How to Identify and Drop Unused Indexes

Indexes play a crucial role in SQL Server performance, but unused indexes can add unnecessary overhead and reduce efficiency. In this guide, we’ll explore how to identify and handle unused indexes in SQL Server to optimize your database performance.

Why Unused Indexes Are a Problem

Unused indexes consume disk space and add maintenance overhead. Every data modification (INSERT, UPDATE, DELETE) has to maintain these indexes, leading to higher CPU and I/O usage. Additionally, unused indexes can complicate the query optimization process by providing unnecessary options for the SQL Server optimizer, resulting in inefficient execution plans.

How to Find Unused Indexes Manually

SQL Server provides a view called sys.dm_db_index_usage_stats, which tracks how often an index has been used since the last SQL Server restart or database restore. This view helps identify potentially unused indexes.

Here’s an example query to identify unused indexes:

SELECT OBJECT_NAME(S.[OBJECT_ID]) AS [Table Name],  
  I.name AS [Index Name],  
  I.index_id AS [Index ID],  
  USER_SEEKS,  
  USER_SCANS,  
  USER_LOOKUPS,  
  USER_UPDATES  
FROM sys.dm_db_index_usage_stats AS S  
  INNER JOIN sys.indexes AS I ON I.OBJECT_ID = S.OBJECT_ID  
WHERE OBJECTPROPERTY(S.[OBJECT_ID],'IsUserTable') = 1  
  AND I.index_id = S.index_id  
  AND USER_SEEKS = 0  
  AND USER_SCANS = 0  
  AND USER_LOOKUPS = 0  
ORDER BY OBJECT_NAME(S.[OBJECT_ID]), I.name;

This query provides a list of indexes that have not been used since the statistics were last reset, helping you identify indexes that might be good candidates for removal.

⚠️ Caution: The statistics are reset when a database is restored, a server is restarted, or when working with replica databases that aren't queried. An index that appears unused may still be critical for specific processes.

How to Drop Unused Indexes

Once you’ve identified an unused index, it’s generally safe to remove it unless there are specific circumstances (e.g., the index is used in reporting or on replica servers).

To drop an unused index, use the following SQL command:

DROP INDEX [IX_IndexName] ON [TableName];
GO

Make sure to review index usage patterns carefully before dropping any indexes, especially in environments with frequent database restores or replicas.

How Aireforge Identifies Unused Indexes

Aireforge automates the process of identifying unused indexes by analyzing index usage data and applying strict checks to ensure indexes are only flagged as unused when enough data is available.

Here's how Aireforge handles unused indexes:

  1. Server Uptime Check:

    • Aireforge first checks whether the server has been running for at least 90 days to ensure sufficient data is available to assess index usage. If the uptime is less than this threshold, the check is skipped to avoid incorrectly flagging indexes that may not have had a chance to be used.

  2. Index Usage Monitoring:

    • Aireforge analyzes index activity using the sys.dm_db_index_usage_stats system view, which tracks index reads (including seeks, scans, and lookups).
    • For each index, Aireforge calculates the total number of reads. If an index has zero reads, it is flagged as unused.

  3. Primary Key and Unique Index Exclusions:

    • Primary keys and unique indexes are automatically excluded from the unused index check, as these indexes are critical for ensuring data integrity. Aireforge will mark these as "not checked" to indicate that they have been deliberately excluded from being flagged as unused.

  4. Unused Index Identification:

    • If an index has zero reads, it is flagged as unused. Aireforge then generates a SQL script that can be used to drop the index safely, ensuring easy cleanup.

  5. Creation Date Consideration:

    • If the index or table was created recently (i.e., after the server start date), Aireforge will add a note highlighting that the lack of usage could be due to the recent creation, not because the index is unnecessary.

  6. Script Generation:

    • Aireforge provides a SQL script for unused indexes, allowing you to drop these indexes easily if they are confirmed to be no longer needed.

Conclusion

By regularly reviewing and removing unused indexes, you can significantly improve SQL Server performance, reduce resource consumption, and optimize database efficiency. Aireforge makes this process straightforward by automatically identifying unused indexes, generating scripts for cleanup, and ensuring primary and unique indexes are left untouched.