Clustered vs Non-Clustered Index in a Database

When it comes to indexing in a database, understanding the difference between clustered and non-clustered indexes is essential.

Clustered Index:

A clustered index determines the physical order of the data in a table. The rows in a table are stored in the order of the clustered index key. Each table can have only one clustered index, and it is often referred to as the primary key index.

Non-Clustered Index:

Unlike a clustered index, a non-clustered index does not alter the physical order of the table's rows. Instead, it creates a separate structure where the index key values are stored along with pointers to the actual data rows. A table can have multiple non-clustered indexes.

Main Differences:

• Clustered index determines the physical order of data, while non-clustered does not.
• Each table can have only one clustered index but multiple non-clustered indexes.
• Clustered indexes are generally faster for retrieving data in range queries due to the physical ordering of rows.
• Non-clustered indexes are beneficial for improving query performance on columns not covered by the clustered index.

Understanding when to use a clustered or non-clustered index based on the database design and query patterns is crucial for optimizing database performance.