Here are some interview questions related to query hints in Azure SQL Database, along with sample answers:
1. What are query hints in SQL Server or Azure SQL Database, and why are they used?
Answer: Query hints are directives provided to the SQL query optimizer to influence the execution of a query. They override the default behavior of the optimizer to improve query performance or address specific scenarios.
Examples of uses:
- Force the use of specific indexes.
- Avoid parameter sniffing issues.
- Optimize resource utilization by limiting memory or CPU usage.
- Control parallelism or join strategies.
2. What is the purpose of the OPTION (RECOMPILE)
query hint?
Answer:
OPTION (RECOMPILE)
forces the query optimizer to discard the cached execution plan and create a new plan every time the query is executed.- It is used to handle parameter sniffing issues where cached plans are not optimal for certain parameter values.
Example:
SELECT *
FROM Orders
WHERE CustomerID = @CustomerID
OPTION (RECOMPILE);
This ensures the query plan is always optimized for the current parameter value.
3. When would you use the FORCESEEK
query hint?
Answer:
- The
FORCESEEK
hint forces the query optimizer to use an index seek instead of a scan, even if a scan might seem optimal. - It is useful when you know that using a seek will result in better performance (e.g., for highly selective queries).
Example:
SELECT *
FROM Customers WITH (FORCESEEK)
WHERE CustomerID = 123;
4. What does the MAXDOP
query hint do, and when should you use it?
Answer:
- The
MAXDOP
hint specifies the maximum degree of parallelism (number of CPU cores) the query can use. - Use it to control CPU usage for resource-intensive queries or avoid contention in multi-tenant environments.
Example:
SELECT *
FROM LargeTable
OPTION (MAXDOP 2);
This restricts the query to use a maximum of 2 CPU cores.
5. What is the difference between LOOP JOIN
, MERGE JOIN
, and HASH JOIN
query hints?
Answer: These hints force the optimizer to use specific join algorithms:
LOOP JOIN
: Suitable for smaller datasets or indexed tables.- Example:
SELECT * FROM Orders o JOIN Customers c ON o.CustomerID = c.CustomerID OPTION (LOOP JOIN);
- Example:
MERGE JOIN
: Ideal for large, sorted datasets.- Example:
SELECT * FROM Orders o JOIN Customers c ON o.CustomerID = c.CustomerID OPTION (MERGE JOIN);
- Example:
HASH JOIN
: Best for unsorted and large datasets.- Example:
SELECT * FROM Orders o JOIN Customers c ON o.CustomerID = c.CustomerID OPTION (HASH JOIN);
- Example:
6. How do you force the use of a specific index with a query hint?
Answer: Use the INDEX
hint to specify the index the optimizer must use.
Example:
SELECT *
FROM Products WITH (INDEX (IX_ProductName))
WHERE ProductName = 'Widget';
This forces the query to use the IX_ProductName
index instead of relying on the optimizer.
7. What does the OPTIMIZE FOR
query hint do, and when is it useful?
Answer:
- The
OPTIMIZE FOR
hint forces the query optimizer to optimize the query for a specific value of a parameter, even if the query is executed with different parameter values. - It is commonly used to address parameter sniffing issues.
Example:
SELECT *
FROM Orders
WHERE CustomerID = @CustomerID
OPTION (OPTIMIZE FOR (@CustomerID = 123));
This optimizes the query as if @CustomerID
is always 123
, regardless of the actual parameter value.
8. How does FAST
query hint improve performance?
Answer:
- The
FAST N
hint tells the optimizer to return the firstN
rows as quickly as possible, rather than optimizing for the entire result set. - It’s useful for applications requiring immediate partial results (e.g., paginated data).
Example:
SELECT *
FROM Orders
OPTION (FAST 10);
This returns the first 10 rows faster but might not optimize for the full query execution.
9. What is the difference between QUERYTRACEON
and other query hints?
Answer:
QUERYTRACEON
enables specific trace flags for debugging or performance tuning during query execution.- Example:
SELECT * FROM Orders OPTION (QUERYTRACEON 8649);
This forces parallel execution regardless of query cost.
10. What does the USE HINT
query hint do, and how is it different from traditional hints?
Answer:
- The
USE HINT
option introduces new hints without requiring specific syntax for each hint. - Example:
SELECT * FROM Customers OPTION (USE HINT ('DISABLE_PARAMETER_SNIFFING'));
Common USE HINT
values:
'DISABLE_PARAMETER_SNIFFING'
: Disables parameter sniffing.'ENABLE_QUERY_OPTIMIZER_HOTFIXES'
: Enables query optimizer hotfixes.
11. Scenario Question:
You notice that a query performs poorly due to parameter sniffing. What query hints would you use to resolve this?
Answer:
- Option 1: Use
OPTION (RECOMPILE)
to force the optimizer to create a new execution plan for each execution.SELECT * FROM Orders WHERE CustomerID = @CustomerID OPTION (RECOMPILE);
- Option 2: Use
OPTIMIZE FOR UNKNOWN
to generate a generic plan.SELECT * FROM Orders WHERE CustomerID = @CustomerID OPTION (OPTIMIZE FOR UNKNOWN);
- Option 3: Use
OPTIMIZE FOR
to optimize for a specific parameter value.SELECT * FROM Orders WHERE CustomerID = @CustomerID OPTION (OPTIMIZE FOR (@CustomerID = 123));
12. Scenario Question:
You need to restrict a query from using more than one CPU core. How would you do this?
Answer: Use the MAXDOP
query hint to limit the degree of parallelism.
Example:
SELECT *
FROM LargeTable
OPTION (MAXDOP 1);
This forces the query to run on a single CPU core, reducing resource contention.
13. Scenario Question:
You have a query that is scanning a large table unnecessarily. How would you ensure it uses an index?
Answer: Use the INDEX
query hint to force the optimizer to use a specific index.
Example:
SELECT *
FROM Customers WITH (INDEX (IX_CustomerName))
WHERE CustomerName = 'John Doe';
This ensures the query uses the IX_CustomerName
index instead of performing a full table scan.