Parallelism

SQL Server can use parallelism to help execute some expensive queries faster by using several processors simultaneously. However, even when a query might get better performance by using parallel plans, it could still use more resources than a similar serial plan.

For the query optimizer to consider parallel plans, the SQL Server installation must have access to at least two processors or cores or a hyper-threaded configuration. In addition, both the affinity mask and the maximum degree of parallelism advanced configuration options must allow the use of at least two processors, which they do by default. Finally, as explained later in this section, SQL Server will only consider parallelism for serial queries whose cost exceeds the configured cost threshold for parallelism, where the default value is 5.

The affinity mask configuration option specifies which processors are eligible to run SQL Server threads, and the default value of 0 means that all the processors can...