Skip to content

Latest commit

 

History

History
209 lines (151 loc) · 15.9 KB

query-store-hints.md

File metadata and controls

209 lines (151 loc) · 15.9 KB
Raw
title description author ms.author ms.date ms.service ms.subservice ms.topic helpviewer_keywords dev_langs monikerRange
Query Store hints
Learn about the Query Store hints feature, which can be used to shape query plans without changing application code.
WilliamDAssafMSFT
wiassaf
01/08/2024
sql
performance
conceptual
Query Store hints
TSQL
=azuresqldb-current||=azuresqldb-mi-current||>=sql-server-ver16||>=sql-server-linux-ver16

Query Store hints

[!INCLUDE sqlserver2022-asdb-asdbmi]

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.

Query Store hints are available in [!INCLUDE ssazure-sqldb] and [!INCLUDE ssazuremi-md]. Query Store hints are also a feature introduced to SQL Server in [!INCLUDE sssql22-md].

Caution

Because the SQL Server Query Optimizer typically selects the best execution plan for a query, we recommend only using hints as a last resort for experienced developers and database administrators. For more information, see Query Hints.

Watch this video for an overview of Query Store hints:

[!VIDEO https://channel9.msdn.com/Shows/Data-Exposed/Query-Store-Hints-in-Azure-SQL-Database/player?WT.mc_id=dataexposed-c9-niner]

Overview

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

If an optimal plan isn't selected, a developer or 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 [!INCLUDE tsql] code to inject a query hint. The [!INCLUDE tsql] 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.

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. [!INCLUDE sssql16-md] first introduced Query Store, and now it's enabled by default in [!INCLUDE sssql22-md], [!INCLUDEssazuremi-md], and [!INCLUDE ssazure-sqldb].

:::image type="complex" source="media/query-store-hints.png" alt-text="The workflow for Query Store Hints."::: First the query is executed, then captured by Query Store. Then the DBA creates a Query Store hint on a query. Thereafter, the query is executed using the Query Store hint. :::image-end:::

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:
  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 is not 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.

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.

Note

For a complete list of hints that are supported, 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:

Attribute Description
QueryStoreStatementHintText Actual Query Store hint(s) applied to the query
QueryStoreStatementHintId Unique identifier of a query hint
QueryStoreStatementHintSource Source of Query Store hint (ex: "User")

Note

These XML elements are available via the output of the [!INCLUDE tsql] commands SET STATISTICS XML and SET SHOWPLAN XML.

Query Store hints and feature interoperability

  • Query Store hints override other hard-coded statement level hints and plan guides.
  • Queries always execute. Opposing Query Store hints are ignored that would otherwise cause an error.
  • If Query Store hints contradict, SQL Server does not block query execution, and Query Store hint is not applied.
  • Simple parameterization - Query Store hints are not supported for statements that qualify for simple parameterization.
  • Forced parameterization - The RECOMPILE hint is not compatible with forced parameterization set at the database level. If the database has forced parameterization set, and the RECOMPILE hint is part of the hints string set in Query Store for a query, SQL Server ignores the RECOMPILE hint and will apply any other hints if they are applied.
    • Additionally, SQL Server issues a warning (error code 12461) stating that the RECOMPILE hint was ignored.
    • For more information on forced parameterization use case considerations, see Guidelines for Using Forced Parameterization.
  • Manually created Query Store hints are exempt from cleanup. The hint and the query will not be cleaned up from Query Store by the automatic retention of the capture policy.
    • Queries can be manually removed by users, which would also remove 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 cleanup by automatic retention of the capture policy, DOP feedback and memory grant feedback data is also cleaned up.
    • You can manually create the same Query Store hint that CE feedback implemented, and then the query with the hint would no longer be subject to clean up by the automatic retention of the capture policy.

Query Store hints and availability groups

For more information, see Query Store for secondary replicas.

  • Prior to [!INCLUDE sssql22-md], Query Store hints can be applied against the primary replica of an availability group.
  • Starting with [!INCLUDE sssql22-md], when Query Store for secondary replicas is enabled, Query Store hints are also replica-aware for secondary replicas in availability groups.
  • You can add a Query Store hint to a specific replica or replica set when you have Query Store for secondary replicas enabled. In sys.sp_query_store_set_query_hints, this is set by the @query_hint_scope parameter, which was introduced in [!INCLUDE sssql22-md].
  • Find the available replica sets by querying sys.query_store_replicas.
  • Find plans forced on secondary replicas with sys.query_store_plan_forcing_locations.

Query Store hints best practices

  • Complete index and statistics maintenance before evaluating queries for potential new Query Store hints.
  • Test your application database on the latest compatibility level before using Query Store hints.
    • For example, Parameter Sensitive Plan (PSP) optimization was introduced in [!INCLUDE sssql22-md] (compatibility level 160), which uses multiple active plans per query to address nonuniform data distributions. If your environment cannot use the latest compatibility level, Query Store hints using the RECOMPILE hint can be used on any supporting compatibility level.
  • Query Store hints override SQL Server query plan behavior. It is recommended to only use Query Store hints when it is necessary to address performance related issues.
  • It is recommended to reevaluate Query Store hints, statement level hints, plan guides, and Query Store forced plans any time data distributions change and during database migrations projects. Changes in data distribution might cause Query Store hints to generate suboptimal execution plans.

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.

:::code language="tsql" source="../../../sql-server-samples/samples/features/query-store/query_store_hints_demo.sql":::

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