Query Hint is a tool in SQL query optimization that forces databases to execute queries in a specified manner, suitable for unstable performance, existing optimization directions, temporary solutions, and specific business needs. Common types include table prompts (such as NOLOCK, READPAST, INDEX), query-level prompts (such as RECOMPILE, MAXDOP, LOOP JOIN) and combination use, but attention should be paid to prioritizing verification in the test environment, monitoring the execution plan, reviewing the effectiveness regularly, and keeping documentation.

Query Hint is a very useful tool in SQL query optimization, but many people are not clear about its usage scenarios and methods. Simply put, Query Hint is a means of forcing databases to perform queries in a specified way. It can improve performance in certain situations, but can also have side effects, especially if its impact is not fully understood.

Here are some key points you really need to know.

What is Query Hint?

Query Hint is a directive written in SQL statements to guide the database engine to execute queries. They are usually used to override the default query optimization behavior, such as forcing an index, changing the connection method, or controlling the parallelism.

For example:

 SELECT * FROM Orders WITH (NOLOCK)

Here WITH (NOLOCK) is a typical table-level hint that tells the database not to add shared locks and allows reading of uncommitted data.

When should I consider using Query Hint?

Using Query Hint should be a conscious choice, not arbitrary addition. Here are some common applicable scenarios:

• Query performance is unstable : When a certain SQL performs greatly at different times, it may be due to changes in statistical information, which may cause fluctuations in execution plans.
• There is already a clear optimization direction : for example, you know that an index is more suitable, but the optimizer is not selected.
• Temporary solution : As a short-term optimization method when structures cannot be modified immediately or statistics are updated.
• Specific business logic requirements : If report-class queries need to avoid lock upgrades, you can use WITH (NOLOCK) .

?? Note: Query Hint has higher priority than the optimizer's judgment. If used improperly, it may lead to performance degradation or even data consistency problems.

Common Query Hint and usage suggestions

1. Table Hints

This type of prompts act on tables or views, the most common ones are WITH (NOLOCK) , WITH (READPAST) , WITH (INDEX(...)) , etc.

• NOLOCK : Suitable for read-only queries, avoid blocking, but will read dirty data.
• READPAST : Skip locked rows, often used for queue processing.
• INDEX : Forces the use of specific indexes, suitable for index selection where significantly better than the default.

Example:

 SELECT * FROM Customers WITH (INDEX(IX_Customers_Name))

2. Query-Level Hints

These prompts are placed at the end of the SQL statement to control how the entire query is executed.

• OPTION (RECOMPILE) : Recompile the execution plan every time it is executed, suitable for situations where the parameter value is large and the plan is not good.
• OPTION (MAXDOP n) : limits parallelism and prevents resource contention.
• OPTION (LOOP JOIN) or HASH JOIN : Forced connection method, suitable for scenarios where the optimal connection strategy is known.

Example:

 SELECT * FROM Orders o JOIN Customers c ON o.CustomerID = c.CustomerID
OPTION (LOOP JOIN)

3. Combination of query prompts

In some cases, multiple hints can be combined to achieve the purpose, such as:

 SELECT * FROM Sales WITH (NOLOCK, INDEX(IX_Sales_Date))

However, be aware that not all hints can coexist, and some combinations may report errors.

Notes on using Query Hint

• Test environment first : Do not use unverified hints directly in production environments.
• Monitor the execution plan : After using hint, be sure to check whether the actual execution plan meets expectations.
• Regular review : As data grows and structure changes, the once-effective hint may no longer apply.
• Documentation : Explain why this hint is used to facilitate subsequent maintenance.

Basically that's it. Although Query Hint is powerful, it is an "advanced gameplay" and is not recommended for beginners to use it frequently. Mastering the basic skills of query optimization, such as index design and statistical information management, is often more reliable than relying on Hint.

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