SQL Server Query Hints and Optimizer Overrides - Advanced Usage


Introduction

SQL Server query hints and optimizer overrides are advanced techniques used to influence the query execution plan. This guide explores the advanced usage of query hints and overrides with sample code for performance optimization.


1. OPTION (RECOMPILE)

The 

OPTION (RECOMPILE)
hint forces SQL Server to recompile the query plan each time it's executed. This can be useful for dynamic queries or when the selectivity of the query varies significantly.

-- Force recompilation of the query
SELECT *
FROM Customers
WHERE Country = 'USA'
OPTION (RECOMPILE);

2. OPTION (HASH JOIN)

The 

OPTION (HASH JOIN)
hint instructs the optimizer to use a hash join algorithm for the query, which can be beneficial in certain scenarios.

-- Use hash join for the query
SELECT *
FROM Orders o
JOIN OrderDetails od ON o.OrderID = od.OrderID
OPTION (HASH JOIN);

3. OPTION (OPTIMIZE FOR)

The 

OPTION (OPTIMIZE FOR)
hint allows you to specify a parameter value to optimize the query plan based on a specific value.

-- Optimize the query for a specific parameter value
DECLARE @CustomerId INT = 123;
SELECT *
FROM Orders
WHERE CustomerID = @CustomerId
OPTION (OPTIMIZE FOR (@CustomerId = 123));

4. OPTION (USE HINT)

The 

OPTION (USE HINT)
hint enables you to specify a hint or query plan guide to influence the optimizer's choices.

-- Use a specific query hint
SELECT *
FROM Products
WHERE CategoryID = 5
OPTION (USE HINT('FORCE_DEFAULT_CARDINALITY_ESTIMATION'));

Conclusion

Advanced usage of SQL Server query hints and optimizer overrides provides greater control over query performance. By using hints like 

OPTION (RECOMPILE)
, 
OPTION (HASH JOIN)
, 
OPTION (OPTIMIZE FOR)
, and 
OPTION (USE HINT)