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sys.dm_exec_query_optimizer_info (Transact-SQL)

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Applies to: SQL Server Azure SQL Database Azure SQL Managed Instance Azure Synapse Analytics Analytics Platform System (PDW)

Returns detailed statistics about the operation of the SQL Server query optimizer. You can use this view when tuning a workload to identify query optimization problems or improvements. For example, you can use the total number of optimizations, the elapsed time value, and the final cost value to compare the query optimizations of the current workload and any changes observed during the tuning process. Some counters provide data that is relevant only for SQL Server internal diagnostic use. These counters are marked as "Internal only."

Note

To call this from Azure Synapse Analytics or Analytics Platform System (PDW), use the name sys.dm_pdw_nodes_exec_query_optimizer_info. This syntax is not supported by serverless SQL pool in Azure Synapse Analytics.

Name Data type Description
counter nvarchar(4000) Name of optimizer statistics event.
occurrence bigint Number of occurrences of optimization event for this counter.
value float Average property value per event occurrence.
pdw_node_id int The identifier for the node that this distribution is on.

Applies to: Azure Synapse Analytics, Analytics Platform System (PDW)

Permissions

SQL Server 2019 (15.x) and earlier versions, and Azure SQL Managed Instance, require VIEW SERVER STATE permission.

SQL Server 2022 (16.x) and later versions, requires VIEW SERVER PERFORMANCE STATE permission on the server.

On Azure SQL Database Basic, S0, and S1 service objectives, and for databases in elastic pools, the server admin account, the Microsoft Entra admin account, or membership in the ##MS_ServerStateReader## server role is required. On all other SQL Database service objectives, either the VIEW DATABASE STATE permission on the database, or membership in the ##MS_ServerStateReader## server role is required.

Remarks

sys.dm_exec_query_optimizer_info contains the following properties (counters). All occurrence values are cumulative and are set to 0 at system restart. All values for value fields are set to NULL at system restart. All value-column values that specify an average use the occurrence value from the same row as the denominator in the calculation of the average. All query optimizations are measured when SQL Server determines changes to dm_exec_query_optimizer_info, including both user-generated and system-generated queries. Execution of an already-cached plan doesn't change values in dm_exec_query_optimizer_info, only optimizations are significant.

Counter Occurrence Value
optimizations Total number of optimizations. Not applicable
elapsed time Total number of optimizations. Average elapsed time per optimization of an individual statement (query), in seconds.
final cost Total number of optimizations. Average estimated cost for an optimized plan in internal cost units.
trivial plan Internal only Internal only
tasks Internal only Internal only
no plan Internal only Internal only
search 0 Internal only Internal only
search 0 time Internal only Internal only
search 0 tasks Internal only Internal only
search 1 Internal only Internal only
search 1 time Internal only Internal only
search 1 tasks Internal only Internal only
search 2 Internal only Internal only
search 2 time Internal only Internal only
search 2 tasks Internal only Internal only
gain stage 0 to stage 1 Internal only Internal only
gain stage 1 to stage 2 Internal only Internal only
timeout Internal only Internal only
memory limit exceeded Internal only Internal only
insert stmt Number of optimizations that are for INSERT statements. Not applicable
delete stmt Number of optimizations that are for DELETE statements. Not applicable
update stmt Number of optimizations that are for UPDATE statements. Not applicable
contains subquery Number of optimizations for a query that contains at least one subquery. Not applicable
unnest failed Internal only Internal only
tables Total number of optimizations. Average number of tables referenced per query optimized.
hints Number of times some hint was specified. Hints counted include: JOIN, GROUP, UNION and FORCE ORDER query hints, FORCE PLAN set option, and join hints. Not applicable
order hint Number of times where join order was forced. This counter isn't restricted to the FORCE ORDER hint. Specifying a join algorithm within a query, such as an INNER HASH JOIN, also forces the join order, which increments the counter. Not applicable
join hint Number of times the join algorithm was forced by a join hint. The FORCE ORDER query hint doesn't increment this counter. Not applicable
view reference Number of times a view is referenced in a query. Not applicable
remote query Number of optimizations where the query referenced at least one remote data source, such as a table with a four-part name or an OPENROWSET result. Not applicable
maximum DOP Total number of optimizations. Average effective MAXDOP value for an optimized plan. By default, effective MAXDOP is determined by the max degree of parallelism server configuration option, and might be overridden for a specific query by the value of the MAXDOP query hint.
maximum recursion level Number of optimizations in which a MAXRECURSION level greater than 0 was specified with the query hint. Average MAXRECURSION level in optimizations where a maximum recursion level was specified with the query hint.
indexed views loaded Internal only Internal only
indexed views matched Number of optimizations where one or more indexed views are matched. Average number of views matched.
indexed views used Number of optimizations where one or more indexed views are used in the output plan after being matched. Average number of views used.
indexed views updated Number of optimizations of a DML statement that produce a plan that maintains one or more indexed views. Average number of views maintained.
dynamic cursor request Number of optimizations in which a dynamic cursor request was specified. Not applicable
fast forward cursor request Number of optimizations in which a fast-forward cursor request was specified. Not applicable
merge stmt Number of optimizations that are for MERGE statements. Not applicable

Examples

A. View statistics on optimizer execution

What are the current optimizer execution statistics for this instance of SQL Server?

SELECT * FROM sys.dm_exec_query_optimizer_info;

B. View the total number of optimizations

How many optimizations are performed?

SELECT occurrence AS Optimizations
FROM sys.dm_exec_query_optimizer_info
WHERE counter = 'optimizations';

C. Average elapsed time per optimization

What is the average elapsed time per optimization?

SELECT ISNULL(value,0.0) AS ElapsedTimePerOptimization
FROM sys.dm_exec_query_optimizer_info
WHERE counter = 'elapsed time';

D. Fraction of optimizations that involve subqueries

What fraction of optimized queries contained a subquery?

SELECT (
    SELECT CAST(occurrence AS FLOAT)
    FROM sys.dm_exec_query_optimizer_info
    WHERE counter = 'contains subquery'
) / (
    SELECT CAST(occurrence AS FLOAT)
    FROM sys.dm_exec_query_optimizer_info
    WHERE counter = 'optimizations'
) AS ContainsSubqueryFraction;

E. View the total number of hints during optimization

How many hints are counted when FORCE ORDER is included as a query hint?

-- Check hint count before query execution
SELECT ISNULL('', 0) AS [Before],
    [counter],
    occurrence
FROM sys.dm_exec_query_optimizer_info
WHERE [counter] IN (
    'hints',
    'order hint',
    'join hint'
);

SELECT poh.PurchaseOrderID,
    poh.OrderDate,
    pod.ProductID,
    pod.DueDate,
    poh.VendorID
FROM Purchasing.PurchaseOrderHeader AS poh
INNER MERGE JOIN Purchasing.PurchaseOrderDetail AS pod
    ON poh.PurchaseOrderID = pod.PurchaseOrderID
OPTION (
    FORCE ORDER,
    RECOMPILE
);

-- check hint count after query execution
SELECT ISNULL('', 0) AS [After],
    [counter],
    occurrence
FROM sys.dm_exec_query_optimizer_info
WHERE [counter] IN (
    'hints',
    'order hint',
    'join hint'
);

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