The present application is directed to a data management system. The data management system is implemented within a computer system that includes one or more processors, one or more electronic memories, and a memory-mapped file facility that electronically stores data. The data management system, implemented by data management system control routines that are executed by the one or more processors, retrieves a particular projection of a multi-level, multi-dimension data set stored within the ONP-tree data structure that contains the multi-level, multi-dimension data set.