SQL Server Indexing: Clustered vs Non Clustered Index Explained

Last updated on September 11th, 2023 at 05:59 pm

If you are new to SQL Server, you might wonder what an index is and why it’s important. Indexes are crucial for improving query performance and are essential for efficient data retrieval.

In this article, we will discuss the differences between clustered and non-clustered indexes in SQL Server.

What is an Index?

In SQL Server, an index is an object that is used to speed up the data retrieval process by allowing queries to locate data in a table quickly. Think of it as a roadmap for your data, where each index entry contains a value and a pointer to the location of the corresponding row.

Indexes can be created on one or more columns of a table, and they can be either clustered or non-clustered.

What is a Clustered index?

A clustered index is used to sort and store the data in a table based on the index key values. 

In other words, A clustered index is a type of database index that determines the physical order of data rows within a table. This means that the data in the table is physically stored on disk in the same order as the clustered index key.

• A table can have only one clustered index, which defines the table data alphabetically, just like a dictionary.
• A clustered index is usually created on a primary key or a column with unique values.

What is a Non clustered index?

In SQL Server, a non-clustered index is a database structure that enhances data retrieval speed from a table. It creates a separate data structure that stores index key values and pointers to the actual data rows within a table. Non-clustered indexes are like a phone book where you can look up data based on the values of the index.

• A table can have multiple non-clustered indexes, but the data in the table remains unsorted.
• Unlike clustered indexes, non-clustered indexes don’t dictate the physical order of data rows in the table. Still, they enhance query performance by allowing efficient data retrieval based on the indexed columns.

Explanation: 

Think of a non-clustered index like the index at the back of a book. You don’t read every page from the start when you want to find information in a book quickly. Instead, you go to the index, find the page number where the topic you’re interested in is mentioned, and then flip to that page.

In SQL Server, a non-clustered index is like that index in the book. It’s a separate list that keeps track of key information (like the words in an index and their page numbers) to help you find data in a table faster. 

Instead of scanning the entire table, which can be like reading the entire book, SQL Server uses this index to pinpoint where the data you’re looking for is stored. This makes searching for data in a large table much quicker and more efficient.

Unlike a clustered index, which rearranges the physical order of the book’s pages, a non-clustered index doesn’t change how the data is physically stored in the table. It’s just a mechanism that helps you find things faster, like an index in a book without reordering the pages.

Comparison Table: Clustered vs Non Clustered Index

Here is a comparison table to help you understand the differences between clustered and non-clustered indexes:

How to Create Clustered and Non-Clustered Index in SQL Server?

Here are some examples of clustered and non-clustered indexes in SQL Server.

Example of Clustered Index:

Consider a table named Students. The table contains the following columns: StudentID, StudentName, Age, Gender, and Course.

Here, StudentID is the primary key column.

-- Creating Students Table
CREATE TABLE Students (
  StudentID INT PRIMARY KEY,
  StudentName VARCHAR(50),
  Age INT,
  Gender VARCHAR(10),
  Course VARCHAR(50)
);

-- Inserting records into Students Table

INSERT INTO Students (StudentID, StudentName, Age, Gender, Course)
VALUES (1, 'Shekh Ali', 20, 'Male', 'Computer Science'),
       (2, 'Jyoti Sharma', 19, 'Female', 'Mathematics'),
       (3, 'David Lee', 21, 'Male', 'Chemistry'),
       (4, 'kalpana Singh', 18, 'Female', 'English'),
       (5, 'Mukesh Kumar', 22, 'Male', 'History'),
       (6, 'Jennifer Davis', 20, 'Female', 'Art');

Once you run the above SQL Query, the ‘Students’ table will look like this:

To create a clustered index on the StudentID column, we can use the following SQL query:

-- Creating a clustered index on the 'StudentID' column
CREATE CLUSTERED INDEX IX_Students_StudentID ON Students(StudentID);

Note: If you have a primary key column in the table, then a Clustered Index will be created on that column by default. Creating a clustered index on the same column might throw an error like this.

“Cannot create more than one clustered index on table ‘Students’. Drop the existing clustered index ‘PK__Students__32C52A79C3C6D85B’ before creating another.”

So, First we need to drop the existing clustered index before creating the new one. Here is the SQL Query.

-- Drop the existing clustered index on the 'StudentID' column
ALTER TABLE Students
DROP CONSTRAINT PK__Students__32C52A79C3C6D85B

-- Create a new clustered index on the 'StudentID' column
CREATE CLUSTERED INDEX IX_Students_StudentID ON Students(StudentID)

The above SQL query will create a clustered index on the ‘StudentID’ column, which will be used to physically order the data in the table based on the values in the ‘StudentID’ column.

Example of Non-Clustered Index:

Consider the same table ‘Students’. If we want to create a non-clustered index on the ‘Course’ column, we can use the following SQL query:

-- Creating a non-clustered index on the 'Course' column.
CREATE NONCLUSTERED INDEX IX_Students_Course ON Students(Course);

The above SQL query will create a non-clustered index on the ‘Course’ column, which will be used to create a separate data structure that stores the values of the ‘Course’ column along with reference to the location of the corresponding row in the table.

By creating non-clustered indexes on frequently used columns, we can improve the performance of queries that search for data based on those columns.

How to find which indexes are created on the table?

We can use the following SQL query to find out which indexes are created on a particular table:

Syntax:

EXEC sp_helpindex 'TableName';

Here, ‘TableName’ is the name of the table for which we want to see the list of indexes.

Once we run the above SQL query, we can see the Clustered index is created on the StudentID column, and the Non-Clustered index is created on the Course column of the Customers table.

When to Use a Clustered or Non-Clustered Index?

Clustered indexes should be used when we want to improve the performance of queries that retrieve a range of values from a table based on the primary key column or a column with unique values.

Non-clustered indexes should be used when we want to improve the performance of queries that search for data based on frequently used columns other than the primary key column.

Advantages of Clustered Index

The following are some of the advantages of using a clustered index in a SQL Server:

1. Faster Data Retrieval: Clustered indexes store data in a sorted manner, allowing for faster data retrieval. When a query is executed against a table with a clustered index, SQL Server uses the Index to quickly locate the required data, resulting in faster query performance.
2. Avoidance of Table Scans: Since data in a clustered index is sorted, SQL Server can quickly skip over large portions of the table that do not contain the data being searched for. This avoids the need for a table scan, which can be time-consuming and resource-intensive.
3. Improved Data Locality: Clustered indexes store data in the same order as the Index, which can improve data locality. That means the data is stored physically close to related data, resulting in faster retrieval and improved performance.
4. Clustered Index is the Primary Key: When a table has a clustered index, the Index is also the primary key. It ensures that each row in the table is uniquely identified, making it easier to maintain referential integrity.
5. Efficient Data Modification: When data is added or modified in a table with a clustered index, SQL Server only needs to update the Index and the affected data pages. It makes modifications more efficient than with non-clustered indexes.

Overall, using a clustered index can result in faster query performance and more efficient data retrieval, making it a popular choice for improving the performance of SQL Server databases.

Advantages of Non Clustered Index.

The following are some of the advantages of using a non-clustered index in a SQL Server:

1. Multiple Indexes per Table: A clustered index can be only one in a table, but a table can have multiple non-clustered indexes. It allows for more flexible indexing strategies, enabling the optimization of specific queries.
2. Faster Query Performance: Non-clustered indexes allow SQL Server to quickly locate data based on the columns included in the index. It can improve query performance, particularly when searching for data not covered by a clustered index.
3. Reduced Disk Space Usage: Non-clustered indexes typically require less disk space than clustered indexes since they do not store the actual data. It can be particularly beneficial when working with large tables.
4. Efficient Sorting: Non-clustered indexes can be used to sort data based on the indexed columns. It can be particularly beneficial when working with large datasets, as sorting can be time-consuming.
5. Covering Indexes: Non-clustered indexes can be used as covering indexes, meaning that all of the columns required by a query are included in the index. It can significantly improve query performance since SQL Server does not need to access the actual data pages.

Overall, non-clustered indexes can improve query performance, reduce disk space usage, and offer more flexible indexing strategies than clustered indexes. 

However, they may not be as efficient as clustered indexes for retrieving large amounts of data since they require more disk I/O.

Disadvantages of Clustered and Non-Clustered Index.

There are certain disadvantages to both clustered and non-clustered indexes in SQL Server:

Disadvantages of Clustered Index:

• Slower Write Performance: SQL Server must adjust the index to maintain the sort order since the data in a clustered index is physically sorted when a new row is inserted, or an existing row is updated. It can lead to slower write performance compared to non-clustered indexes.
• Limited Number of Clustered Indexes per Table: SQL Server allows only one clustered index per table. If you need to sort the data in multiple ways, you may need to create multiple tables or use non-clustered indexes.
• Disk Space Usage: Clustered indexes can use more disk space than non-clustered indexes since they store the actual data. Itis can be particularly problematic when working with large tables.

Disadvantages of Non-clustered Index:

• Slower Query Performance: Although non-clustered indexes can improve query performance, they can be slower than clustered indexes when retrieving large amounts of data. This is because non-clustered indexes require more disk I/O to retrieve data.
• More Disk Space Usage: Although non-clustered indexes typically require less disk space than clustered indexes, they can still consume a significant amount of disk space when working with large tables.
• Additional Maintenance Overhead: Since non-clustered indexes are separate from the actual data, they require additional maintenance overhead to keep them synchronized with the data.

Overall, both clustered and non-clustered indexes have their advantages and disadvantages. The choice of which index to use depends on the specific needs of the application and the nature of the data being stored.

Indexing Best Practices

Indexing is a critical aspect of database design, and it is essential to follow some best practices to ensure optimal database performance. Here are some of the best practices for indexing in SQL Server:

1. Identify the columns that need indexing: Not all columns in a table require indexing, and indexing every column can lead to performance issues. Identify the columns frequently used in queries and crucial for database performance. These columns should be indexed.
2. Choose the appropriate index type: As discussed earlier, SQL Server offers different types of indexes, such as clustered, non-clustered, and full-text indexes. Choose the appropriate index type based on the requirements of the database.
3. Use index filters: Index filters can help reduce the size of the index and improve query performance. Index filters are conditions that limit the number of rows that are included in an index.
4. Keep indexes updated: Indexes should be kept up-to-date to ensure optimal database performance. Indexes should be rebuilt or reorganized periodically to maintain optimal performance.
5. Be careful when dropping indexes: Dropping an index can significantly impact database performance. Before dropping an index, make sure to identify any queries that use the index and determine if the index is no longer needed.

References: MSDN-Clustered vs Non Clustered Index

By following these best practices, you can ensure the optimal performance of your SQL Server database.

FAQs

Here are some frequently asked questions (FAQs) related to clustered and non-clustered indexes in SQL Server:

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Shekh Ali

Meet Sheikh Ali, a skilled Software engineer and passionate a blogger. He enjoys sharing his expertise in C#, Java, SQL, Design Patterns, and other related topics through his writing. When he's not coding, Sheikh Ali can often indulge in his passions for reading books and fitness training.

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