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cost threshold for parallelism Option

Use the cost threshold for parallelism option to specify the threshold at which Microsoft SQL Server creates and runs parallel plans for queries. SQL Server creates and runs a parallel plan for a query only when the estimated cost to run a serial plan for the same query is higher than the value set in cost threshold for parallelism. The cost refers to an estimated elapsed time in seconds required to run the serial plan on a specific hardware configuration. Only set cost threshold for parallelism on symmetric multiprocessors.

Longer queries usually benefit from parallel plans; the performance advantage negates the additional time required to initialize, synchronize, and terminate parallel plans. The cost threshold for parallelism option is actively used when a mix of short and longer queries is run. The short queries run serial plans, whereas the longer queries use parallel plans. The value of cost threshold for parallelism determines which queries are considered short, and they should therefore be run using serial plans.

In certain cases, a parallel plan may be chosen even though the query's cost plan is less than the current cost threshold for parallelism value. This can happen because the decision to use a parallel or serial plan is based on a cost estimate provided before the full optimization is complete.

The cost threshold for parallelism option can be set to any value from 0 through 32767. The default value is 5.

SQL Server ignores the cost threshold for parallelism value under the following conditions:

  • Your computer has only one processor.

  • Only a single CPU is available to SQL Server because of the affinity mask configuration option.

  • The max degree of parallelismoption is set to 1.

    NoteNote

    If MAXDOP query option is used, then SQL Server will not ignore the cost-threshold-for-parallelism value.

The cost threshold for parallelism option is an advanced option. If you are using the sp_configure system stored procedure to change the setting, you can change cost threshold for parallelism only when show advanced options is set to 1. The setting takes effect immediately (without a server restart).

The following example sets the cost threshold for parallelism to 10 seconds.

sp_configure 'show advanced options', 1;

GO

reconfigure;

GO

sp_configure 'cost threshold for parallelism', 10;

GO

reconfigure;

GO

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