Query Store hints

Applies to: SQL Server 2022 (16.x) and later versions Azure SQL Database Azure SQL Managed Instance SQL database in Microsoft Fabric Preview

This article outlines how to apply query hints using the Query Store. Query Store hints provide an easy-to-use method for shaping query plans without changing application code.

• For more information on configuring and administering with the Query Store, see Monitor performance by using the Query Store.
• For information on discovering actionable information and tune performance with the Query Store, see Tune performance with the Query Store.
• For information about operating the Query Store in Azure SQL Database, see Operating the Query Store in Azure SQL Database.

Watch this video for an overview of Query Store hints:

Overview

Ideally, the Query Optimizer selects an optimal execution plan for a query.

If an optimal plan isn't selected, a developer or database administrator (DBA) might wish to manually optimize for specific conditions. Query hints are specified via the OPTION clause and can be used to affect query execution behavior. While query hints help provide localized solutions to various performance-related issues, they do require a rewrite of the original query text. Database administrators and developers might not always be able to make changes directly to Transact-SQL code to add a query hint. The Transact-SQL might be hard-coded into an application or automatically generated by the application. Previously, a developer might have to rely on plan guides, which can be complex to use.

Query Store hints solve this problem by letting you inject a query hint into a query without modifying the query Transact-SQL text directly. For information on which query hints can be applied, see Supported query hints.

When to use Query Store hints

As the name suggests, this feature extends and depends on the Query Store. Query Store enables the capturing of queries, execution plans, and associated runtime statistics. Query Store greatly simplifies the overall performance tuning customer experience. SQL Server 2016 (13.x) first introduced Query Store, and now it's enabled by default in SQL Server 2022 (16.x), Azure SQL Managed Instance, Azure SQL Database, and SQL database in Microsoft Fabric Preview.

Examples where Query Store hints can help with query-level performance issues:

• Recompile a query on each execution.
• Cap the memory grant size for a bulk insert operation.
• Limit the maximum degree of parallelism when updating statistics.
• Use a Hash join instead of a Nested Loops join.
• Use compatibility level 110 for a specific query while keeping everything else in the database at compatibility level 150.
• Disable row goal optimization for a SELECT TOP query.

To use Query Store hints:

1. Identify the Query Store query_id of the query statement you wish to modify. You can do this in various ways:

   • Querying the Query Store catalog views (Transact-SQL).
   • Using SQL Server Management Studio built-in Query Store reports.
   • Using Azure portal Query Performance Insight for Azure SQL Database.

2. Execute sys.sp_query_store_set_hints with the query_id and query hint string you wish to apply to the query. This string can contain one or more query hints. For complete information, see sys.sp_query_store_set_hints.

Once created, Query Store hints are persisted and survive restarts and failovers. Query Store hints override hard-coded statement-level hints and existing plan guide hints.

If a query hint contradicts what is possible for query optimization, query execution isn't blocked and the hint isn't applied. In the cases where a hint would cause a query to fail, the hint is ignored, and the latest failure details can be viewed in sys.query_store_query_hints.

Before you use Query Store hints

Consider the following before you start using Query Store hints.

• Complete statistics maintenance and index maintenance (if necessary) before evaluating queries for potential new Query Store hints. Statistics maintenance, and to a lesser degree index maintenance might resolve the problem that requires a query hint otherwise.
• Before using Query Store hints, test your application database on the latest compatibility level to see if that solves the problem that requires a query hint.
  • For example, Parameter Sensitive Plan (PSP) optimization was introduced in SQL Server 2022 (16.x) under compatibility level 160. It uses multiple active plans per query to address nonuniform data distributions. If your environment can't use the latest compatibility level, Query Store hints using the RECOMPILE hint can be used on any supporting compatibility level.
• Query Store hints override the Database Engine default query plan behavior. You should only use Query Store hints when it's necessary to address performance related issues.
• You should reevaluate Query Store hints, statement level hints, plan guides, and Query Store forced plans any time data volume and distribution changes and during database migrations projects. Changes in data volume and distribution might cause Query Store hints to generate suboptimal execution plans.

Query Store hints system stored procedures

To create or update hints, use sys.sp_query_store_set_hints. Hints are specified in a valid string format N'OPTION (...)'.

• When you create a Query Store hint, if no Query Store hint exists for a specific query_id, a new Query Store hint is created.
• When you create or update a Query Store hint, if a Query Store hint already exists for a specific query_id, the last value provided overrides previously specified values for the associated query.
• If a query_id doesn't exist, an error is raised.

For a complete list of hints that are supported as a Query Store hint, see sys.sp_query_store_set_hints.

To remove hints associated with a query_id, use sys.sp_query_store_clear_hints.

Execution Plan XML attributes

When hints are applied, the following result set appears in the StmtSimple element of the Execution plan in XML format:

Query Store hints and feature interoperability

• Query Store hints override other hard-coded statement level hints and plan guides.
• Except for the ABORT_QUERY_EXECUTION hint, queries with Query Store hints always execute. Opposing Query Store hints are ignored that would otherwise cause an error.
• If Query Store hints contradict, the Database Engine doesn't block query execution, and Query Store hint isn't applied.
• Query Store hints aren't supported for statements that qualify for simple parameterization.
• The RECOMPILE hint isn't compatible with forced parameterization set at the database level. If a database has forced parameterization set, and the RECOMPILE hint is part of the Query Store hints for a query, the Database Engine ignores the RECOMPILE hint and applies any other hints if they're specified.
  • The Database Engine issues a warning (error code 12461) stating that the RECOMPILE hint was ignored.
  • For more information about forced parameterization use case considerations, see Guidelines for Using Forced Parameterization.
• Manually created Query Store hints are exempt from Query Store cleanup. The hint and the query aren't cleaned up by the automatic retention capture policy.
  • Queries can be manually removed by users. That also removes the associated Query Store hint.
  • Query Store hints automatically generated by the CE Feedback are subject to clean up by the automatic retention of the capture policy.
  • DOP feedback and memory grant feedback shape query behavior without using Query Store hints. When queries are cleaned up by the automatic retention capture policy, DOP feedback and memory grant feedback data is also cleaned up.
  • If you create the same Query Store hint that CE feedback implemented manually, the query with the hint is no longer subject to clean up by the automatic retention capture policy.

Query Store hints and secondary replicas

Query Store hints have no effect on secondary replicas unless Query Store for secondary replicas is enabled. For more information, see Query Store for readable secondaries.

• In SQL Server 2022 (16.x) and earlier versions, Query Store hints can be applied on the primary replica only.
• In SQL Server 2022 (16.x) and later versions, when Query Store for secondary replicas is enabled, Query Store hints can be applied on secondary replicas in availability groups.
• You can add a Query Store hint to be in effect only on a specific replica group when you have Query Store for secondary replicas enabled. To do this, use the @replica_group_id parameter when calling sys.sp_query_store_set_query_hints. Conversely, you can remove a Query Store hint from a specific replica group using sys.sp_query_store_clear_query_hints.
• Find the available replica groups by querying sys.query_store_replicas.
• Find plans forced on secondary replicas with sys.query_store_plan_forcing_locations.

Examples

A. Query Store hints demo

The following walk-through of Query Store hints in Azure SQL Database uses an imported database via a BACPAC file (.bacpac). Learn how to import a new database to an Azure SQL Database server, see Quickstart: Import a bacpac file to a database in Azure SQL Database or Azure SQL Managed Instance.

-- ************************************************************************ --
-- Query Store hints demo

-- Demo uses "PropertyMLS" database which can be imported from BACPAC here:
-- https://github.com/microsoft/sql-server-samples/tree/master/samples/features/query-store

-- Email QSHintsFeedback@microsoft.com for questions\feedback
-- ************************************************************************ --

/*
    Demo prep, connect to the PropertyMLS database
*/

ALTER DATABASE [PropertyMLS] SET QUERY_STORE CLEAR;
ALTER DATABASE CURRENT SET QUERY_STORE = ON;
ALTER DATABASE CURRENT SET QUERY_STORE  (QUERY_CAPTURE_MODE = ALL);
GO

-- Should be READ_WRITE
SELECT actual_state_desc
FROM sys.database_query_store_options;
GO

/*
    You can verify Query Store Hints in sys.query_store_query_hints.
    Checking if any already exist (should be none).
*/
SELECT	query_hint_id,
        query_id,
        query_hint_text,
        last_query_hint_failure_reason,
        last_query_hint_failure_reason_desc,
        query_hint_failure_count,
        source,
        source_desc
FROM sys.query_store_query_hints;
GO

/*
     The PropertySearchByAgent stored procedure has a parameter
     used to filter AgentId.  Looking at the statistics for AgentId,
     you will see that there is a big skew for AgentId 101.
*/
SELECT	hist.range_high_key AS [AgentId],
        hist.equal_rows
FROM sys.stats AS s
CROSS APPLY sys.dm_db_stats_histogram(s.[object_id], s.stats_id) AS hist
WHERE s.[name] = N'NCI_Property_AgentId';


-- Show actual query execution plan to see plan compiled.
-- Agent with many properties will have a scan with parallelism.
EXEC [dbo].[PropertySearchByAgent] 101;

-- Agents with few properties still re-use this plan (assuming no recent plan eviction).
EXEC [dbo].[PropertySearchByAgent] 4;


/*
    Now let's find the query_id associated with this query.
*/
SELECT query_sql_text, q.query_id
FROM sys.query_store_query_text qt
INNER JOIN sys.query_store_query q ON
    qt.query_text_id = q.query_text_id
WHERE query_sql_text like N'%ORDER BY ListingPrice DESC%' and query_sql_text not like N'%query_store%';
GO

/*
     We can set the hint associated with the query_id returned in the previous result set, as below.
     Note, we can designate one or more query hints
*/
EXEC sp_query_store_set_hints @query_id=5, @value = N'OPTION(RECOMPILE)';
GO

/*
    You can verify Query Store Hints in sys.query_store_query_hints
*/
SELECT	query_hint_id,
        query_id,
        query_hint_text,
        last_query_hint_failure_reason,
        last_query_hint_failure_reason_desc,
        query_hint_failure_count,
        source,
        source_desc
FROM sys.query_store_query_hints;
GO

-- Execute both at the same time and show actual query execution plan.
-- You should see two different plans, one for AgentId 101 and one for AgentId 4.
EXEC [dbo].[PropertySearchByAgent] 101;
EXEC [dbo].[PropertySearchByAgent] 4;
GO

SELECT	query_hint_id,
        query_id,
        query_hint_text,
        last_query_hint_failure_reason,
        last_query_hint_failure_reason_desc,
        query_hint_failure_count,
        source,
        source_desc
FROM sys.query_store_query_hints;
GO

/*
    We can remove the hint using sp_query_store_clear_query_hints
*/
EXEC sp_query_store_clear_hints @query_id = 10;
GO

/*
    That Query Store Hint is now removed
*/
SELECT	query_hint_id,
        query_id,
        query_hint_text,
        last_query_hint_failure_reason,
        last_query_hint_failure_reason_desc,
        query_hint_failure_count,
        source,
        source_desc
FROM sys.query_store_query_hints;
GO

-- Execute both at the same time and show actual query execution plan.
-- You should see one plan again.
EXEC [dbo].[PropertySearchByAgent] 101;
EXEC [dbo].[PropertySearchByAgent] 4;
GO

B. Identify a query in Query Store

The following example queries sys.query_store_query_text and sys.query_store_query to return the query_id for an executed query text fragment.

In this demo, the query we're attempting to tune is in the SalesLT sample database:

SELECT * FROM SalesLT.Address as A
INNER JOIN SalesLT.CustomerAddress as CA
on A.AddressID = CA.AddressID
WHERE PostalCode = '98052' ORDER BY A.ModifiedDate DESC;

Query Store doesn't immediately reflect query data to its system views.

Identify the query in the Query Store system catalog views:

SELECT q.query_id, qt.query_sql_text
FROM sys.query_store_query_text qt
INNER JOIN sys.query_store_query q ON
    qt.query_text_id = q.query_text_id
WHERE query_sql_text like N'%PostalCode =%'
  AND query_sql_text not like N'%query_store%';
GO

In the following samples, the previous query example in the SalesLT database was identified as query_id 39.

Once identified, apply the hint to enforce a maximum memory grant size in percent of configured memory limit to the query_id:

EXEC sys.sp_query_store_set_hints @query_id= 39, @query_hints = N'OPTION(MAX_GRANT_PERCENT=10)';

You can also apply query hints with the following syntax, for example the option to force the legacy cardinality estimator:

EXEC sys.sp_query_store_set_hints @query_id= 39, @query_hints = N'OPTION(USE HINT(''FORCE_LEGACY_CARDINALITY_ESTIMATION''))';

You can apply multiple query hints with a comma-separated list:

EXEC sys.sp_query_store_set_hints @query_id= 39, @query_hints = N'OPTION(RECOMPILE, MAXDOP 1, USE HINT(''QUERY_OPTIMIZER_COMPATIBILITY_LEVEL_110''))';

Review the Query Store hint in place for query_id 39:

SELECT query_hint_id, query_id, query_hint_text, last_query_hint_failure_reason, last_query_hint_failure_reason_desc, query_hint_failure_count, source, source_desc
FROM sys.query_store_query_hints
WHERE query_id = 39;

Finally, remove the hint from query_id 39, using sp_query_store_clear_hints.

EXEC sys.sp_query_store_clear_hints @query_id = 39;

Related content

• sys.query_store_query_hints (Transact-SQL)
• sys.sp_query_store_set_hints (Transact-SQL)
• sys.sp_query_store_clear_hints (Transact-SQL)
• Save an Execution Plan in XML Format
• Display and save execution plans
• Query hints (Transact-SQL)
• Best practices for monitoring workloads with Query Store
• Query Store hints best practices
• Monitor performance by using the Query Store
• Configure the max degree of parallelism (MAXDOP) in Azure SQL Database