Parallel external sort–merge is an alternative to range partitioning. Suppose that a relation has already been partitioned among disks D0,D1, . . .,Dn−1 (it does not matter how the relation has been partitioned). Parallel external sort–merge then works this way:

1. Each processor Pi locally sorts the data on disk Di.
2. The system then merges the sorted runs on each processor to get the final sorted output.