Understanding Non Clustered Index in SQL Server

Hey Dev! Are you having trouble understanding non clustered index in SQL Server? Well, no need to worry because we got you covered. In this article, we will discuss everything you need to know about non clustered indexes in SQL Server. So, let’s dive in!

What is a Non Clustered Index?

A non clustered index is a type of index in SQL Server that is used to improve querying performance. In simple terms, it is a data structure that allows you to quickly search for specific data within a table. Unlike clustered indexes, non clustered indexes do not reorganize the physical order of the table. Instead, they create a separate structure that stores the indexed columns and their corresponding row addresses.

Let’s take a closer look at how non clustered indexes work.

How Non Clustered Indexes Work

When you create a non clustered index on a table, SQL Server creates a separate structure that contains the indexed columns and their corresponding row addresses. This structure is known as the index tree. The index tree is similar to a binary search tree, where each level of the index tree contains a subset of the rows in the table.

When you query the table using a non clustered index, SQL Server uses the index tree to quickly locate the relevant rows. It first navigates to the appropriate level of the index tree based on the search criteria. Then, it scans the leaf nodes of the index tree to retrieve the row addresses that match the search criteria. Finally, it uses these row addresses to access the corresponding rows in the table.

Overall, non clustered indexes can greatly improve query performance by allowing you to quickly search for specific data within a table.

Creating a Non Clustered Index

Now that you have an understanding of how non clustered indexes work, let’s take a look at how to create one in SQL Server.

Syntax

To create a non clustered index, you must specify the name of the index, the name of the table on which to create the index, and the columns to include in the index. You can also specify the sort order for each column (ascending or descending) using the ASC or DESC keywords.

Here is an example of how to create a non clustered index on the Employee table for the LastName column:

“`CREATE NONCLUSTERED INDEX IX_Employee_LastName ON Employee (LastName ASC);“`

This will create a non clustered index named IX_Employee_LastName on the Employee table for the LastName column in ascending order.

Considerations When Creating Non Clustered Indexes

When creating a non clustered index, there are some considerations to keep in mind to ensure optimal performance.

Number of Indexes

It’s important to keep the number of non clustered indexes on a table to a minimum. Each non clustered index requires additional disk space and can slow down insert, update, and delete operations on the table. As a general rule, you should limit the number of non clustered indexes to no more than 5-10 per table.

Index Key Size

The size of the index key is also an important factor to consider when creating non clustered indexes. The larger the index key, the more disk space required to store the index and the slower the index becomes. As a general rule, you should limit the size of the index key to no more than 16 bytes.

Data Distribution

The distribution of data within a table can also affect the performance of non clustered indexes. If the data is heavily skewed towards certain values, the index may not be very selective and may not provide much performance benefit. In some cases, it may be better to create multiple non clustered indexes on subsets of the data rather than a single index on the entire table.

When to Use a Non Clustered Index

Non clustered indexes are most useful for tables with a large number of rows and frequent querying. They are particularly useful for queries that involve sorting, grouping, or searching for specific values within a table.

However, it’s important to keep in mind that non clustered indexes come with some overhead. They require additional disk space and can slow down insert, update, and delete operations on the table. As a general rule, you should only create non clustered indexes when they will provide a significant performance benefit for your queries.

Conclusion

And that’s a wrap Dev! You now have a better understanding of non clustered indexes in SQL Server. We discussed what they are, how they work, how to create them, and when to use them. By following the best practices for creating and using non clustered indexes, you can greatly improve the performance of your queries.

FAQ

What is the difference between a clustered and non clustered index?

A clustered index reorganizes the physical order of the table based on the indexed column, while a non clustered index creates a separate structure that stores the indexed columns and their corresponding row addresses.

How many non clustered indexes should I create on a table?

As a general rule, you should limit the number of non clustered indexes to no more than 5-10 per table.

When should I use a non clustered index?

Non clustered indexes are most useful for tables with a large number of rows and frequent querying. They are particularly useful for queries that involve sorting, grouping, or searching for specific values within a table.