Can a Clustered Index Be Non-Unique, and What Are the Performance Implications?

Can Clustered Indexes Be Non-Unique?

In general, clustered indexes are strongly recommended to be unique. However, it is possible to create a clustered index on a non-unique column.

Behavior of a Non-Unique Clustered Index

When a clustered index is non-unique, SQL Server generates an internal uniqueifier value to differentiate duplicate keys. This uniqueifier value adds some overhead in calculation and storage.

Performance Implications

The performance impact of a non-unique clustered index depends on various factors such as:

• Table size
• Insert rate
• Frequency of index usage

The overhead of the uniqueifier may become noticeable if:

• The table contains a large amount of data
• There is a high rate of inserts
• The index is frequently used in selects without covering indexes

Recommendation

To maximize performance, it is generally advisable to create clustered indexes on unique columns. If a non-unique clustered index is necessary due to specific application requirements, the potential performance implications should be carefully considered.

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