How to Handle Infrequently Used Indexes

Indexes are essential for speeding up query performance in SQL Server, but little used indexes, those that are only used for a small proportion of queries, can introduce unnecessary overhead. This warning is raised when SQL Server's statistics indicate that the index is infrequently used in relation to the total number of queries on the table.

Why Infrequently Used Indexes Can Be a Problem

Maintaining an index comes with a performance cost. Every time data is modified (INSERT, UPDATE, DELETE), the index needs to be updated as well. If an index is only used in a small fraction of the queries on the table, the cost of maintaining it may outweigh the performance benefits it provides. Dropping an infrequently used index can reduce overhead and improve the overall efficiency of your database.

⚠️ Note: This warning is based on SQL Server's statistics, which are reset after a database restore or server restart. If your database is frequently restored, restarted, or a read-only replica, these statistics may not reflect the actual usage of the index.

How to Find Little Used 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 indexes that are used infrequently.

Here’s an example query to identify little used indexes based on the total number of reads:

SELECT 
    OBJECT_NAME(I.[object_id]) AS [Table Name],
    I.name AS [Index Name],
    SUM(S.USER_SEEKS + S.USER_SCANS + S.USER_LOOKUPS) AS [Total Reads],
    SUM(S.USER_UPDATES) AS [Total Writes],
    CASE 
        WHEN SUM(S.USER_SEEKS + S.USER_SCANS + S.USER_LOOKUPS) = 0 THEN 'Unused'
        ELSE 'Infrequently Used'
    END AS [Usage Status]
FROM 
    sys.indexes AS I
    LEFT JOIN sys.dm_db_index_usage_stats AS S 
        ON I.[object_id] = S.[object_id] 
        AND I.index_id = S.index_id
WHERE 
    OBJECTPROPERTY(I.[object_id], 'IsUserTable') = 1
    AND I.is_primary_key = 0  -- Exclude primary keys
    AND I.is_unique = 0       -- Exclude unique indexes
GROUP BY 
    OBJECT_NAME(I.[object_id]), I.name
HAVING 
    SUM(S.USER_SEEKS + S.USER_SCANS + S.USER_LOOKUPS) < 100  -- Example threshold for "little used"
ORDER BY 
    [Total Reads] ASC;

This query provides a list of indexes that are used infrequently, helping you identify indexes that might be candidates for removal or consolidation.

⚠️ 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 or infrequently used may still be critical for specific processes.

Suggested Action

If an index is found to be little used, consider dropping it to reduce maintenance overhead. However, make sure that the queries relying on this index are not critical or can still run efficiently without it. Always review the impact on query performance before removing any index.

How to Drop an Infrequently Used Index

You can drop an infrequently used index using the following SQL command:

DROP INDEX [IX_IndexName] ON [TableName];
GO

Ensure you carefully evaluate the queries relying on this index before dropping it, as this action may impact their performance.

How Aireforge Handles Infrequently Used Indexes

Aireforge automates the identification of infrequently used indexes by analyzing index usage data and applying a configurable percentage threshold for flagging indexes as "little used."

Here's how Aireforge handles the check:

  1. Server Uptime Check:

    • Aireforge 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 percentage of total reads that the index contributes to. If the index is used in less than a specified percentage (e.g., 5%) of total reads, it is flagged as little used.

  3. Configurable Threshold:

    • The percentage threshold for flagging an index as "little used" is configurable, allowing you to tailor the check based on your environment. For example, Aireforge may be set to flag indexes used in less than 5% of total reads.

  4. Primary Key and Unique Index Exclusions:

    • Primary keys and unique indexes are automatically excluded from the little used 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 little used.

  5. Script Generation:

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

Conclusion

Identifying and addressing infrequently used indexes can help reduce database overhead and improve performance. Aireforge streamlines this process by automatically identifying little used indexes based on a configurable percentage threshold, ensuring that your database remains efficient and well-maintained.