Improve the performance by Execution Plan

Improve the performance by Execution Plan

Introduction

Developers ask a common quest “How to improve the performance of a SQL Query”. It is not so easy to answer as lot of factors is related to it. There are some general guidelines that we can follow to improve the overall performance of a query.

But I recommended the execution plan to understand the performance of the query. I preferred execution plan when I am building the query block step by step.
In this article I am trying to show a basic strategy, how to improve a query by observing the query plan.
Prerequisite
To understand this article, we have a very good knowledge of Index, Index Scan, Index Seek, Table scan etc. Please follow the related tropics of this article, to complete this.
Improving Query
To understand it properly, I am taken an example.
Step-1 [ Creating The Base Table ]
— Creating the Base Table
IF OBJECT_ID(‘Emp_Dtls’) IS NOT NULL
   BEGIN
      DROP TABLE Emp_Dtls;
   END
GO  
CREATE TABLE Emp_Dtls
       (EMPID    INT          NOT NULL IDENTITY,
        EMPNAME  VARCHAR(50)  NOT NULL,
        EMPGRADE VARCHAR(1)   NOT NULL,
        EMPDEPT  VARCHAR(30)  NOT NULL);
GO
Step-2 [ Inserting the Records ]
DECLARE @i INT=1;
BEGIN TRY
BEGIN TRAN
WHILE (@i <= 50000)
BEGIN
      INSERT INTO Emp_Dtls
             (EMPNAME, EMPGRADE, EMPDEPT) 
      VALUES(‘Developer-‘+CONVERT(VARCHAR, @i),‘C’,‘DEV’);
      SET @i=@i+1;
END
SET @i=1;  
WHILE (@i <= 50000)
BEGIN
      INSERT INTO Emp_Dtls
             (EMPNAME, EMPGRADE, EMPDEPT) 
      VALUES(‘Devlivery Mgr-‘+CONVERT(VARCHAR, @i),‘B’,‘DM’);
      SET @i=@i+1;
END  
SET @i=1;  
WHILE (@i <= 50000)
BEGIN
      INSERT INTO Emp_Dtls
             (EMPNAME, EMPGRADE, EMPDEPT) 
      VALUES(‘Manager-‘+CONVERT(VARCHAR, @i),‘A’,‘MGR’);
      SET @i=@i+1;
END  
COMMIT TRAN
END TRY
BEGIN CATCH
ROLLBACK TRAN
END CATCH
Step-3 [ See the Actual Execution Plan ]
— Execution Plan-1 [ Table Scan ]
SELECT * FROM Emp_Dtls; 

As there is NO INDEX defined it going to TABLE SCAN. So the performance of the SQL Query is worst. We have to improve the performance of the Query.
Step-4 [ Create Clustered Index ]
As there is no index over here and the attribute “EMPID” of Table objects “EMP_DTLS” has INTEGER data type, so it is a good candidate key for CLUSTERED INDEX. Now we are going to create the CLUSTERED INDEX on it.
— Create Custered Index 
IF EXISTS(SELECT *
          FROM   sys.sysindexes
          WHERE  id = OBJECT_ID(‘Emp_Dtls’)
                 AND name =‘IX_CLUS_Emp_Dtls’)
   BEGIN
      DROP INDEX Emp_Dtls.IX_CLUS_Emp_Dtls;
   END      
GO
CREATE CLUSTERED INDEX IX_CLUS_Emp_Dtls
ON  Emp_Dtls(EMPID);
Aster creating the CLUSTERED INDEX we are going to see the EXECUTION plan again that it Improves or NOT.
— Execution Plan-2 [ Clustered Index Scan ]
SELECT * FROM Emp_Dtls;
Now we can see that there is Clustered Index Scan. So the performance is little bit improve. At least it uses the CLSUTERED INDEX.
 Step-5[ Putting WHERE conditions in Query ]
— Execution Plan-3 [ Using WHERE Conditions ]
SELECT EMPID, EMPNAME FROM Emp_Dtls WHERE EMPGRADE=‘A’; 
 As the “EMPGRADE” is used in the WHERE conditions we are going to make a NON CLUSTERED Index on it.
— Now Create Non Clustered Index on EMPGRADE
IF EXISTS(SELECT *
          FROM   sys.sysindexes
          WHERE  id = OBJECT_ID(‘Emp_Dtls’)
                 AND name =‘IX_NONCLUS_EMPGRADE’)
   BEGIN
      DROP INDEX Emp_Dtls.IX_NONCLUS_EMPGRADE;
   END      
GO
CREATE NONCLUSTERED INDEX IX_NONCLUS_EMPGRADE
ON  Emp_Dtls(EMPGRADE); 
GO
Now again see the execution plan.
— Execution Plan-4
SELECT EMPID, EMPNAME FROM Emp_Dtls WHERE EMPGRADE=‘A’; 
Here again the clustered Index is used. The non clustered index that we created is not used here. Why?
You can extend the functionality of nonclustered indexes by adding nonkey columns to the leaf level of the nonclustered index. By including nonkey columns, you can create nonclustered indexes that cover more queries.
In this Example the “EMPNAME” is a NONKEY Columns.
Step-6[ Solve the Problem ]
— Non clustered Index with Incluse
IF EXISTS(SELECT *
          FROM   sys.sysindexes
          WHERE  id = OBJECT_ID(‘Emp_Dtls’)
                 AND name =‘IX_NONCLUS_EMPGRADE_EMPNAME’)
   BEGIN
      DROP INDEX Emp_Dtls.IX_NONCLUS_EMPGRADE_EMPNAME;
   END      
GO
CREATE NONCLUSTERED INDEX IX_NONCLUS_EMPGRADE_EMPNAME
ON Emp_Dtls(EMPGRADE) INCLUDE(EMPNAME);
Now again see the Execution Plan.
— Execution Plan-5
SELECT EMPID, EMPNAME FROM Emp_Dtls WHERE EMPGRADE=‘A’;

Now the desired output came and it is INDEX SEEK.
Related Tropics
Hope you like it.
Posted by: MR. JOYDEEP DAS