Understanding the Query Optimizer
The SQL Server Query Optimizer is a cost-based engine that determines the most efficient way to execute a T‑SQL query. It evaluates multiple execution plans, estimates their costs, and selects the plan with the lowest estimated cost.
Key Concepts
- Logical Plan: Abstract representation of the query using relational algebra operators.
- Physical Plan: Concrete implementation of the logical plan with specific algorithms (e.g., index seek, hash join).
- Cost Model: Estimates resources such as CPU, I/O, and memory required for each operator.
- Statistics: Data distribution information that guides cardinality estimates.
Cost Estimation Process
Controlling the Optimizer
While the optimizer works automatically, you can influence its decisions with hints and configuration options.
-- Forcing a specific join type
SELECT *
FROM dbo.TableA a
JOIN dbo.TableB b
ON a.ID = b.ID
OPTION (HASH JOIN);
Use hints sparingly; unintended side effects may degrade performance.
Common Performance Issues
- Out‑of‑date statistics leading to inaccurate cardinality estimates.
- Parameter sniffing causing suboptimal plans for varied parameter values.
- Missing or unused indexes forcing costly scans.
- Implicit conversions preventing index usage.
Run SET STATISTICS IO ON; and SET STATISTICS TIME ON; to investigate.