SQL Performance Tuning Questions

 Frequently asked in architect interviews.

Q1: Query is taking 10 minutes. What will you check?

• Execution plan
• Indexes
• Joins
• Statistics

Q2: Why avoid SELECT *?

Reads unnecessary columns.

Q3: Clustered vs Non-Clustered Index?

Clustered = physical ordering.
Non-clustered = separate lookup structure.

Q4: Why is index not being used?

• Function in WHERE clause
• Small table
• Outdated statistics

Q5: How do you identify bottlenecks?

Execution plan analysis.