Identify and Drop Hypothetical Indexes

Hypothetical indexes in SQL Server are special indexes created by performance tuning tools like the Index Tuning Wizard and Database Tuning Advisor. These indexes are not used in actual query execution but are maintained by SQL Server for optimization analysis. While these indexes should be deleted once the tuning process is complete, errors can leave them behind, cluttering your database and wasting resources.

What Are Hypothetical Indexes?

Hypothetical indexes are created by SQL Server's tuning tools to analyze potential query optimizations. Unlike regular indexes, hypothetical indexes don't store data or contribute to query performance. They are only used for optimization purposes during performance tuning.

However, if a crash or error occurs during the tuning process, these indexes may not be automatically removed, leading to unnecessary clutter and potential confusion when analyzing your index usage.

Why Should You Drop Hypothetical Indexes?

Leaving hypothetical indexes in your SQL Server database provides no benefit. In fact, they can cause several issues:

  • Wasted Resources: SQL Server still maintains hypothetical indexes, which adds unnecessary overhead during maintenance tasks.
  • Clutter: They can clutter your index list, making it harder to assess the real indexes in your system.
  • Confusion: Because these indexes aren't visible in tools like SQL Server Management Studio (SSMS), they can lead to confusion when you're auditing your index usage.

How to Identify and Drop Hypothetical Indexes in SQL Server

If you're sure the Database Tuning Advisor or Index Tuning Wizard isn't running, it's a good idea to drop any leftover hypothetical indexes to clean up your environment.

Identify Hypothetical Indexes:

You can identify hypothetical indexes by querying the sys.indexes system view. Hypothetical indexes can be filtered using the is_hypothetical column.

SELECT name, object_id, index_id
FROM sys.indexes
WHERE is_hypothetical = 1;

Drop Hypothetical Indexes:

Once you've identified the hypothetical indexes, you can drop them using the following SQL command:

DROP INDEX [IX_Name] ON [TableName];
GO

Removing hypothetical indexes will not impact your query performance, as they are not used for actual data retrieval.

Best Practices for Managing Hypothetical Indexes

  • Check Before Dropping: Always ensure the Database Tuning Advisor or Index Tuning Wizard isn't running before dropping hypothetical indexes. These indexes are only needed for analysis and serve no purpose once the tuning process is complete.
  • Regular Audits: Include a review of hypothetical indexes in your regular database audits to keep your system clean and optimized.

How Aireforge Handles Hypothetical Indexes

Aireforge automatically identifies hypothetical indexes that are no longer needed and flags them for review and removal. Here’s how Aireforge handles these indexes:

  1. Index Detection: Aireforge scans all the indexes in a table and flags those marked as hypothetical by checking the is_hypothetical property.

  2. Automated Cleanup: Aireforge generates a SQL script to drop the hypothetical index, allowing you to clean up your environment efficiently.

  3. Low Severity Warnings: Hypothetical indexes are flagged with a low-severity warning, as they don’t directly affect query performance but can add clutter and confusion.

  4. No Impact on Performance: Aireforge ensures that removing hypothetical indexes won’t impact database performance, as these indexes are not used in actual query execution.

Conclusion

Hypothetical indexes are a byproduct of performance tuning processes and should be cleaned up after the analysis is complete. They don't contribute to query performance but can clutter your database and add unnecessary maintenance overhead. Aireforge makes it easy to identify and remove these indexes, keeping your system lean and efficient.