Method and apparatus for storing, retrieving, and processing customer-oriented data sets in which relatively large sets of customers and their transactions or the like may be quickly and efficiently analyzed. A multi-dimensional access structure is utilized in which each cell representing one dimension element or a combination of dimension elements may include a list of customers who have made purchases or other transactions associated with that cell. Each customer record in a list may summarize predetermined information pertaining to that customer's behavior in the cell. Such records may be sorted by customer number to enable efficient combining of multiple lists. In applications wherein the number of such cells is prohibitively large, only a portion of these cells may actually include materialized lists. Additionally, a user-specified subset of the cells may be efficiently populated from input data, and lists which are not materialized may be materialized on demand from other lists in an efficient manner.