sys.dm_db_missing_index_group_stats_query (Transact-SQL)

Straipsnis
02/27/2023

Applies to: SQL Server 2019 (15.x) Azure SQL Database Azure SQL Managed Instance

Returns information about queries that needed a missing index from groups of missing indexes, excluding spatial indexes. More than one query may be returned per missing index group. One missing index group may have several queries that needed the same index.

In Azure SQL Database, dynamic management views cannot expose information that would impact database containment or expose information about other databases the user has access to. To avoid exposing this information, every row that contains data that doesn't belong to the connected tenant is filtered out.

Column name	Data type	Description
group_handle	int	Identifies a group of missing indexes. This identifier is unique across the server. The other columns provide information about all queries for which the index in the group is considered missing. An index group contains only one index. Can be joined to `index_group_handle` in sys.dm_db_missing_index_groups.
query_hash	binary(8)	Binary hash value calculated on the query and used to identify queries with similar logic. You can use the query hash to determine the aggregate resource usage for queries that differ only by literal values.
query_plan_hash	binary(8)	Binary hash value calculated on the query execution plan and used to identify similar query execution plans. You can use query plan hash to find the cumulative cost of queries with similar execution plans. Will always be 0x000 when a natively compiled stored procedure queries a memory-optimized table.
last_sql_handle	varbinary(64)	Is a token that uniquely identifies the batch or stored procedure of the last compiled statement that needed this index. The `last_sql_handle` can be used to retrieve the SQL text of the query by calling the dynamic management function sys.dm_exec_sql_text.
last_statement_start_offset	int	Indicates, in bytes, beginning with 0, the starting position of the query that the row describes within the text of its batch or persisted object for the last compiled statement that needed this index in its SQL batch.
last_statement_end_offset	int	Indicates, in bytes, beginning with 0, the ending position of the query that the row describes within the text of its batch or persisted object for the last compiled statement that needed this index in its SQL batch.
last_statement_sql_handle	varbinary(64)	Is a token that uniquely identifies the batch or stored procedure of the last compiled statement that needed this index. Used by Query Store. Unlike `last_sql_handle`, `sys.query_store_query_text` references the `statement_sql_handle` used by the Query Store catalog view sys.query_store_query_text. If Query Store was not enabled when the query was compiled, returns 0.
user_seeks	bigint	Number of seeks caused by user queries that the recommended index in the group could have been used for.
user_scans	bigint	Number of scans caused by user queries that the recommended index in the group could have been used for.
last_user_seek	datetime	Date and time of last seek caused by user queries that the recommended index in the group could have been used for.
last_user_scan	datetime	Date and time of last scan caused by user queries that the recommended index in the group could have been used for.
avg_total_user_cost	float	Average cost of the user queries that could be reduced by the index in the group.
avg_user_impact	float	Average percentage benefit that user queries could experience if this missing index group was implemented. The value means that the query cost would on average drop by this percentage if this missing index group was implemented.
system_seeks	bigint	Number of seeks caused by system queries, such as auto stats queries, that the recommended index in the group could have been used for. For more information, see Auto Stats Event Class.
system_scans	bigint	Number of scans caused by system queries that the recommended index in the group could have been used for.
last_system_seek	datetime	Date and time of last system seek caused by system queries that the recommended index in the group could have been used for.
last_system_scan	datetime	Date and time of last system scan caused by system queries that the recommended index in the group could have been used for.
avg_total_system_cost	float	Average cost of the system queries that could be reduced by the index in the group.
avg_system_impact	float	Average percentage benefit that system queries could experience if this missing index group was implemented. The value means that the query cost would on average drop by this percentage if this missing index group was implemented.

Remarks

Information returned by sys.dm_db_missing_index_group_stats_query is updated by every query execution, not by every query compilation or recompilation. Usage statistics are not persisted and are 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 the usage statistics after server recycling. Use the sqlserver_start_time column in sys.dm_os_sys_info to find the last database engine startup time. You can also persist missing indexes with Query Store.

Note

The result set for this DMV is limited to 600 rows. Each row contains one missing index. If you have more than 600 missing indexes, you should address the existing missing indexes so you can then view the newer ones.

Permissions

To query this dynamic management view, users must be granted the VIEW SERVER STATE permission or any permission that implies the VIEW SERVER STATE permission.

Permissions for SQL Server 2022 and later

Requires VIEW SERVER PERFORMANCE STATE permission on the server.

Examples

The following examples illustrate how to use the sys.dm_db_missing_index_group_stats_query dynamic management view.

A. Find the latest query text for the top 10 highest anticipated improvements for user queries

The following query returns the last recorded query text for the 10 missing indexes that would produce the highest anticipated cumulative improvement, in descending order.

SELECT TOP 10 
    SUBSTRING
    (
            sql_text.text,
            misq.last_statement_start_offset / 2 + 1,
            (
            CASE misq.last_statement_start_offset
                WHEN -1 THEN DATALENGTH(sql_text.text)
                ELSE misq.last_statement_end_offset
            END - misq.last_statement_start_offset
            ) / 2 + 1
    ),
    misq.*
FROM sys.dm_db_missing_index_group_stats_query AS misq
CROSS APPLY sys.dm_exec_sql_text(misq.last_sql_handle) AS sql_text
ORDER BY misq.avg_total_user_cost * misq.avg_user_impact * (misq.user_seeks + misq.user_scans) DESC;

Next steps

Learn more about the missing index feature and related concepts in the following articles:

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