• SQL Server provides parallel queries processing 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.
• After exchange operators are inserted, the result is a parallel-query execution plan.
• A parallel query processing 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.
• A serial execution plan is considered faster than any possible parallel execution plan for the particular query.

Configure the max degree of parallelism option

• In Object Explorer, right-click a server and select Properties.
• Click the Advanced node.
• In the Max Degree of Parallelism box, select the maximum number of processors to use in parallel plan execution.