SQL performance tuning is a critical skill for database professionals, especially when interviewing at top MNCs like Sapient, TCS, Deloitte, Accenture, and others. Companies look for candidates who can optimize queries, improve database efficiency, and troubleshoot performance bottlenecks.
In this blog, we’ll cover 50+ SQL performance tuning interview questions categorized into Basic, Indexing, Query Optimization, Database Design, Troubleshooting, Advanced, Scenario-Based, and Behavioral questions.
1. Basic SQL Performance Tuning Questions
Q1. What is SQL performance tuning, and why is it important?
Answer:
SQL performance tuning involves optimizing SQL queries and database structures to improve speed, efficiency, and resource usage. It’s important because:
- Reduces query execution time
- Lowers CPU, memory, and I/O overhead
- Enhances user experience
- Saves costs on cloud database resources
Learn more about query optimization: SQL Query Optimization Techniques
Q2. How do you identify performance bottlenecks in SQL queries?
Answer:
- Use execution plans (EXPLAIN in MySQL, Execution Plan in SQL Server)
- Monitor slow query logs
- Check database performance metrics (CPU, I/O, locks)
- Use profiling tools (SQL Server Profiler, Oracle AWR)
Read about execution plans: Understanding SQL Execution Plans
Q3. What are the most common causes of poor SQL performance?
Answer:
- Missing or inefficient indexes
- Poorly written JOINs and subqueries
- Full table scans due to lack of filtering
- Locking and blocking issues
- Outdated statistics leading to bad query plans
Related reading: Common SQL Performance Issues
Q4. How do you optimize SQL queries for better performance?
Answer:
- Use proper indexing
- **Avoid SELECT *** (fetch only needed columns)
- Optimize JOINs (use INNER JOIN over WHERE clauses)
- Limit subqueries (replace with JOINs where possible)
- Use pagination (LIMIT/OFFSET or FETCH NEXT)
Optimization guide: SQL Query Optimization Best Practices
Q5. What is the difference between query optimization and indexing?
Answer:
- Query optimization focuses on rewriting queries for efficiency (e.g., avoiding subqueries, using JOINs).
- Indexing involves creating data structures (indexes) to speed up data retrieval.
Deep dive: Indexing vs Query Tuning
2. Indexing Questions
Q1. What is indexing, and how does it improve query performance?
Answer:
An index is a database structure that speeds up data retrieval by allowing faster lookup (like a book index).
Benefits:
- Reduces full table scans
- Speeds up WHERE, JOIN, ORDER BY operations
Indexing explained: Database Indexing for Performance
Q2. What types of indexes are available in SQL?
Answer:
- Clustered Index (reorders table storage, only one per table)
- Non-Clustered Index (separate structure, multiple allowed)
- Unique Index (ensures no duplicate values)
- Composite Index (on multiple columns)
- Covering Index (includes all columns needed for a query)
Index types comparison: Clustered vs Nonclustered Indexes
Q3. How do you determine which columns to index?
Answer:
- Columns frequently used in WHERE, JOIN, ORDER BY, GROUP BY
- High-selectivity columns (columns with many unique values)
- Avoid over-indexing (too many indexes slow down INSERT/UPDATE)
Index selection guide: Choosing the Right Indexes
Q4. What is the difference between index scanning and index seeking?
Answer:
- Index Scan: Reads entire index (slower, used when no useful filter exists)
- Index Seek: Directly jumps to matching rows (faster, uses WHERE conditions efficiently)
Visual explanation: Index Scan vs Seek
Q5. How do you maintain indexes for optimal performance?
Answer:
- Rebuild/Reorganize fragmented indexes
- Update statistics for accurate query planning
- Monitor unused indexes and remove redundant ones
Index maintenance guide: SQL Server Index Maintenance
3. Query Optimization Questions
Q1. How do you optimize SQL queries using EXPLAIN/EXECUTION PLAN?
Answer:
- Run EXPLAIN ANALYZE (PostgreSQL) or SHOW PLAN (SQL Server)
- Look for full table scans, missing indexes, expensive sorts
- Optimize steps with high cost percentages
Execution plan tutorial: How to Read Execution Plans
Q2. What is the difference between WHERE and HAVING clauses?
Answer:
- WHERE filters rows before aggregation
- HAVING filters rows after aggregation (used with GROUP BY)
Clause comparison: WHERE vs HAVING
Q3. How do you optimize JOIN operations?
Answer:
- Use INNER JOIN instead of WHERE for better readability and performance
- Ensure join columns are indexed
- Avoid cartesian products (always specify join conditions)
JOIN optimization: SQL JOIN Performance Tips
Q4. What is the impact of subqueries on performance?
Answer:
- Correlated subqueries (executed row-by-row) are slow
- Replace with JOINs or CTEs (Common Table Expressions) where possible
Subquery alternatives: Rewriting Subqueries as JOINs
Q5. How do you optimize aggregate functions (SUM, AVG, MAX)?
Answer:
- Filter data first (reduce rows before aggregation)
- Use indexed columns in GROUP BY
- Consider materialized views for frequent aggregations
Aggregation optimization: Optimizing GROUP BY
4. Scenario-Based Questions
Q1. You have a slow-running query joining multiple tables. How would you optimize it?
Answer:
- Check join conditions and ensure proper indexing
- Replace subqueries with JOINs
- Use query hints if needed (e.g., FORCE INDEX in MySQL)
Case study: Optimizing Complex Joins
Q2. Your database has high CPU usage. What would you do?
Answer:
- Identify top CPU-consuming queries
- Optimize expensive operations (sorts, joins, aggregations)
- Check for blocking/locking issues
Troubleshooting guide: High CPU Diagnosis
Q3. How would you optimize a query with multiple subqueries?
Answer:
- Replace with JOINs or CTEs
- Use temporary tables for intermediate results
- Ensure subqueries are not correlated
Subquery optimization: Converting Subqueries to JOINs
5. Behavioral Questions
Q1. Describe a time you optimized a slow SQL query.
Answer: (Example)
- Situation: A report query took 30+ seconds.
- Action: Analyzed execution plan, added a composite index, and replaced a subquery with a JOIN.
- Result: Reduced runtime to < 1 second.
Real-world example: Query Optimization Case Study
Q2. How do you stay updated on SQL tuning techniques?
Answer:
- Follow database blogs (SQL Server Central, Oracle Docs)
- Attend webinars and conferences (PASS Summit, AWS re:Invent)
- Experiment with new features (Query Store, In-Memory OLTP)
Learning resources: SQL Performance Resources
Final Thoughts
SQL performance tuning is a must-have skill for database roles. Mastering these questions will help you ace interviews at top MNCs like TCS, Deloitte, Accenture, and Sapient.
🔹 Pro Tip: Practice real-world query optimization on platforms like LeetCode, HackerRank, and SQLZoo.
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