How is a non-clustered index different from a clustered index?

22 Sep 2023

Hashtags: #NonClusteredIndex #ClusteredIndex

When it comes to database management, indexes play a crucial role in improving query performance. Among the various types of indexes, two commonly used ones are the non-clustered index and the clustered index. While both serve the purpose of improving performance, they have distinct characteristics and are used in different scenarios. In this article, we will explore the difference between a non-clustered index and a clustered index, and understand when to use each.

Non-clustered Index

A non-clustered index is an index structure that is separate from the actual data in a table. It consists of a copy of selected columns or key columns that are frequently used for searching, sorting, or filtering data. The non-clustered index stores a reference to the location of the actual data rows.

Here are some key points to know about non-clustered indexes:

The non-clustered index is stored separately from the actual data, allowing for faster search operations.
It contains a copy of the selected columns or key columns that are defined in the index.
Multiple non-clustered indexes can be created on a single table.
Non-clustered indexes are ideal for columns that are frequently used in WHERE clauses or JOIN operations.
They are suitable for tables with less primary key or unique columns.

Clustered Index

A clustered index determines the physical order of data in a table. It defines the arrangement of data rows on the disk based on the values of one or more key columns. In simpler terms, the clustered index organizes the data in the table in a specific order, making it faster to retrieve data based on that order.

Here are some key points to know about clustered indexes:

A table can have only one clustered index.
It changes the physical storage order of the data in a table, based on the key column(s) defined in the index.
The leaf nodes of the clustered index contain the actual data rows.
Clustered indexes are particularly useful for tables that are frequently queried for range-based searches.
They are ideal for columns with high selectivity, such as primary key or unique columns.

Conclusion

In summary, the main difference between a non-clustered index and a clustered index lies in their structure and purpose. A non-clustered index is stored separately and contains a copy of selected columns, allowing for efficient search operations. On the other hand, a clustered index defines the physical order of data in a table and is particularly useful for range-based searches. Understanding the difference between these indexes is essential for efficiently optimizing database performance.

So, the next time you’re working on optimizing your database, consider the differences between a non-clustered index and a clustered index, and choose the one that suits your specific requirements.

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