When Should I Use a Non-Unique Clustered Index in SQL Server?

Understanding the Non-Uniqueness of Clustered Indexes

When creating a clustered index, it's typical to assume that the indexed column is unique. However, it's important to address the potential scenarios where a clustered index might not be unique.

Consequences of Non-Unique Clustered Indexes

When a clustered index is non-unique, SQL Server inserts a uniqueifier value into the index to distinguish duplicate rows. While this ensures row uniqueness, it introduces an additional overhead in calculation and storage. Depending on the table size, insert rate, and index usage, this overhead can potentially impact performance.

Best Practices

To optimize performance, it's generally recommended to make your clustered indexes unique by using a truly unique column. However, there may be exceptions where using a non-unique clustered index might make sense.

For example, in a scenario where you prioritize grouping rows by a logical partition rather than uniqueness, creating a clustered index on a non-unique partition column may be appropriate. In such cases, the performance impact of the uniqueifier should be carefully considered against the benefits of logical partitioning.

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