Specifying Max Degree of Parallelism in SQL Server for a Query


By:   |   Updated: 2006-08-24   |   Comments   |   Related: More > Query Optimization


Problem

The Max Degree of Parallelism or MAXDOP is a configuration indicating how the SQL Server optimizer will use the CPUs.  This is a server wide configuration that by default uses all of the CPUs to have the available portions of the query executed in parallel.  MAXDOP is very beneficial in a number of circumstances, but what if you have a reporting like query that runs in an OLTP system that monopolizes much of the CPU and adversely affects typical OLTP transactions.  How can you scale back the CPU usage just for this query?

Solution

Luckily SQL Server 2000 and 2005 provide the OPTION clause which among other configurations, offers the opportunity to specify the MAXDOP for the specific query.  This means that a fixed number of CPUs is used for the specific query overriding the server configuration. 

If you leave MAXDOP enabled, which is the default, and need to disable it on particular queries, leverage the OPTION code to achieve non parallel processing.  This may benefit the situation where OLTP processing is impacted by reporting or batch processing.  If you have already disabled MAXDOP, then the OPTION clause can enhance particular queries. 

How do I configure the Max Degree of Parallelism at the server level?

SQL Server 2000 SQL Server 2005
  1. Open SQL Server 2000 Enterprise Manager
  2. Once the tool loads, navigate to the intended server and right click on server name
  3. Select the 'Properties' option
  4. Select the 'Processor' tab
  5. Review the configurations for the 'Parallelism' section
  1. Open SQL Server 2005 Management Studio
  2. Once the tool loads, navigate to the intended server in the Object Explorer and right click on server name
  3. Select the 'Properties' option
  4. Select the 'Advanced' page from the left navigation
  5. Review the configurations for the 'Parallelism' section
How to configure the number of processors available for parallel queries (Enterprise Manager) How to: Configure the Number of Processors Available for Parallel Queries (SQL Server Management Studio)

 

How can I specify the Max Degree of Parallelism at the query level?

SQL Server 2000 SQL Server 2005
SELECT *
FROM Pubs.dbo.Authors
OPTION (MAXDOP 1)
 
SELECT *
FROM Pubs.dbo.Authors
OPTION (MAXDOP 1) ;
 
OPTION Clause Query Hint (Transact-SQL)

 

Next Steps


Last Updated: 2006-08-24


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About the author
MSSQLTips author Jeremy Kadlec Jeremy Kadlec is the Co-Founder, Editor and Author at MSSQLTips.com, CTO @ Edgewood Solutions and a six time SQL Server MVP.

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