A grouping mechanism is provided which can return distinct groups of entries of data that satisfy a users query in a non-blocking fashion. Each distinct group can normaly be returned to the user substantially concurrently with the following entries being received and processed. The grouping mechanism supports an overflow mechanism which can transfer parts of the data between a primary memory to a secondary memory to alleviate shortage of primary memory. This non-blocking mechanism is useful as a part of a dataflow model data processing system.