non-transitory computer-readable storage medium of claim 25 , wherein: said left-side expression of said rule identifies one or more cells in a second set of rows, wherein said second set of rows is included in said relation; said window function specifies one or more partition columns by which to partition said second set of rows into partitions; the instructions that cause said step of executing said query comprise instructions which, when executed by the one or more processors, cause the one or more processors to perform: based on said left-side expression, determining said second set of rows in said relation; and based on said one or more partition columns, partitioning said second set of rows into one or more subsets of rows; the instructions that cause said step of producing said first set of rows comprise instructions which, when executed by the one or more processors, cause the one or more processors to perform producing said first set of rows based on at least one of said one or more subsets of rows.
28. The non-transitory computer-readable storage medium of claim 27 , wherein: each of said one or more cells in said second set of rows holds a value that is determined by said right-side expression of said rule; and the instructions that cause said step of executing said query further comprise instructions which, when executed by the one or more processors, cause the one or more processors to perform: for each cell of said one or more cells, computing the value of said right-side expression of said rule based on information generated during said step of evaluating said window function.