Actually SQL Server Query Optimizer is really one of thesmart tools which used to find or generate best execution plan for query or storedprocedure and make the task easy for DBA. Basically it uses Cost BasedOptimizer (CBO) to do this task, even sometime feels to have specific ExecutionPlan for query so Microsoft have provided many different hints at differentlevel.
Query Optimizer (QO) used to save execution plan when itcompiles Stored Procedure first time and use the same execution plan when thesame stored procedure is being used again so that you can get rid of overheadof generating execution plan again and again.
When index statistics changes heavily, QO used to recompilethe statement to get best plan for your stored procedure but if you are surethat the existing Plan would works fine than one should go for OPTION KEEPFIXEDPLAN for that statement so that you can save your resources and boost upperformance.
I will demonstrate this with few queries run in SQL ServerManagement Studio (SSMS) and capture the results in Profiler.
I will select following Events in Profiler while creatingnew Trace for my server.
Stored Procedure
- SP:Completed
- SP:Recompile
- SP:smtpCompleted
- SP:smtpStarting
Now execute following TSQL in your SSMS.
–create onedatabase which you can delete after running this example
create database SQLHub
GO
USE SQLHub
GO
–if orderstable 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
–creatingnonclustered index
CREATE NONCLUSTERED INDEXIDX_ORD ON Orders(amount)
GO
–inserting only1 record in our table
INSERT INTO orders VALUES(GETDATE(),1001,111)
GO
–creating SPwhich 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 followingevent captured
Now, make change in Index statistics by inserting so manyrecords in table so that we can test the effect in profiler after executing SPagain.
–inserting50000 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 SPagain
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 youagain 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 smallchange in Stored Procedure and will use “KEEPFIXED PLAN” hint so that we canavoid 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 belowstatement other than that, same code as previous SP
OPTION (KEEPFIXED PLAN);
GO
exec spGet_ord
GO
After recreating table along with fresh records, we aregoing to see what happened while executing SP in above statement.
Since our table now has only 1 record so it is time tochange statistics with bulk insert as follows again.
–inserting100000 fack rows into table (BTW, thank to Jacob Sebastian, copy in INSERTscript 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 showRecompilation.
You can even add few more thousands of rows and execute SPagain, you won’t get recompilation.
Reference: Ritesh Shah
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