Tag Archives: hint

Playing with OPTION KEEPFIXED PLAN hint in SQL Server

My previous article was on Compilation & Recompilation of Stored Procedure. After reading that article, two of my reader sent me an email about more clarification on some terminology and that is the base of writing this article.
Actually SQL Server Query Optimizer is really one of the smart tools which used to find or generate best execution plan for query or stored procedure and make the task easy for DBA. Basically it uses Cost Based Optimizer (CBO) to do this task, even sometime feels to have specific Execution Plan for query so Microsoft have provided many different hints at different level.
Query Optimizer (QO) used to save execution plan when it compiles Stored Procedure first time and use the same execution plan when the same stored procedure is being used again so that you can get rid of overhead of generating execution plan again and again.
When index statistics changes heavily, QO used to recompile the statement to get best plan for your stored procedure but if you are sure that the existing Plan would works fine than one should go for OPTION KEEPFIXED PLAN for that statement so that you can save your resources and boost up performance.
I will demonstrate this with few queries run in SQL Server Management Studio (SSMS) and capture the results in Profiler.
I will select following Events in Profiler while creating new Trace for my server.
Stored Procedure
  • SP:Completed
  • SP:Recompile
  • SP:smtpCompleted
  • SP:smtpStarting

 

Now execute following TSQL in your SSMS.
–create one database which you can delete after running this example
create database SQLHub
GO
USE SQLHub
GO
–if orders table is already there. you can delete it than create new one with name “Orders”
IF OBJECT_ID(‘orders’, ‘U’) IS NOT NULL BEGIN
      DROP TABLE orders
END
GO
–creating table
CREATE TABLE orders(OrderID INT IDENTITY, OrderDate DATETIME, Amount MONEY, Refno INT)
GO
–creating nonclustered index
CREATE NONCLUSTERED INDEXIDX_ORD ON Orders(amount)
GO
–inserting only 1 record in our table
INSERT INTO orders VALUES(GETDATE(),1001,111)
GO
–creating SP which will SELECT all records from Orders table where Amount is 1001
CREATE PROC spGet_ord
AS
SELECT * FROM orders WHERE Amount=1001
GO
–execute SP
exec spGet_ord
GO
If you will see in profiler than you would get following event captured
Now, make change in Index statistics by inserting so many records in table so that we can test the effect in profiler after executing SP again.
–inserting 50000 fack rows into table
INSERT INTO orders(OrderDate, Amount, Refno)
SELECT TOP 50000
      DATEADD(minute, ABS(a.object_id % 50000 ), CAST(‘2010-02-01’ AS DATETIME)),
      ABS(a.object_id % 10),
      CAST(ABS(a.object_id) AS VARCHAR)
FROM sys.all_objects a
CROSS JOIN sys.all_objects b
GO
–execute SP again
exec spGet_ord
GO
Now, if you see the profiler, you will see “SP:Recompile” event as your statistics are updated so it need recompilation of statement. If you again few more thousands record in same table and execute the same SP again, you will see recompilation again.
Now, after droping table and SP, we will make one small change in Stored Procedure and will use “KEEPFIXED PLAN” hint so that we can avoid recompilation.
drop proc spGet_ord
GO
drop table orders
go
–creating table
CREATE TABLE orders(OrderID INT IDENTITY, OrderDate DATETIME, Amount MONEY, Refno INT)
GO
CREATE NONCLUSTERED INDEXIDX_ORD ON Orders(amount)
GO
INSERT INTO orders VALUES(GETDATE(),1001,111)
GO
CREATE PROC spGet_ord
AS
SELECT * FROM orders WHERE Amount=1001
–adding below statement other than that, same code as previous SP
OPTION (KEEPFIXED PLAN);
GO
exec spGet_ord
GO
After recreating table along with fresh records, we are going to see what happened while executing SP in above statement.
Since our table now has only 1 record so it is time to change statistics with bulk insert as follows again.
–inserting 100000 fack rows into table (BTW, thank to Jacob Sebastian, copy in INSERT script from one of his demo to generate big table)
INSERT INTO orders(OrderDate, Amount, Refno)
SELECT TOP 50000
      DATEADD(minute, ABS(a.object_id % 50000 ), CAST(‘2010-02-01’ AS DATETIME)),
      ABS(a.object_id % 10),
      CAST(ABS(a.object_id) AS VARCHAR)
FROM sys.all_objects a
CROSS JOIN sys.all_objects b
GO
exec spGet_ord
GO
–if you wish, you can uncomment below code and delete SQLHub database
–use master
–go
–drop database sqlhub
Here is the screen capture of Profiler which doesn’t show Recompilation.

 

You can even add few more thousands of rows and execute SP again, you won’t get recompilation.
Reference: Ritesh Shah
 
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