# Clustered Indexes: A Definition > [!Note] > Clustered Indexes define the physical ordering of table rows, ensuring that related data is stored contiguously for optimal retrieval. A **Clustered Index** organizes the actual data pages of a table in the order of its key. This means that the table itself becomes the index, and the rows are stored on disk following the **B-tree** structure defined by the index key. By aligning the physical sequence of rows with the logical sequence of key values, range scans and ordered retrieval operations become highly efficient, as the database engine can traverse leaf-level pages sequentially without random I/O. Because only one Clustered Index can exist per table, choosing the right column—or combination of columns—is crucial. The selected key should reflect the most common query patterns, particularly those involving **range queries**, **sorting**, or **grouping**. A poorly chosen key may lead to excessive page splits or fragmentation, degrading performance over time. To mitigate this, appropriate **fill factors** and periodic **index maintenance** operations, such as rebuilding or reorganizing, help preserve the integrity and performance of the index structure. In write-heavy environments, the overhead of maintaining a Clustered Index during **INSERT**, **UPDATE**, and **DELETE** operations must be balanced against the read-performance gains. Updates to indexed columns can trigger page movements, and large insert streams may induce page splits if new rows fall between existing ones. Understanding the trade-offs between data modification costs and query acceleration is essential when designing schemas that leverage Clustered Indexes effectively. --- ## References - Microsoft Docs: _Clustered Index Architecture and Design Guidelines_ - Garcia-Molina, H., Ullman, J. D., & Widom, J. _Database Systems: The Complete Book_ - 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.