Techniques are provided for bitmap-based computation of a COUNT(DISTINCT) function, where the bitmaps are generated based on ranks of target expression values. According to an embodiment, the ranks are computed using the DENSE_RANK function. The bitmaps may be maintained in a materialized view. Bitmap data that represents the ranks for target expression values occurring in data for a given group is divided across multiple bucket bitmaps, each corresponding to a distinct sub-range of the ranks. According to an embodiment, target expression value ranks are computed relative to partitions of the target expression values. When these partitions correspond to a subset (not necessarily strict) of the target query grouping keys for a query rewrite, the resulting bitmaps allow computation of multiple levels of aggregation from the single set of bitmaps.