What Happens When a Clustered Index Isn't Unique in SQL Server?

Clustered Indexes and Uniqueness: Unraveling the Mystery

A clustered index organizes table data based on its key values. While it's generally advisable for clustered indexes to be unique, this is not a strict requirement. The question arises: what happens if a clustered index is non-unique?

If a clustered index is not unique, SQL Server inserts a uniqueifier value to each duplicate key. This ensures that every row is uniquely identified internally. However, this additional step introduces some performance overhead due to the calculation and storage of the uniqueifier value.

The impact of this overhead depends on various factors:

• Table data size: Larger tables may experience more significant overhead.
• Insert rate: High insert rates can increase the overhead of maintaining unique identifiers.
• Index usage: If the clustered index is heavily used in queries without covering indexes, the overhead becomes more noticeable.

It's worth noting that in certain scenarios, creating a non-unique clustered index may be a viable option. For instance, when the data is naturally organized in a non-unique manner or to improve performance in specific query patterns.

However, in most cases, it's recommended to create unique clustered indexes to avoid the performance implications of maintaining uniqueifiers. This ensures optimal indexing efficiency and supports optimal query performance.

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