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SQL Server 2016 Database Scoped Configurations for MAXDOP, Procedure Cache and Cross Database Queries


By:   |   Last Updated: 2016-05-04   |   Comments   |   Related Tips: More > SQL Server 2016

Problem

In an earlier tip SQL Server 2016 Database Scoped Configuration, we provided an overview of the important parameters that can be set at the database level which were previously available at the instance level only. In this tip we are going to explore this functionality in more detail.

Solution

Max Degree of Parallelism as a Database Scoped Configuration

By default, Max Degree of Parallelism (MAXDOP) is set to value 0 at the instance level, as shown below.

Max Degree of Parallelism at an Instance Level

For demo purposes I am going to use the AdventureWorks2016CTP3 database. If we check the database scope parameter default values we can see 0 = Disabled and 1 = Enabled.

Querying the sys.database_scoped_configurations DMV

If we run the below query with the default options we see the following execution plan.

SELECT *
FROM Sales.SalesOrderDetail sod
INNER JOIN Production.Product p ON sod.ProductID = p.ProductID
ORDER BY Style
GO

Review the query plan MAX DOP setting

We can see that a parallel execution plan is being created for the query with a Degree of Parallelism of 4.

Now suppose we don't want the Max Degree of Parallelism to be 4 and we want to generate a serial plan. To do so we would set the MAXDOP=1 for this database as shown below.

ALTER DATABASE SCOPED CONFIGURATION SET MAXDOP = 1;

And we verify this setting as shown below.

Validate the MAXDOP Database Scoped Configuration

Now if we run the same query, we can see this time a serial plan is generated and the optimizer shows why a parallel plan is not selected.

Degree of Parallelism Setting for a Query

Similarly if we set MAXDOP=2, we can see this setting effects the query execution.

Degree of Parallelism Setting after setting MAXDOP=2

Now we will set MAXDOP = 0 ( default value ) and see the Cardinality Estimation is set to 130.

Cardinality Estimation Model Version

ALTER DATABASE SCOPED CONFIGURATION SET LEGACY_CARDINALITY_ESTIMATION = ON;

Cardinality Estimation Model Version set to 70

So we can see here that the database engine is now using the legacy cardinality estimation instead of cardinality estimation for SQL Server 2016.

As we can see a value of 70 after setting the Legacy Cardinality estimator ON. This indicates that the cardinality estimation model being used is pre-SQL Server 2016. The cardinality estimator had its last major change back in SQL Server 7.0 (70).

Clear SQL Server Database Procedure Cache

Let's see the current procedure cache for our demo database AdventureWorks2016CTP3 with the following query:

SELECT
    d.name AS [db_name],
    count(*) as nb_cached_entries
FROM sys.dm_exec_cached_plans AS cp
CROSS APPLY (
    SELECT CAST(pa.value AS INT) AS database_id
    FROM sys.dm_exec_plan_attributes(cp.plan_handle) AS pa
    WHERE pa.attribute = N'dbid'
) AS DB
INNER JOIN sys.databases AS d
    ON d.database_id = DB.database_id
	where d.database_id=8 --database id for AdventureWorks2016CTP3
GROUP BY d.name
ORDER BY d.name

Query the Procedure Cache

Now if we clear the procedure cache on the AdventureWorks2016CTP3 database we can see 0 rows returned i.e. procedure cache has been cleared.

ALTER DATABASE SCOPED CONFIGURATION CLEAR PROCEDURE_CACHE;
GO

Validate that the Procedure Cache has been cleared

We can see the there are no records returned now i.e. the database level cache has been cleared. This is a very interesting option because previously we could only clear the procedure cache at the instance level.

Cross Database Queries and MAXDOP as a Database Scoped Configuration

Now we will see the MAXDOP behavior when more than one database is involved in a SQL Server query. To test I have created another test database and exported the data for the production.product table.

Example 1 - MAXDOP Disabled for the Database Scope Configuration

If both the databases are configured with MAXDOP=0 as the Database Scope Configuration, then the instance level MAXDOP setting is used as shown below.

SELECT *
FROM AdventureWorks2016CTP3.Sales.SalesOrderDetail sod
INNER JOIN Testdb.Production.Product p ON sod.ProductID = p.ProductID
ORDER BY Style
GO

Review the Degree of Parallelism for Queries Issued from two databases

We can see in both databases that the query is being executed with MAXDOP=4.

Example 2 - Different MAXDOP values for the Database Scope Configuration

Now we will change the MAXDOP configuration for both databases, so they are using different MAXDOP values.

Use AdventureWorks2016CTP3
ALTER DATABASE SCOPED CONFIGURATION SET MAXDOP = 3
GO
Use TestDB
ALTER DATABASE SCOPED CONFIGURATION SET MAXDOP = 2
GO

Different MAXDOP values for the Database Scope Configuration

So we can see if the same query is being executed against the AdventureWorks2016CTP3 database, MAXDOP is 3 while executed against the TestDB it is 2 which is per our settings.

Example 3 - Different MAXDOP values for the Database Scope Configuration

Use TestDB
go
ALTER DATABASE SCOPED CONFIGURATION SET MAXDOP = 1

Serial query plan when the query being executed in the TestDB database

Here we get a serial plan for the query being executed in the TestDB database as we have set MAXDOP = 1 for TestDB.

The SQL Server database where the T-SQL code executes will dictate the MAXDOP setting. So be careful while configuring this option on production servers and enable this setting with proper testing of the workload.

Next Steps


Last Updated: 2016-05-04


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About the author
MSSQLTips author Rajendra Gupta Rajendra is a Consultant DBA with 9+ years of extensive experience in database administration including large critical OLAP, OLTP, Reporting and SharePoint databases.

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