In Microsoft SQL Server, the concept of parallel execution is known as parallelism. SQL Server automatically decides whether to use parallelism based on factors such as query cost and available system resources. However, you can use query hints to influence the decision-making process. The relevant query hint for parallelism is OPTION (MAXDOP).
Here's how you can use the OPTION (MAXDOP) hint in SQL Server:
SELECT * FROM TableName OPTION (MAXDOP 4);
In this example:
- TableName is the name of the table from which you want to retrieve data.
- MAXDOP 4 specifies the maximum degree of parallelism. In this case, SQL Server is instructed to use up to 4 parallel workers to execute the query. You can adjust the number according to your system's resources and query requirements.
It's important to note the following when using the MAXDOP hint:
1. The MAXDOP hint affects the maximum degree of parallelism for the entire query.
2. Setting MAXDOP to 0 (or not specifying it) allows SQL Server to automatically determine the degree of parallelism based on server configuration and query cost.
3. Setting MAXDOP to 1 disables parallelism for the query.
4. Specifying a high degree of parallelism may not always lead to better performance and can potentially degrade performance due to increased resource contention and overhead.
5. You should test and benchmark different MAXDOP settings to find the optimal value for your workload and system configuration.
Additionally, you can also configure the maximum degree of parallelism at the server level using the max degree of parallelism option in SQL Server Configuration Manager or with the sp_configure system stored procedure. This server-wide setting controls the maximum number of processors that SQL Server can use for parallel execution plans.
