How to Identify and Handle Superseded Indexes

Indexes are essential for boosting query performance, but superseded indexes, which are made redundant by more comprehensive indexes, can create unnecessary overhead. Dropping these redundant indexes helps optimize your SQL Server by simplifying index management and reducing resource consumption.

What Are Superseded Indexes?

A superseded index occurs when a broader index covers all the same columns and queries that a narrower index handles. For example, if there’s an index on columns A and B but a broader index on columns A, B, and C also exists, the wider index can handle all queries, making the narrower index redundant. Retaining both indexes leads to inefficiency and redundant resource use.

Why Superseded Indexes Are a Problem

  • Maintenance Overhead: Maintaining a superseded index during data modifications (INSERT, UPDATE, DELETE) wastes valuable CPU and I/O resources.
  • Optimizer Confusion: The presence of unnecessary indexes provides more options for SQL Server’s query optimizer to evaluate, which can lead to suboptimal execution plans.
  • Wasted Storage: Keeping multiple redundant indexes consumes disk space and adds complexity without improving performance.

How to Handle Superseded Indexes in SQL Server

Dropping Superseded Indexes If an index is fully covered by a more comprehensive index, you can safely drop the narrower index to reduce overhead. For instance, if a wider index covers columns A, B, and C, and another index only covers A and B, the latter is redundant and can be removed.

Merging Superseded Indexes If a superseded index has additional included columns that the more comprehensive index doesn't, consider merging them by creating a new index that combines both.

Suggested Actions

  • Drop Superseded Indexes: If the wider index (e.g., A, B, C) covers all queries handled by the superseded index (e.g., A, B), drop the narrower index to streamline your strategy.
  • Merge Indexes: If the superseded index includes extra columns not in the wider index, consider merging them to create a single, optimized index.

How to Drop a Superseded Index in SQL Server

To drop a superseded index, use this SQL command:

DROP INDEX [IX_IndexName] ON [TableName];
GO

Before dropping any index, ensure it’s not required for specific queries or use cases, especially in complex environments with many dependencies.

How Aireforge Handles Superseded Indexes

Aireforge automates the identification of superseded indexes by analyzing index structures to identify when one index is fully covered by another broader index. This process ensures that redundant indexes are flagged and can be dropped, optimizing your database performance.

Here's how Aireforge handles the check:

  1. Index Tree Structure:

    • Aireforge builds a tree of indexes for each table using the key and included columns to determine relationships between indexes. This structure helps identify cases where a broader index supersedes a narrower one.

  2. Comprehensive Coverage Detection:

    • If an index is completely covered by another index, Aireforge flags the narrower index as superseded. This includes both key columns and included columns in the analysis, ensuring that the broader index can handle all the queries that the narrower index was designed for.

  3. Maintenance Overhead Evaluation:

    • Aireforge considers the maintenance cost associated with each superseded index, highlighting those that could be dropped to reduce unnecessary overhead during data modifications.

  4. Merging Suggestions:

    • In cases where an index is partially superseded (e.g., it has additional included columns that the broader index lacks), Aireforge suggests merging the indexes by adding the missing columns to the broader index.

  5. Script Generation:

    • Aireforge generates a SQL script to drop superseded indexes or to merge indexes when necessary, allowing you to streamline your index strategy quickly and efficiently.

Conclusion

Superseded indexes create unnecessary complexity and resource consumption in your SQL Server environment. Aireforge simplifies the process of identifying and handling these indexes by automatically flagging them and generating scripts to drop or merge them. This ensures your database remains lean and optimized without redundant indexes.