Explanation of Unused Indexes in SQL Server

Unused Indexes are indexes that exist in a table but are rarely or never used by SQL Server for reading data. These indexes consume storage and slow down write operations (INSERT, UPDATE, DELETE) because SQL Server has to keep them updated, even if they’re not helping in queries.

SQL Server tracks index usage, and you can identify unused indexes using the system view:

sys.dm_db_index_usage_stats

1. SELECT   
2.     OBJECT_NAME(i.object_id) AS TableName,  
3.     i.name AS IndexName,  
4.     i.index_id,  
5.     s.user_seeks,  
6.     s.user_scans,  
7.     s.user_lookups,  
8.     s.user_updates  
9. FROM   
10.     sys.indexes AS i  
11. LEFT JOIN   
12.     sys.dm_db_index_usage_stats AS s   
13.     ON i.object_id = s.object_id AND i.index_id = s.index_id  
14. WHERE   
15.     i.type_desc = 'NONCLUSTERED'  
16.     AND s.user_seeks IS NULL   
17.     AND s.user_scans IS NULL   
18.     AND s.user_lookups IS NULL  
19.     AND i.is_primary_key = 0  
20.     AND i.is_unique_constraint = 0;  

This shows nonclustered indexes that have never been used (no seeks, scans, or lookups).

Conclusion:

Unused indexes can negatively impact performance by slowing down data modification and increasing storage. Regularly checking and removing them helps keep your database optimized. But be careful—review usage trends over time before dropping any index.