Applies to: SQL Server (all supported versions) Azure SQL Database Azure SQL Managed Instance
Returns information about database table columns that are missing an index.
sys.dm_db_missing_index_columns is a dynamic management function.
An integer that uniquely identifies a missing index. It can be obtained from the following dynamic management objects:
|Column name||Data type||Description|
|column_id||int||ID of the column.|
|column_name||sysname||Name of the table column.|
|column_usage||varchar(20)||How the column is used by the query. The possible values and their descriptions are:
EQUALITY: Column contributes to a predicate that expresses equality, of the form:
table.column = constant_value
INEQUALITY: Column contributes to a predicate that expresses inequality, for example, a predicate of the form: table.column > constant_value. Any comparison operator other than "=" expresses inequality.
INCLUDE: Column is not used to evaluate a predicate, but is used for another reason, for example, to cover a query.
Information returned by
sys.dm_db_missing_index_columns is updated when a query is optimized by the query optimizer, and is not persisted. Missing index information is kept only until the database engine is restarted. Database administrators should periodically make backup copies of the missing index information if they want to keep it after server recycling. Use the
sqlserver_start_time column in sys.dm_os_sys_info to find the last database engine startup time.
If a transaction creates or drops a table, the rows containing missing index information about the dropped objects are removed from this dynamic management object, preserving transaction consistency.
Users must be granted the VIEW SERVER STATE permission or any permission that implies the VIEW SERVER STATE permission to query this dynamic management function.
The following example runs a query against the
Address table and then runs a query using the
sys.dm_db_missing_index_columns dynamic management view to return the table columns that are missing an index.
USE AdventureWorks2012; GO SELECT City, StateProvinceID, PostalCode FROM Person.Address WHERE StateProvinceID = 9; GO SELECT mig.*, statement AS table_name, column_id, column_name, column_usage FROM sys.dm_db_missing_index_details AS mid CROSS APPLY sys.dm_db_missing_index_columns (mid.index_handle) INNER JOIN sys.dm_db_missing_index_groups AS mig ON mig.index_handle = mid.index_handle ORDER BY mig.index_group_handle, mig.index_handle, column_id; GO
Missing index suggestions should be combined when possible with one another, and with existing indexes in the current database. Learn how to apply these suggestions in tune nonclustered indexes with missing index suggestions.
Learn more about the missing index feature in the following articles: