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How JOIN Order Can Increase Performance in SQL Queries

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Introduction

All developers are very much concerned about  performance. If someone say that this increase performance, all the developer are running behind it. It is not a bad practice at all. Rather as per my point of view we must span all our effort related improve the performance of query.
“One common question that we find that, if we change the ordering of table join in case of inner join will effect or increase performance”
To understand it lets take a simple example of Inner join. There is two tables named Table-A and Table-B. We can us the Inner Join on both the table.
Like this
 FROM [Table-A]  AS a INNER JOIN [Table-B] AS b ON a.IDNO = b.IDNO
OR
 FROM [Table-B]  AS a INNER JOIN [Table-A] AS b ON  a.IDNO = b.IDNO

Which one is best for performance?

To answer this question we all know that whenever a SQL Query is executed the MS SQL server create several query plans with different join Order and choose the best one.
That means the Join order that we are writing in the query may not be executed by execution plan. May be different join order is used by the execution plan. In the above case the execution plan decide which Join order he will chose depends on best possible costing of execution.
Here [Table-A] JOIN [Table-B] or [Table-B] JOIN [Table-A], MS SQL Server knows it well that both are same.

To understand it Details Lets take an Example

Step-1 [ Create Base Table and Insert Some Records ]

 -- Item Master

IF OBJECT_ID(N'dbo.tbl_ITEMDTLS', N'U')IS NOT NULL
  BEGIN
  DROP TABLE [dbo].[tbl_ITEMDTLS];
  END
GO
CREATE TABLE [dbo].[tbl_ITEMDTLS]
  (
  ITEMCD  INT  NOT NULL IDENTITY PRIMARY KEY,
  ITEMNAME  VARCHAR(50) NOT NULL
  ) 
GO

-- Inserting Records

INSERT INTO [dbo].[tbl_ITEMDTLS]
  (ITEMNAME)
VALUES ('ITEM-1'),('ITEM-2'),('ITEM-3');
 
-- Item UOM Master

IF OBJECT_ID(N'dbo.tbl_UOMDTLS', N'U')IS NOT NULL
  BEGIN
  DROP TABLE [dbo].[tbl_UOMDTLS];
  END
GO
CREATE TABLE [dbo].[tbl_UOMDTLS]
  (
  UOMCD  INT  NOT NULL IDENTITY PRIMARY KEY,
  UOMNAME  VARCHAR(50) NOT NULL
  ) 
GO

-- Inserting Records

INSERT INTO  [dbo].[tbl_UOMDTLS]
  (UOMNAME)
VALUES ('KG'),('LTR'),('GRM');
GO 

-- Transaction Table 

IF OBJECT_ID(N'dbo.tbl_SBILL', N'U')IS NOT NULL
  BEGIN
  DROP TABLE [dbo].[tbl_SBILL];
  END
GO
CREATE TABLE [dbo].[tbl_SBILL]
  (
  TRID  INT  NOT NULL IDENTITY PRIMARY KEY,
  ITEMCD  INT  NOT NULL,
  UOMCD  INT  NOT NULL,
  QTY  DECIMAL(18,3) NOT NULL,
  RATE  DECIMAL(18,2) NOT NULL,
  AMOUNT  AS QTY * RATE
  );
GO

-- Foreign Key Constraint

ALTER TABLE  [dbo].[tbl_SBILL]
ADD CONSTRAINT  FK_ITEM_tbl_SBILL FOREIGN KEY(ITEMCD) REFERENCES [dbo].[tbl_ITEMDTLS](ITEMCD);
GO
ALTER TABLE  [dbo].[tbl_SBILL]
ADD CONSTRAINT  FK_UOMCD_tbl_SBILL FOREIGN KEY(UOMCD) REFERENCES [dbo].[tbl_UOMDTLS](UOMCD);

-- Insert Records

INSERT INTO [dbo].[tbl_SBILL]
  (ITEMCD, UOMCD, QTY, RATE)
VALUES (1, 1, 20, 2000),(2, 3, 23, 1400);  

Step-2 [ Now Make Some JOIN  ]

 SELECT b.TRID, b.ITEMCD, a.ITEMNAME, b.UOMCD,
  c.UOMNAME, b.QTY, b.RATE, b.AMOUNT
FROM  [dbo].[tbl_ITEMDTLS] AS a
  INNER JOIN  [dbo].[tbl_SBILL] AS b ON a.ITEMCD = b.ITEMCD
  INNER JOIN  [dbo].[tbl_UOMDTLS]  AS c ON b.UOMCD  = c.UOMCD;
Here  [tbl_ITEMDETAILS] JOIN [tbl_SALES] JOIN [tbl_UOMDETAILS]
If we look at the Execution Plan
We find that
[tbl_SALES] JOIN [tbl_ITEMDETAILS] JOIN [tbl_UOMDETAILS]
Step-2 [ Now we need to Force Order Hint to maintain Join Order ]
 SELECT b.TRID, b.ITEMCD, a.ITEMNAME, b.UOMCD,
  c.UOMNAME, b.QTY, b.RATE, b.AMOUNT
FROM  [dbo].[tbl_ITEMDTLS] AS a
  INNER JOIN  [dbo].[tbl_SBILL] AS b ON a.ITEMCD = b.ITEMCD
  INNER JOIN  [dbo].[tbl_UOMDTLS]  AS c ON b.UOMCD  = c.UOMCD
OPTION ( QUERYRULEOFF JoinCommute);
For this we need the FORCE ORDER Hint.
The query optimizer uses different rules to evaluate different plan and one of the rules is called JoinCommute. We can turn it off using the undocumented query hint QUERYRULEOFF.
Hope you like it.

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Published at DZone with permission of Joydeep Das, DZone MVB. See the original article here.

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