Clustered Index vs Non-Clustered Index in a Database

In a database, a clustered index and a non-clustered index are two types of indexing structures used to organize and retrieve data efficiently. Understanding the difference between the two is crucial for optimizing database performance.

Clustered Index

A clustered index determines the physical order of data rows in a table. When a clustered index is created on a table, the rows are stored on disk in the same order as the index key. This means that the data in a table can only have one clustered index. The leaf nodes of a clustered index contain the actual data rows.

Non-Clustered Index

A non-clustered index does not dictate the physical order of data rows on disk. Instead, it stores the index key values along with pointers to the corresponding data rows. This allows for multiple non-clustered indexes to be created on a table. The leaf nodes of a non-clustered index do not contain the actual data rows but rather pointers to them.

Key Differences

1. A clustered index determines the physical order of data rows, while a non-clustered index does not.
2. Only one clustered index can exist on a table, while multiple non-clustered indexes can be created.
3. The leaf nodes of a clustered index contain the actual data rows, while the leaf nodes of a non-clustered index contain pointers to the data rows.
4. Clustered indexes are generally faster for retrieving data, as they eliminate the need for additional lookups to fetch data.
5. Non-clustered indexes are useful for improving query performance on columns that are frequently searched but not used for sorting or grouping data.

When designing a database schema and optimizing query performance, understanding when to use a clustered index versus a non-clustered index is essential for efficient data retrieval and management.