SQL Non-clustered index

A non-clustered index in SQL is a data structure that improves the performance of queries by providing a quick access path to the data. Unlike a clustered index, which determines the physical order of the data in a table, a non-clustered index creates a separate structure that contains a sorted copy of the indexed column(s) and a reference to the corresponding table rows. This allows for efficient searching and retrieval of data based on the indexed columns.

Key points

Here are some key points about SQL non-clustered indexes:

Structure: A non-clustered index is typically implemented as a balanced tree structure, such as a B-tree or a B+ tree. This structure organizes the indexed values in a hierarchical manner, allowing for efficient search operations.

Indexed Columns: You can create a non-clustered index on one or more columns of a table. When defining the index, you specify the column(s) to be indexed and the sort order (ascending or descending) for each column.

Data Duplication: Unlike a clustered index, a non-clustered index does not dictate the physical order of the data. Instead, it creates a separate structure that points to the corresponding table rows. This means that the indexed columns are duplicated in both the index and the table, which requires additional storage space.

Improved Query Performance: Non-clustered indexes can significantly improve query performance, especially for queries that involve filtering, sorting, or joining operations based on the indexed columns. The index allows the database engine to quickly locate the relevant rows, reducing the need for scanning the entire table.

Trade-Offs: While non-clustered indexes offer performance benefits, they come with some trade-offs. The presence of indexes can impact the performance of data modification operations, such as INSERT, UPDATE, and DELETE, as the indexes need to be maintained alongside the data. Additionally, the additional storage required for indexes can impact disk space usage.

Index Selection: The decision to create a non-clustered index should be based on the query patterns and the specific requirements of your application. It’s essential to analyze the workload and identify the columns that are frequently used in queries to determine the most effective indexes to create.

Index Maintenance: Non-clustered indexes may need periodic maintenance to ensure optimal performance. This includes monitoring index fragmentation, which can occur due to data modifications, and rebuilding or reorganizing indexes to eliminate fragmentation and optimize query execution.


The syntax for creating a non-clustered index in SQL varies slightly depending on the specific database management system you are using. However, the general syntax follows a similar pattern. Here’s a basic example:

ON table_name (column1, column2, ...)

In the above syntax:

CREATE NONCLUSTERED INDEX is the statement used to create a non-clustered index.
index_name is the name you want to assign to the index.
table_name is the name of the table on which the index will be created.
(column1, column2, …) specifies the column(s) to be included in the index.


Let’s consider a practical example to illustrate the usage of a non-clustered index. Suppose we have a table called Customers with the following structure:

CREATE TABLE Customers (
    FirstName VARCHAR(50),
    LastName VARCHAR(50),
    Email VARCHAR(100)

Now, let’s create a non-clustered index on the LastName column to improve the performance of queries that involve searching customers by their last name:

ON Customers (LastName)

In the above example, we created a non-clustered index called idx_Customers_LastName on the LastName column of the Customers table. This index will help speed up queries that involve filtering or sorting data based on the last name of customers.

Once the non-clustered index is created, the database optimizer can utilize it to quickly locate the relevant data rows based on the indexed column(s), resulting in improved query performance.

It’s important to note that creating too many non-clustered indexes on a table can negatively impact performance during data modification operations (such as insert, update, or delete), as the indexes need to be maintained. Therefore, it’s crucial to carefully evaluate the query patterns and select the appropriate columns for indexing to strike a balance between improved query performance and the overhead of maintaining the indexes.

In summary, a non-clustered index in SQL provides a separate structure for efficient data access based on the indexed columns. It improves query performance by enabling quick searching, sorting, and joining operations. However, it requires additional storage space and can impact data modification operations. Proper analysis and maintenance are necessary to utilize non-clustered indexes effectively.