Query Optimization

Introduction

Query optimization turns slow queries into fast ones. The toolkit: pick selective filters, use indexes, write sargable predicates, avoid unnecessary work, and read execution plans.

Understanding the topic

Core concepts to understand:

• Filter selectively — most rows out early.
• Use indexes — columns in WHERE/JOIN/ORDER BY.
• Avoid functions on indexed columns.
• Choose the right join type by size.
• Read EXPLAIN ANALYZE — the truth.

Syntax reference

Visual workflow / architecture:

Slow query checklist
 □ Selective WHERE?
 □ Indexed join columns?
 □ Functions on indexed cols?  → rewrite
 □ Right plan in EXPLAIN?
 □ Fresh statistics (ANALYZE)?

Real-world use

Most production incidents end with 'we needed an index' or 'stats were stale'. Optimization is daily work for senior backend devs.

Best practices

• Always EXPLAIN ANALYZE before assumptions.
• Add indexes only after measuring.
• Keep statistics current.

Hands-on exercise

Interview preparation — practice these questions:

• Q1. Steps to debug a slow query.
• Q2. How do you decide to add an index?
• Q3. Stats and ANALYZE.
• Q4. Sargable predicate.
• Q5. Cost-based optimizer.