1. A parallel database-management system comprising: two or more computer systems, each including multiple processors, shared memory, and one or more of internal mass-storage and attached mass-storage; a workload-management-services component that monitors usage of computational resources in the parallel database-management system and that provides a query-processing-task-management interface; and a query-execution engine that receives query-processing requests associated with one of a number of services from host computers and accesses the workload-management-services component to determine whether to immediately launch execution of query-processing tasks corresponding to the received query-processing requests or to place the query-processing requests on wait queues for subsequent execution based on the current usage of computational resources within the parallel database-management system.