Non-Clustered Index in SQL Server

Non-Clustered Index in SQL Server with Examples

In this article, I am going to discuss the Non-Clustered Index in SQL Server with examples. Please read our previous article before proceeding to this article where we discussed the Clustered Index in SQL Server with some examples. As part of this article, we are going to discuss the following pointers.

  1. What is the Non-Clustered Index in SQL Server?
  2. Example to Understand the SQL Server Non-Clustered Index.
  3. Understanding the Composite Non-Clustered Index.
  4. What are the differences between a clustered and a non-clustered index in SQL Server?
  5. When should we create indexes on a table?
  6. Which Index is faster, Clustered or Non-Clustered Index?
What is SQL Server Non-Clustered Index?

In the Non-Clustered Index, the arrangement of data in the index page or table will be different from the arrangement of data in the actual table. The data is stored in one place and the index is stored in another place. Moreover, the index will have pointers to the storage location of the actual data.

In order to understand this better, please have a look at the following diagram which shows the B-Tree structure of the non-clustered index. Both clustered and non-clustered index has the same B-Tree structure (i.e. having the Root Node, Intermediate Node, and Leaf Nodes). The only difference between the clustered and non-clustered index is how the leaf nodes are worked. In the case of the Clustered Index, the leaf node has the actual data. On the other hand, the non-clustered index leaf-node has a Row Identifier (RID) and this Row ID pointing to different things based on the situations.

Non-Clustered Index in SQL Server

Situation1:

If your table does not have a clustered index, them the Row Identifier (RID) of the non-clustered index pointing to the heap table. A heap table is nothing but a table without indexes. In the heap table, it will search record row by row until it finds the data.

Situation2:

If your table has a clustered index, then the Row Identifier (RID) of the non-clustered index will point to the clustered index key and that indexed key is used to search the data.

How many non-clustered indexes can create in a table in SQL Server?

As the non-clustered indexes are created separately from the actual data, so, a table can have more than one non clustered index in SQL Server.

In the index table, the data is stored either in the ascending or descending order of the index key which does not make any effect or changes to the actual data stored in the table.

In SQL Server we can create a maximum of 249 non clustered indexes for a table.

Understanding the Non-Clustered Index in SQL Server:

Let us understand the non-clustered index with an example. Please create the following tblOrder table by executing the below SQL Script. Please note that we don’t have any indexes at the moment in the table.

CREATE TABLE tblOrder
(
 Id INT,
 CustomerId INT,
 ProductId Varchar(100),
 ProductName VARCHAR(50)
)
GO

Now insert some data into the table. In order to do this, here, we are inserting some mock data using a loop. Please execute the following query.

DECLARE @i int = 0
WHILE @i < 3000 
BEGIN
    SET @i = @i + 1
 if(@i < 500)
 Begin
     INSERT INTO tblOrder VALUES (@i, 1, 'Product - 10120', 'Laptop')
 end
 Else if(@i < 1000)
 Begin
     INSERT INTO tblOrder VALUES (@i, 3, 'Product - 1020', 'Mobile')
 End
    Else if(@i < 1500)
 Begin
     INSERT INTO tblOrder VALUES (@i, 2, 'Product - 101', 'Desktop')
 End
 Else if(@i < 2000)
 Begin
     INSERT INTO tblOrder VALUES (@i, 3, 'Product - 707', 'Pendrive')
 End
    Else if(@i < 2500)
 Begin
     INSERT INTO tblOrder VALUES (@i, 2, 'Product - 999', 'HD')
 End
  Else if(@i < 3000)
 Begin
     INSERT INTO tblOrder VALUES (@i, 1, 'Product - 100', 'Tablet')
 End
END

Now, execute the following query to get the data by ProductId.

SELECT * FROM tblOrder WHERE ProductId = ‘Product – 101’;

After executing the above SQL Query, let us click on the Display Estimated Execution Plan button or simply click (Ctrl + L) which will open the Display Estimated Execution Plan window as shown below.

Display Estimated Execution Plan for Table Scan

As you can see in the above image, here, the database search engine uses a table scan to get the required data.

Creating Non-clustered Index in SQL Server:

Please execute the following SQL Query which will create a non-clustered index on the ProductId column of tblOrder table.

CREATE NONCLUSTERED INDEX IX_tblOrder_ProductId
ON dbo.tblOrder (ProductId)
INCLUDE ([Id],[CustomerId],[ProductName])
GO

Once you created the non-clustered index, now execute the following query and check the execution plan.

SELECT * FROM tblOrde WHERE ProductId = ‘Product – 101’;

Following is the execution plan. As you can see, it now uses a non clustered index to get the required data.

Display Estimated Execution Plan

Can we create a composite Non-Clustered Index in SQL Server?

Yes. It is also possible to create a composite non-clustered index (non-clustered index based on more than one column) in SQL Server like composite Clustered Index. Sometimes, the Display Estimated Execution Plan gives you the information for creating the missing indexes which can improve the performance of the search query.

Let us understand this with an example. We want to fetch the details from the order table where CustomerId = 3 and ProductName = ‘Pendrive’ by executing the following query.

SELECT * FROM tblOrder WHERE CustomerId = 3 and ProductName = ‘Pendrive’;

Once, you execute the above query, now open the Display Estimated Execution Plan window as shown below.

Can we create composite Non-Clustered Index in SQL Server?

Now, right-click on the window and select the Missing Index Details option which will give you the information to create the missing indexes. In our example, it will give us information to create a composite non-clustered index based on CustomerId and ProductName.

So, let us create one non-clustered index based on the CustomerId in Ascending order and ProductName in Descending Order by executing the following query.

CREATE NONCLUSTERED INDEX IX_tblOrder_CustomerId_ProductName 
ON tblOrder(CustomerId ASC, ProductName DESC)
INCLUDE ([Id],[ProductId]);

Once you create the above composite non-clustered index, now execute the below query and see the execution plan which should use the composite non-clustered index to fetch the data.

SELECT * FROM tblOrder WHERE CustomerId = 3 and ProductName = ‘Pendrive’;

What is the difference between clustered and non-clustered index?

We can create only one clustered index per table whereas we can create more than one non-clustered indexes per table.

In the clustered index, the leaf node actually has the actual data and in the case of a non-clustered index, the leaf node actually points to the leaf node of a clustered index.

The SQL Server Clustered Index determines the storage order of the rows in the table and hence does not require additional disk space whereas a Non-Clustered Index in SQL Server is stored separately from the table so additional storage space is required.

When should we create indexes on a table?

We need to create indexes on table columns provided those columns are frequently used in where condition or order by clause in a select query.

It is not advised creating an index on each and every column because a number of indexes can degrade the performance of the database also because every modification we make on the data should be reflected in all the index tables. How index impacts the performance of DML operations that we will discuss in our next article.

Which Index is faster, Clustered or Non-Clustered Index?

The Clustered Index is slightly faster than the Non-Clustered Index. This is because, in the case of the clustered index, the leaf node actually has the actual data and hence when we search any data, it directly gets the data from the leaf node. On the other hand, in the case of a Non-Clustered Index, the leaf node actually points to the leaf node of clustered index and hence there is an extra look up from the Non-Clustered Index to the actual table (leaf node of a clustered index) to fetch the actual data.

In the next article, I am going to discuss How Index Impacts DML Operations in SQL Server with examples. Here, in this article, I try to explain the SQL Server Non-Clustered Index step by step with some examples. I hope you enjoy this article.

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