Welcome to our blog post on SQL Server query hints! In this article, we will discuss some important concepts related to query hints and how they can be used to optimize your SQL Server queries.

Before we dive into the details, let’s quickly recap the basics of query hints. Query hints are special instructions that you can add to your SQL queries to provide guidance to the SQL Server query optimizer. These hints can help improve query performance by influencing the execution plan chosen by the optimizer.

Now, let’s explore three important concepts related to query hints:

1. Expecting Seek and getting a Scan

One common scenario in SQL Server is when you expect a seek operation on an index but end up with a scan instead. This can happen due to various reasons, such as outdated statistics or incorrect query formulation. By using query hints, you can explicitly specify the index to be used, ensuring that the desired seek operation is performed.

2. Creating an index for improved optimization

Indexes play a crucial role in optimizing query performance. By creating appropriate indexes on the relevant columns, you can significantly improve the execution time of your queries. Query hints can be used to specify the index to be used, ensuring that the optimizer chooses the most efficient plan.

3. Implementing the query hint

Once you have identified the query hint that can improve the performance of your query, you can implement it by adding the hint to your SQL statement. This can be done using the “WITH” clause followed by the appropriate hint keyword.

Now, let’s test your understanding of these concepts with a small quiz:

Quiz:

1) You have the following query:

DECLARE @UlaChoice TinyInt
SET @Type = 1
SELECT * FROM LegalActivity WHERE UlaChoice = @UlaChoice

You have a nonclustered index named IX_Legal_Ula on the UlaChoice field. The Primary key is on the ID field and called PK_Legal_ID. 99% of the time, the value of the @UlaChoice is set to ‘YP101’. What query will achieve the best optimization for this query?

1. SELECT * FROM LegalActivity WHERE UlaChoice = @UlaChoice WITH(INDEX(X_Legal_Ula))
2. SELECT * FROM LegalActivity WHERE UlaChoice = @UlaChoice WITH(INDEX(PK_Legal_ID))
3. SELECT * FROM LegalActivity WHERE UlaChoice = @UlaChoice OPTION (Optimize FOR(@UlaChoice = ‘YP101’))

2) You have the following query:

SELECT * FROM CurrentProducts WHERE ShortName = 'Yoga Trip'

You have a nonclustered index on the ShortName field, and the query runs an efficient index seek. You change your query to use a variable for ShortName, and now you are using a slow index scan. What query hint can you use to get the same execution time as before?

1. WITH LOCK
2. FAST
3. OPTIMIZE FOR MAXDOP
4. READONLY

Now, let’s reveal the answers:

Answers:

1) The correct answer is option a) SELECT * FROM LegalActivity WHERE UlaChoice = @UlaChoice WITH(INDEX(X_Legal_Ula)). By specifying the index IX_Legal_Ula, we can achieve the best optimization for this query.

2) The correct answer is option d) READONLY. By using the READONLY hint, we can get the same execution time as before, even when using a variable for ShortName.

We hope you found this quiz helpful in testing your knowledge of SQL Server query hints. If you have any feedback or questions, please leave them in the comments section below.

Thank you for reading!