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Utilizing query hints and plan forcing with the SQL Query Store

In the world of database optimization, query hints and plan forcing are techniques used to influence the execution plan chosen by the database optimizer. These techniques can be especially useful when you want to ensure the use of a specific execution plan that you know performs well for a particular query.

In this blog post, we will explore how to utilize query hints and plan forcing in SQL Server using the SQL Query Store feature. The Query Store feature, introduced in SQL Server 2016, allows you to monitor and manage the performance of your queries over time.

Table of Contents

Understanding Query Hints

A query hint is a directive used to provide instructions to the query optimizer on how to generate an execution plan for a specific query. It can specify which indexes to use, which join algorithms to employ, or other optimizations to take into account.

Query hints can be useful when you know that the optimizer may not choose the best execution plan for a query due to statistical inaccuracies or lack of knowledge about specific data characteristics. However, it’s important to use query hints judiciously as they can limit the optimizer’s ability to adapt to changing data or environment conditions.

Using Query Hints in SQL Server

In SQL Server, you can specify query hints by using the OPTION clause in your queries. Here’s an example that forces the use of an index hint:

SELECT * 
FROM MyTable WITH (INDEX(IndexName))
WHERE Column1 = 'Value';

In this example, the WITH (INDEX(IndexName)) hint tells the optimizer to use the specified index, IndexName, when executing the query. This can be particularly useful when you know that the specified index performs better for the given query than the default index chosen by the optimizer.

Plan Forcing with the Query Store

With the SQL Query Store, you can take plan forcing to the next level by persisting a specific execution plan for a query, ensuring it’s always used regardless of updates to statistics or environmental changes. This feature provides greater control over query performance and makes it easier to maintain consistent execution plans.

To force a specific execution plan with the Query Store, you can use the FORCED_PLAN query hint. Here’s an example:

SELECT * 
FROM MyTable
OPTION (USE HINT('FORCE_LEGACY_CARDINALITY_ESTIMATION'));

In this example, the USE HINT clause references the specific query hint, FORCE_LEGACY_CARDINALITY_ESTIMATION, which forces the use of the legacy cardinality estimation algorithm. This can be useful when you encounter issues with the optimizer’s default cardinality estimation model.

Monitoring and Managing Plan Forcing

To monitor and manage plan forcing with the Query Store, you can use the Query Store reports and DMVs (Dynamic Management Views). These provide valuable insights into query performance and allow you to identify queries that might benefit from plan forcing.

By analyzing the query execution plans and performance metrics provided by the Query Store, you can determine when plan forcing is necessary. You can also identify instances where a forced plan may not be optimal and needs to be adjusted or removed.

Conclusion

Query hints and plan forcing offer valuable tools for optimizing query performance in SQL Server. By leveraging the SQL Query Store feature, you can monitor and manage plan forcing to ensure consistent and efficient execution plans.

Remember to use query hints and plan forcing judiciously, as they can have both positive and negative impacts on query performance. Regularly monitor and evaluate the effectiveness of forced plans to ensure you are achieving the desired optimization results.

#sql #query-optimization

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