How to Find Missing Indexes in SQL Server

Introduction
The Importance of Missing Indexes
Identifying Missing Indexes with T-SQL
T-SQL description
The Formula Behind the Scenes
Conclusion

Introduction

In the world of managing databases, making sure your queries run fast is super important. One big way to do this is by having the right indexes set up in your database. But what if you're missing the perfect index? Don't worry! SQL Server has a way to find out which indexes are missing, so you can make your queries even faster.

The Importance of Missing Indexes

When the SQL Server query optimizer generates an execution plan for a query, it aims to find the optimal access path. However, if the ideal index doesn’t exist, the optimizer settles for the next best option or resorts to a table scan. Nevertheless, it stores details about these missing indexes.
Suggested indexes derived from such analysis should be rigorously tested to ensure they don’t inadvertently degrade performance.

Identifying Missing Indexes with T-SQL

This SQL query retrieves information from the dynamic management views - sys.dm_db_missing_index_group_stats, sys.dm_db_missing_index_groups and sys.dm_db_missing_index_details, providing the missing indexes information for a given database.

use dbdocs; -- Replace it with your application database 

select
    (user_seeks + user_scans) * avg_total_user_cost * (avg_user_impact * 0.01) as index_advantage,
    dbmigs.last_user_seek,
    dbmid.statement as "table",
    dbmid.equality_columns,
    dbmid.inequality_columns,
    dbmid.included_columns,
    dbmigs.unique_compiles,
    dbmigs.user_seeks,
    dbmigs.user_scans,
    dbmigs.avg_total_user_cost,
    dbmigs.avg_user_impact
from
    sys.dm_db_missing_index_group_stats as dbmigs with (nolock)
    inner join
    sys.dm_db_missing_index_groups as dbmig with (nolock)
        on dbmigs.group_handle = dbmig.index_group_handle
    inner join
    sys.dm_db_missing_index_details as dbmid with (nolock)
        on dbmig.index_handle = dbmid.index_handle
where
    dbmid.database_id = db_id()
order by
    index_advantage desc;

T-SQL description

Now, let's break down each component and understand its significance

Colum               | Description
------------------- | ---------------------------------------------------------
index_advantage     | Calculated benefit of suggested index.
last_user_seek      | Last time a seek operation might have used the index.
table               | Table name.
equality_columns    | Columns that would have been useful based on equality predicate.
inequality_columns  | Columns that would have been useful based on inequality predicate.
included_columns    | Columns that, if included, would have been useful to cover the query.
unique_compiles     | Number of plans that have been compiled/recompiled that might have used the index.
user_seeks          | Number of seek operations in user queries that might have used the index.
user_scans          | Number of scan operations in user queries that might have used the index.
avg_total_user_cost | Average cost saving for the queries that could have been helped bu the index.
avg_user_impact     | Estimated percentatge by which the average query cost would drop, for queries that could use this index..

The Formula Behind the Scenes

The SQL Server Query Optimization Team at Microsoft employs a formula to calculate the overall benefit of a suggested index. This formula, based on columns in the sys.dm_db_missing_index_group_stats, is as follows:
 (user_seeks + user_scans) * avg_total_user_cost * (avg_user_impact * 0.01)  
In essence, this formula takes into account the number of seeks and scans, the cost of these operations, and the impact of the index on query performance to quantify the potential advantage of a missing index.

Conclusion

Uncovering missing indexes in your SQL Server database can be a game-changer in optimizing query performance. By leveraging the provided SQL query and understanding the principles behind it, you’re equipped to identify potential areas for improvement and enhance the overall efficiency of your database operations. Remember, though, always validate suggested indexes before implementing them to ensure they deliver the expected performance benefits without unintended consequences.

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Last updated in July, 2024

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