An important feature of database technology of the nineties is the use of distributed computation for speeding up the execution of complex queries. Today, the use of parallelism is tested in several experimental database architectures and a few commercial systems for conventional select-project-join queries. In particular, hash-based fragmentation is used to distribute data to disks under the control of different processors, in multi-processor architectures without shared memory, in order to perform selections and joins in parallel.