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Difference Between Clustered and Non-Clustered Index

Difference Between Clustered and Non-Clustered Index

Last Updated : 14 May, 2025

Indexing is a critical performance optimization technique in SQL Server that helps speed up data retrieval operations. Understanding the differences between Clustered and Non-Clustered indexes is essential for database administrators and developers looking to optimize query performance.

In this article, we will explain the key differences between these two types of indexes, their characteristics, use cases, and best practices for effective indexing.

Differences Between Clustered and Non-Clustered Index

This table organizes the primary differences between clustered and non-clustered indexes, making it easier to understand when to use each index type based on performance requirements and database structure.

Feature	Clustered Index	Non-Clustered Index
Speed	Faster for range-based queries and sorting.	Slower for range-based queries but faster for specific lookups.
Memory Usage	Requires less memory for operations.	Requires more memory due to additional index structure.
Data Storage	The clustered index stores data in the table itself.	The non-clustered index stores data separately from the table.
Number of Indexes per Table	A table can have only one clustered index.	A table can have multiple non-clustered indexes.
Disk Storage	The clustered index can store data on the disk.	The non-clustered index stores the index structure (B-tree) on disk with pointers to the data pages.
Pointer Storage	Stores pointers to the data blocks, not the data itself.	Stores both the indexed value and a pointer to the actual row in a separate data page.
Leaf Nodes	Leaf nodes contain the actual data itself.	Leaf nodes contain indexed columns and pointers to data.
Data Order	Defines the physical order of the rows in the table.	Defines the logical order of data in the index, not the table.
Index Structure	The data is physically reordered to match the index.	The logical order does not match the physical order of rows.
Primary Key	Primary keys are by default clustered indexes.	Composite keys used with unique constraints are non-clustered.
Size	Typically larger, especially for large primary clustered indexes.	Smaller than clustered indexes, especially when composite.
Use Case	Ideal for range queries and sorting.	Suitable for optimizing lookups and queries on non-primary columns.
Impact on Table	A clustered index directly impacts the table's physical storage order.	A non-clustered index does not affect the physical storage order of the table.

What is a Clustered Index?

A Clustered Index determines the physical order of the data in a table. When a clustered index is created on a column, SQL Server reorders the data in the table based on that index. Because the data is physically stored in the order of the clustered index, a table can only have one clustered index. Typically, a clustered index is created on the primary key by default.

A clustered index is created only when both the following conditions are satisfied:

The data or file, that we are moving into secondary memory should be in sequential or sorted order.
There should be a key value, meaning it can not have repeated values.

Example of Clustered Index

Example of Clustered Index

Consider a table called Student where the Roll_No column is the primary key. This automatically becomes a clustered index. Here, SQL Server automatically creates a clustered index on the Roll_No column. The rows are physically stored in ascending order based on the Roll_No.

Student table

We can have only one clustered index in one table, but we can have one clustered index on multiple columns, and that type of index is called a composite index. Here, the Roll_No column serves as the primary key, automatically becoming the clustered index. The output of querying this table will present data in ascending order of Roll_No.

Non-Clustered Index

A Non-Clustered Index is an index that does not affect the physical order of the data in the table. It is a separate structure from the data table, where the index contains a copy of the indexed columns and a pointer to the actual data. This allows for multiple non-clustered indexes on a table, unlike the clustered index, which can only exist once.

The data is stored in one place, and the index is stored in another place. Since the data and non-clustered index is stored separately, then we can have multiple non-clustered indexes in a table.

Non clustered index — Example of Non Clustered Index

Example of Non-Clustered Index

In the Student table, we could create a non-clustered index on the Name column. Here, roll no is a primary key, hence there is automatically a clustered index. If we want to apply a non-clustered index in the NAME column (in ascending order), then a new table will be created for that column. In this example, a non-clustered index is created on the Name column. SQL Server will create a separate structure containing Name and pointers to the rows where the corresponding data resides.

Query:

CREATE NONCLUSTERED INDEX NIX_FTE_Name
ON Student (Name ASC);

Output

student table — Student table

The row address is used because, if someone wants to search the data for Sudhir, then by using the row address he/she will directly go to that row address and can fetch the data directly.

Conclusion

Understanding the differences between clustered and non-clustered indexes in SQL Server is fundamental to optimizing database performance. Clustered indexes are ideal for range-based queries and sorting, while non-clustered indexes excel in optimizing specific lookups and dynamic queries. By carefully choosing the right type of index for our tables, we can significantly improve query performance and overall system efficiency.

Difference Between Clustered and Non-Clustered Index

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