Database Indexing: Clustered vs Non-Clustered Explained

Databases are one of the crucial aspects in new-age applications, where the efficiency of date retrieval impacts the overall performance. Indexing is a basic technique used in databases to accelerate query execution. There are different types of indexes, primarily, clustered and non-clustered indexes, used in relational database management system.

Thus, it becomes imperative to understand the basic difference between two and make the most of databases and help developers optimize query performance efficiency.

A clustered index defines the physical order of data in a table, while a non-clustered index creates a separate structure to store index data. Both are different and can affect how queries perform. So, keep reading on the article to dive into key differences.

Let’s Get Started!

What is a Clustered Index?

It sorts and saves the data rows in the table based on the indexed column(s). As a table only have one physical order, it can have only one clustered index. This also means that the data is stored in the same physical order when a table has a clustered index.

A clustered index is created only when it meets the following conditions:

• The file or data, you are moving into secondary memory should be in sorted or sequential order.
• There should be a key value, without having repeated values.

Examples of Clustered Index

Imagine you are creating a table called Student where the Roll_No column is the primary key. Now, it will automatically becomes a clustered index. The SQL Server automatically creates a clustered index on the Roll_No column. On the other hand, the rows are physically stored in ascending order depending the Roll_No.

Roll_No	Name	Gender	Mob_No
2	Jack	Male	8987690xxx
1	Ross	Male	8760988xxx
3	Maria	Female	9876022xxx

Here you have only one clustered index in one table, but can have one clustered index on multiple columns, and that type of index is called a composite index.

The Roll_No column is the primary key, becoming the clustered index automatically. The output of querying this table will represent data in ascending order of Roll_No.

What is a Non-Clustered Index?

A non-clustered index maintains a separate structure that holds pointers to the actual rows in the table rather than sorting the data physically. This allows for multiple non-clustered indexes on a table, improving search efficiency for various columns.

In simple words, the data is stored in one place, and the index in another. Since the data and non-clustered index sorted separately, you can ensure multiple non-clustered indexes in a table.

Examples of a Non-Clustered Index

In the Employee table, you can create a non-clustered index on the Name column. Here, department is primary key and there is automatically a clustered index. In case you want to create a non-clustered index in the NAME column in ascending order, a new table will be created for that column.

In the following example, a non-clustered index is created on the Name column. SQL Server will create a separate structure consisting of Name and pointers to the rows.

If you want to search employees by Department, you can create a non-clustered index as follows:

CREATE NONCLUSTERED INDEX idx_department

ON Employees (Department);

This index enables for quick lookups based on Department without tweaking the physical order of data storage.

Key Differences: Clustered Index vs. Non-Clustered Index

Feature	Clustered Index	Non-Clustered Index
Storage Structure	Stores data in the table physically	Stores index separately with pointers to data
Number	Only one per table	Multiple allowed per table
Data Access Speed	Faster for range-based queries	Slower for range queries but efficient for lookups
Primary Key Default	Yes, usually the primary key	No, must be created separately
Insertion/Update Performance	Slower due to reordering of data	Faster since data remains unsorted
Space	No additional space required	Requires additional storage for index structuring
Speed	Faster for range-based queries	Slower for range queries but efficient for lookups
Usage	Best for queries that return large data ranges	Best for queries retrieving specific records

Wrapping Up

Understanding the key difference between clustered and non-clustered indexes in SQL Server to crucial to optimizing database performance. Clustered indexes is great for range-based queries and sorting, while non-clustered indexes are great for optimizing specific lookups and queries. So, carefully choose the right type of index for better efficiency.