Should Clustered Indexes Always Be Unique?

Unique or Not: The Dilemma of Clustered Indexes

Clustered indexes are crucial for optimizing database performance, but do they always have to be unique? Let's delve into the intricacies of clustered indexes and explore the consequences of using non-unique keys.

Firstly, clustered indexes are not inherently required to be unique. However, it is strongly recommended for several reasons. If a clustered index is non-unique, SQL Server creates a uniqueifier to distinguish duplicate keys. While this uniqueifier adds some overhead in storage and calculation, its impact on performance is usually negligible for small tables with low insert rates.

However, for large tables with high insert rates, using a unique clustered index is more beneficial. This is because when a non-unique clustered index is used, new rows can overflow into additional pages, resulting in performance degradation. Therefore, it is prudent to use a unique clustered index to avoid such overflow and maintain optimal performance.

Now, let's address the question of how to make a clustered index unique. The best practice is to create the clustered index on a column or a combination of columns that is naturally unique or has distinct values for each row. This ensures uniqueness without the need for the uniqueifier.

If making the existing clustered index unique is not feasible, an alternative approach is to create a unique non-clustered index. This allows for faster lookups and inserts by utilizing the uniqueifier. However, it does not protect against overflow pages in the clustered index.

In conclusion, it is generally recommended to use unique clustered indexes for optimal database performance. If non-unique clustered indexes are employed, be aware of the potential for overflow pages and the associated performance implications. By carefully considering the data characteristics and performance requirements, you can make informed decisions about whether to use unique or non-unique clustered indexes.

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