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Parallel Query Processing

SQL Server provides parallel queries to optimize query execution and index operations for computers that have more than one microprocessor (CPU). Because SQL Server can perform a query or index operation in parallel by using several operating system threads, the operation can be completed quickly and efficiently.

During query optimization, SQL Server looks for queries or index operations that 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. The exchange operator includes the Distribute Streams, Repartition Streams, and Gather Streams logical operators as subtypes, one or more of which can appear in the Showplan output of a query plan for a parallel query.

After exchange operators are inserted, the result is a parallel-query execution plan. A parallel-query execution plan can use more than one thread. A serial execution plan, used by a nonparallel query, uses only one thread for its execution. The actual number of threads used by a parallel query is determined at query plan execution initialization and is determined by the complexity of the plan and the degree of parallelism. Degree of parallelism determines the maximum number of CPUs that are being used; it does not mean the number of threads that are being used. The degree of parallelism value is set at the server level and can be modified by using the sp_configure system stored procedure. You can override this value for individual query or index statements by specifying the MAXDOP query hint or MAXDOP index option.

The SQL Server query optimizer does not use a parallel execution plan for a query if any one of the following conditions is true:

  • The serial execution cost of the query is not high enough to consider an alternative, parallel execution plan.

  • A serial execution plan is considered faster than any possible parallel execution plan for the particular query.

  • The query contains scalar or relational operators that cannot be run in parallel. Certain operators can cause a section of the query plan to run in serial mode, or the whole plan to run in serial mode.