Tips on Optimizing SQL Server Clustered Indexes

When deciding on whether to create a clustered or non-clustered index, it is often helpful to know what the estimated size of the clustered index will be. Sometimes, the size of a clustered index on a particular column or columns may be very large, leading to database size bloat and increased disk I/O. [7.0, 2000, 2005] Updated 3-5-2004


Clustered index values often repeat many times in a table’s non-clustered storage structures, and a large clustered index value can unnecessarily increase the physical size of a non-clustered index. This increases disk I/O and reduces performance when non-clustered indexes are accessed.

Because of this, ideally a clustered index should be based on a single column (not multiple columns) that is as narrow as possible. This not only reduces the clustered index’s physical size, it also reduces the physical size of non-clustered indexes and boosts SQL Servers overall performance. [6.5, 7.0, 2000, 2005] Updated 10-15-2004


When you create a clustered index, try to create it as a UNIQUE clustered index, not a non-unique clustered index. The reason for this is that while SQL Server will allow you to create a non-unique clustered index, under the surface, SQL Server will make it unique for you by adding a 4-byte “uniqueifer” to the index key to guarantee uniqueness. This only serves to increase the size of the key, which increases disk I/O, which reduces performance. If you specify that your clustered index is UNIQUE when it is created, you will prevent this unnecessary overhead. [7.0, 2000, 2005] Updated 10-15-2004


If possible, avoid adding a clustered index to a GUID column (uniqueidentifier data type). GUIDs take up 16-bytes of storage, more than an Identify column, which makes the index larger, which increases I/O reads, which can hurt performance. While the performance hit will be minimal if you do decide to add a clustered index to a GUID column, every little bit ads up. [7.0, 2000, 2005] Updated 10-15-2004

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