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Object Qualification

I came across an interesting issue recently with NHibernate, now it is widely known I despise ORM’s, in my experience they do a pretty mediocre job at best and at times can be absolutely horrific.  The issue was that the statements being fired at an instance of SQL Server from an application using NHibernate were not schema qualified.  Now this is not a rant at ORM’s as the issue I will show below is experienced with stored procedures, ad-hoc sql and any T-SQL you execute against SQL Server for that matter.  In fact I will be using a stored procedure in the example ;)

Now for those of you that don’t know SQL Server has to do an awfull lot of work before a statement is actually executed, here I want to show you the performance improvements that can be achieved by schema qualifying your objects.  The below quote is from Microsoft and will set the scene for the rest of the post.

"If user "dbo" owns object dbo.mystoredproc, and another user "Harry" runs this stored procedure with the command "exec mystoredproc," the initial cache lookup by object name fails because the object is not owner-qualified. (It is not yet known whether another stored procedure named Harry.mystoredproc exists, so SQL cannot be sure that the cached plan for dbo.mystoredproc is the right one to execute.) SQL Server then acquires an exclusive compile lock on the procedure and makes preparations to compile the procedure, including resolving the object name to an object ID. Before it compiles the plan, SQL Server uses this object ID to perform a more precise search of the procedure cache and is able to locate a previously compiled plan even without the owner qualification.
 If an existing plan is found, SQL Server reuses the cached plan and does not actually compile the stored procedure. However, the lack of owner-qualification forces SQL to perform a second cache lookup and acquire an exclusive compile lock before determining that the existing cached execution plan can be reused. Acquiring the lock and performing lookups and other work that is needed to get to this point can introduce a delay that is sufficient for the compile locks to lead to blocking. This is especially true if a large number of users who are not the stored procedure's owner simultaneously run it without supplying the owner name. Note that even if you do not see SPIDs waiting on compile locks, lack of owner-qualification can introduce delays in stored procedure execution and unnecessarily high CPU utilization."

Script

To demonstrate this I used the below script, I am running SQL Server 2008 R2 developer edition on my local instance and used the AdventureWorks2008R2 databasewhich is available here.

The script creates a schema called Chris in the AdventureWorks2008R2 database, a user SQL365\Chris is created for the login SQL365\Chris with the default schema of Chris.  Finally a procedure called dbo.spGetSalesOrderHeader is created that returns every record from AdventureWorks2008R2.dbo.SalesOrderHeader.

/*
      -----------------------------------------------------------------
      Object Qualification
      -----------------------------------------------------------------
   
      For more SQL resources, check out SQLServer365.blogspot.com

      -----------------------------------------------------------------

      You may alter this code for your own purposes.
      You may republish altered code as long as you give due credit.
      You must obtain prior permission before blogging this code.

      THIS CODE AND INFORMATION ARE PROVIDED "AS IS"
    
      -----------------------------------------------------------------
*/
-- Set Database Context
USEAdventureWorks2008R2;
GO
-- Declare variable
DECLARE @SQL VARCHAR(255)

-- Set variable
SET @SQL = 'CREATE SCHEMA Chris AUTHORIZATION dbo'

-- Create Schema
IF NOT EXISTS ( SELECT  1
                FROM    sys.schemas
                WHERE   name = 'Chris' )
    BEGIN
        EXEC (@SQL)
    END
GO

-- Create user mapped to login with default schema of the above created schema
IF NOT EXISTS ( SELECT  1
                FROM    sys.database_principals
                WHERE   name = 'SQL365\chris' )
    BEGIN
        CREATE USER [SQL365\Chris] FORLOGIN [SQL365\Chris] WITHDEFAULT_SCHEMA =Chris;
    END
GO

-- Create procedure in dbo schema to be executed by the above user
IF EXISTS ( SELECT  1
            FROM    sys.objects
            WHERE   [object_id] =OBJECT_ID(N'[dbo].[spGetSalesOrderHeader]')
                    ANDtype IN ( N'P', N'PC' ) )
    DROP PROCEDURE [dbo].[spGetSalesOrderHeader]
GO
CREATE PROCEDURE dbo.spGetSalesOrderHeader
AS
    BEGIN
        SELECT  SalesOrderID ,
                RevisionNumber ,
                OrderDate ,
                DueDate ,
                ShipDate ,
                [Status] ,
                OnlineOrderFlag ,
                SalesOrderNumber ,
                PurchaseOrderNumber ,
                AccountNumber ,
                CustomerID ,
                SalesPersonID ,
                TerritoryID ,
                BillToAddressID ,
                ShipToAddressID ,
                ShipMethodID ,
                CreditCardID ,
                CreditCardApprovalCode ,
                CurrencyRateID ,
                SubTotal ,
                TaxAmt ,
                Freight ,
                TotalDue ,
                Comment ,
                rowguid ,
                ModifiedDate
        FROM    sales.SalesOrderHeader
    END
GO

I use a great tool SQLQueryStress developed by Adam Machanic (B - T) quite frequently when testing the effects of changes under load, it is ingeniously simple to use and I love it.  I used SQLQueryStress to record the results of executing the procedure and without schema qualification and with schema qualification, I used 4 threads (the number of cores in my laptop) and ran a thousand iterations to get a good average.  Results of which are included below;

Non Schema Qualified



Schema Qualified


As you can see the results are pretty damn conclusive every metric measured by SQLQueryStress saw a performance improvement by schema qualifying objects.

Run Time - 27.01%Improvement
ClientSeconds/Iteration (Avg) - 11.99% Improvement
CPU Seconds/Iteration (Avg) - 1.13% Improvement
Actual Seconds/Iteration (Avg) - 12.54% Improvement

There is no excuse for not schema qualifying your objects, performance improvements like this just cannot be ignored.

Enjoy!

Chris

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