Microsoft® SQL Server™ provides parallel queries to optimize query execution for computers having more than one processor. By allowing SQL Server to perform a query in parallel by using several operating system threads, SQL Server completes complex queries with large amounts of data quickly and efficiently.
During query optimization, SQL Server looks for queries which might benefit from parallel execution. For these queries, SQL Server inserts exchange operators into the query execution plan to prepare the query for parallel execution. An exchange operator is an operator in a query execution plan that provides process management, data redistribution, and flow control. After exchange operators are inserted, the result is a parallel query execution plan. A parallel query execution plan can use more than one thread, whereas a serial execution plan, used by a nonparallel query, uses only a single thread for its execution. The actual number of threads used by a parallel query is determined at query plan execution initialization and is called the degree of parallelism.