1. A computer memory for storing data for access by an application program being executed on a data processing system, comprising: a hierarchical data structure stored in said memory, said data structure storing an ordered set of keys and comprising a root node and a plurality of first level data structures, a subset of said ordered set of keys uniquely associated with respective ones of said first level data structures; and each of said first level data structures having associated therewith a count value representing a number of entries of an associated one of said subsets.