Query hints in SQL Server are a powerful tool that developers can use to manipulate the operation of the Query Optimizer. While they should be used with caution and typically as a last resort, in certain situations, they can be beneficial in improving the performance of SQL queries.
Understanding Query Hints
Query hints are options or strategies forced onto the Query Optimizer to use a specific method to execute a SQL query. The primary reasons to use query hints are:
- To override the default behavior of the Query Optimizer.
- To enhance the performance of a SQL query.
- To control the locking and isolation level of a query.
In Azure SQL, the query hints you can use are the same as those available in SQL Server. However, it should be noted some options have less relevance in a managed environment, where you have less control over the infrastructure.
Guidelines for Using Query Hints
While query hints can occasionally improve performance, they should be used judiciously:
- Advice from the Query Optimizer: Generally, the Query Optimizer will choose the best method to execute a SQL query. It is not wise to override this method unless you are confident that the hint will lead to better performance.
- Maintainability: SQL query hints can make your code more difficult to maintain. Additionally, what works today may not be the best solution tomorrow, particularly in dynamic environments or as data evolves.
- Testing: Always thoroughly test a new query hint to ensure it provides the desired result and does not have any negative impacts.
Examples of Query Hints
USE PLAN
The USE PLAN hint forces the Query Optimizer to use a specified query plan. It is used when you have a query that has different performance each time it executes, due to the different plans chosen by the Query Optimizer.
SQL
SELECT *
FROM Sales.OrderDetails
OPTION (USE PLAN N’Plan_String’)
MAXDOP
The MAXDOP hint specifies the maximum degree of parallelism that SQL Server will use to execute a query to the lower of two values: the value set in configuration, or the number given in the query hint.
SQL
SELECT *
FROM Sales.OrderDetails
OPTION (MAXDOP 2)
OPTIMIZE FOR
The OPTIMIZE FOR hint tells the Query Optimizer to use a particular value for a local variable when creating a plan. This is useful when a stored procedure is normally called with certain parameters, so we want the plan that is optimal for those parameters.
SQL
DECLARE @id INT = 123;
SELECT *
FROM Sales.OrderDetails
WHERE OrderId = @id
OPTION (OPTIMIZE FOR (@id = 456))
Impact of Query Hints on Query Performance
The impact of using query hints on query performance can either be beneficial or detrimental. When used properly, they can guide the Query Optimizer to generate a more efficient execution plan than it might otherwise create. This could result in more efficient use of system resources and shorter response times. However, when used improperly or unnecessarily, they could result in inefficient execution plans that could lead to detrimental performance.
In conclusion, while query hints can be a useful tool in some scenarios, they should only be used after a thorough analysis of the query, its execution plan and substantial testing to ensure they actually provide an expected overall performance benefit. In the context of DP-300 “Administering Microsoft Azure SQL Solutions” exam, understanding when and how to use query hints can be essential in gaining the proficiency required to administer Azure SQL solutions effectively.
Practice Test
In Microsoft Azure SQL, query hints can be used to improve the performance of a query.
- a) True
- b) False
Answer: a) True
Explanation: Query hints in Azure SQL can be used to influence the query plan choice, which can potentially improve the performance of a query.
Query hints are always recommended for improving the performance of a query in Microsoft Azure SQL.
- a) True
- b) False
Answer: b) False
Explanation: Although query hints can help, they’re not always the best solution. They can cause other problems if not used properly, and there are often other ways to optimize query performance.
Using query hints on a production database is always advisable.
- a) True
- b) False
Answer: b) False
Explanation: Query hints should be used with caution, especially in a production environment, because improper use can limit the options of the query optimizer and could potentially degrade performance.
Query hints are used to:
- a) Influence the query optimizer
- b) Change the data in the tables
- c) Backup the database
- d) Download data from the cloud
Answer: a) Influence the query optimizer
Explanation: Query hints are used to influence the choices made by the query optimizer to potentially improve query performance. They don’t change the data in the tables, backup the database, or download data.
Which of the following is a type of Query Hint in Microsoft Azure SQL?
- a) OPTION
- b) FORCE
- c) RESOLVE
- d) FOCUS
Answer: a) OPTION
Explanation: OPTION is a type of query hint in Azure SQL, which allows you to modify the execution plan used by the Query Optimizer.
You can specify multiple query hints in a single SQL statement.
- a) True
- b) False
Answer: a) True
Explanation: A query can contain multiple query hints, although fewer hints are generally better.
The Query Optimizer totally ignores Query hints.
- a) True
- b) False
Answer: b) False
Explanation: The Query Optimizer does not ignore query hints. Rather, it uses the hints to influence the choice of the execution plan.
Query hints can only be specified in the SELECT statements.
- a) True
- b) False
Answer: b) False
Explanation: Query hints can be specified in SELECT, INSERT, UPDATE, DELETE, or MERGE statements.
Overusing Query hints can sometimes limit the Query Optimizer’s ability to choose the optimal plan.
- a) True
- b) False
Answer: a) True
Explanation: Query hints can limit the Query Optimizer’s choices, which might prevent it from choosing the most optimal plan, especially if the query or data changes over time.
Which of the following is not a good use case for query hints?
- a) To influence the query optimizer’s choice of execution plan
- b) To attempt to improve query performance
- c) For all cases because query hints always improve performance
- d) If the default query plan isn’t giving the required performance
Answer: c) For all cases because query hints always improve performance
Explanation: Query hints are not always the best solution for improving query performance and should be used selectively and with caution. They may not always provide the desired performance gain and can sometimes have negative effects.
Interview Questions
What are query hints in Azure SQL Database?
Query hints are mechanisms used to override the choices the query processor makes during the compilation and optimization of queries in Azure SQL Database.
How can you implement a query hint in an Azure SQL query?
Query hints can be implemented by including a hint keyword in the query. For example, FORCESEEK hint is used to force an index seek operation.
What is the purpose of using query hints in Azure SQL Database?
The purpose of using query hints is to influence the query processor’s choices in compiling and optimizing queries to improve query performance.
How does the IGNORE_NONCLUSTERED_COLUMNSTORE_INDEX hint affect query optimization in Azure SQL Database?
This hint instructs the query processor to ignore any nonclustered columnstore indexes when optimizing the query plan.
What does the MAXDOP query hint do in Azure SQL Database?
The MAXDOP hint specifies the maximum number of processors that the query processor can use to execute the query.
How can the USE HINT query hint be used to optimize query performance in Azure SQL Database?
The USE HINT hint allows specifying custom query hints, enabling the use of custom query optimizations.
When should query hints be used in Azure SQL Database queries?
Query hints should be used cautiously and only when necessary, as they can override default query optimization choices made by the query processor.
What happens if a query hint conflicts with the query optimizer’s default behavior in Azure SQL Database?
In case of a conflict between a query hint and the query optimizer’s default behavior, the query hint typically takes precedence.
How can query hints be beneficial in optimizing query performance in Azure SQL Database?
Query hints can be beneficial in scenarios where specific optimizations are necessary to improve query performance that cannot be achieved through standard optimization techniques.
Can query hints be used to force a specific query plan in Azure SQL Database?
Yes, query hints can be used to force a specific query plan by instructing the query processor on how to approach query compilation and optimization.
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