# Key Properties of the Clustered Index Key > [!Note] > `= Selecting an optimal clustered index key involves balancing **uniqueness**, **narrowness**, **immutability**, and an **ever-increasing** pattern to minimize storage overhead, fragmentation, and maintenance costs while maximizing query performance.` When a clustered index key is defined as **Unique**, SQL Server does not need to append an additional 4-byte uniquifier to rows, keeping the index narrower and more efficient. A **Narrow** key—one composed of the fewest possible bytes—reduces the overall size of the B-Tree, improves cache utilization, and lowers I/O overhead during seeks and scans. Because the clustered key is embedded in every nonclustered index, changes to a **Static** key value force updates across all related indexes, driving fragmentation and increasing maintenance work. Ever-Increasing keys (such as identity columns or sequence values) naturally append new rows at the end of leaf pages, preventing page splits and further reducing fragmentation. > [!tip] > Design your clustered key with ever-increasing values to avoid costly page splits and keep your index streamlined over time. --- ## ## References - Korotkevitch, D. (2022). _SQL Server advanced troubleshooting and performance tuning: Best practices and techniques_. O’Reilly Media. - Nevarez, B. (2022). _SQL Server query tuning and optimization: Optimize Microsoft SQL Server 2022 queries and applications_. Packt Publishing.