What is index depth?

Index depth is the number of B-tree levels SQL Server traverses from root to leaf. As an index grows, additional intermediate levels may be required.

A deeper index can require more page accesses for navigation, although SQL Server caches frequently used upper-level pages effectively. Narrow keys allow more index entries per page and can help keep structures smaller.

Index depth should be interpreted together with workload, page count, cache behavior, and query performance rather than treated as an isolated tuning target.