Understanding SQL Server Clustered Index: A Dev’s Guide

As a developer, you might have come across the term “clustered index” in SQL Server. Clustered Index is one of the most vital components when it comes to optimizing the performance of your database. In this journal article, we will discuss in detail about Clustered Index in SQL Server, how it works, how to create and maintain it, its limitations, and much more. So, let’s begin.

What is SQL Server Clustered Index?

A Clustered Index is a type of index in SQL Server that changes the physical order of data in a table based on the indexed columns. It sorts the data rows in a table in a specific order to speed up data retrieval operations. In simple terms, the clustered index determines the physical location of data rows in a table.

When we create a clustered index on a table, it reorganizes the data rows based on the index key, which is a column or a group of columns selected by the developer, in ascending or descending order. It makes it easier for SQL Server to scan and navigate through the data rows, thereby speeding up data retrieval operations.

How does Clustered Index Work?

When a clustered index is created, it organizes the data rows in a table in a b-tree structure. Each level of the b-tree structure is called a page. The first level of the b-tree structure is called the root page, and it contains pointers to the other pages in the b-tree structure. The last level of the b-tree structure is called the leaf page, and it contains the data rows in the table.

The leaf page of the clustered index is the actual data of the table, sorted by the indexed column(s). The leaf page also contains a pointer to the next leaf page, which makes it possible for SQL Server to navigate through the data rows.

What are the Advantages of Clustered Index?

Clustered Index has several advantages, which are as follows:

What are the Limitations of Clustered Index?

Clustered Index has some limitations, which are as follows:

How to Create SQL Server Clustered Index?

Creating clustered index in SQL Server is a simple process. Follow the below steps to create a clustered index on a table:

Step 1: Determine which Column(s) to Index

The first step is to determine which column(s) you want to index. The column(s) you select should be the ones that are frequently used in queries, and the ones that have a high cardinality (i.e., the number of unique values in the column(s))

Step 2: Create the Index

Once you have determined the column(s) to index, you can create the clustered index using the SQL Server Management Studio (SSMS) or Transact-SQL (T-SQL).

Using SSMS:

1. Connect to the SQL Server Instance
2. Expand the Databases folder
3. Expand the database that contains the table you want to create the index on
4. Expand the Tables folder
5. Right-click on the table you want to create the index on and select “Design”
6. Click on the “Indexes/Keys” button at the top of the window
7. Click on the “Add” button to create a new index
8. In the “New Index” window, select “Clustered” as the index type
9. Select the column(s) you want to index
10. Click on the “OK” button to create the index
11. Save the changes to the table by clicking on the “Save” button

Using T-SQL:

Run the below T-SQL command to create a clustered index:

CREATE CLUSTERED INDEX index_name ON table_name (column1, column2);

Replace the “index_name” with the name of the index you want to create, “table_name” with the name of the table you want to create the index on, and “column1, column2” with the name of the column(s) you want to index.

Step 3: Verify the Index

Once you have created the clustered index, you can verify it using SSMS or T-SQL.

Using SSMS:

1. Connect to the SQL Server Instance
2. Expand the Databases folder
3. Expand the database that contains the table you created the index on
4. Expand the Tables folder
5. Right-click on the table and select “Properties”
6. Click on the “Indexes/Keys” tab
7. Verify that the clustered index is listed

Using T-SQL:

Run the below T-SQL command to verify the clustered index:

sp_helpindex table_name;

Replace the “table_name” with the name of the table you created the index on.

How to Maintain SQL Server Clustered Index?

Maintaining the SQL Server Clustered Index is important to ensure optimal performance. The maintenance involves rebuilding or reorganizing the index to maintain the health of the index. SQL Server provides two methods for maintaining the clustered index: Rebuild and Reorganize.

Rebuild

The Rebuild method rebuilds the entire clustered index. It drops the existing index and then recreates it. It also reorganizes the data rows in the table based on the new index key. The Rebuild method is recommended when the index fragmentation is greater than 40%.

Using SSMS:

1. Connect to the SQL Server Instance
2. Expand the Databases folder
3. Expand the database that contains the table you created the index on
4. Expand the Tables folder
5. Right-click on the table and select “Properties”
6. Click on the “Indexes/Keys” tab
7. Select the clustered index you want to rebuild
8. Right-click on the index and select “Rebuild”
9. Click on “OK” to confirm the action

Using T-SQL:

Run the below T-SQL command to rebuild the index:

ALTER INDEX index_name ON table_name REBUILD;

Replace the “index_name” with the name of the index you want to rebuild, and “table_name” with the name of the table that contains the index.

Reorganize

The Reorganize method reorganizes the existing data rows in the clustered index. It does not drop or recreate the index. The Reorganize method is recommended when the index fragmentation is between 10% and 40%.

Using SSMS:

1. Connect to the SQL Server Instance
2. Expand the Databases folder
3. Expand the database that contains the table you created the index on
4. Expand the Tables folder
5. Right-click on the table and select “Properties”
6. Click on the “Indexes/Keys” tab
7. Select the clustered index you want to reorganize
8. Right-click on the index and select “Reorganize”
9. Click on “OK” to confirm the action

Using T-SQL:

Run the below T-SQL command to reorganize the index:

ALTER INDEX index_name ON table_name REORGANIZE;

Replace the “index_name” with the name of the index you want to rebuild, and “table_name” with the name of the table that contains the index.

FAQs

What is the difference between Clustered Index and Non-Clustered Index?

The difference between Clustered Index and Non-Clustered Index is that the Clustered Index changes the physical order of data rows in a table based on the indexed columns, while the Non-Clustered Index does not change the physical order of data rows. It creates a separate data structure that contains the index key and a pointer to the data rows.

Can a table have both Clustered and Non-Clustered Index?

Yes, a table can have both Clustered and Non-Clustered Index. A table can have one Clustered Index and multiple Non-Clustered Index.

What happens if a Clustered Index is created on a column with Duplicate Values?

If a Clustered Index is created on a column with duplicate values, SQL Server adds a uniqueifier to the indexed column to make the values unique. The uniqueifier is a 4-byte binary value added to the indexed column value.

What is the maximum number of columns that can be included in a Clustered Index in SQL Server?

The maximum number of columns that can be included in a Clustered Index in SQL Server is 16.

Can a Clustered Index be dropped in SQL Server?

Yes, a Clustered Index can be dropped in SQL Server using the SSMS or T-SQL.

Conclusion

Clustered Index is a critical component of SQL Server that can improve the performance of your database. In this journal article, we discussed in detail about Clustered Index, how it works, how to create and maintain it, its advantages, limitations, and much more. We hope this article has helped you understand Clustered Index and how it can benefit your database.