Query Optimization — Complete Guide
Query Optimization — Complete Guide: free step-by-step lesson with examples, common mistakes, and interview tips — part of MySQL Tutorial on Toolliyo Academy.
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MySQL Tutorial · Lesson 67 of 100
Query Optimization
Basics ✓ → Advanced
Advanced · 2 — Production · ~10 min · MySQL — Indexing & Performance
What is this?
Query optimization reshapes SQL and schema so the optimizer picks efficient plans: better indexes, sargable WHERE, smaller result sets, avoiding SELECT *, rewriting subqueries to JOINs when helpful.
Why should you care?
Report that ran 45 seconds blocks replication — optimized to 2 seconds saves RDS bill and user patience.
See it live — copy this example
Run in MySQL Workbench or the mysql CLI.
-- Before: non-sargable
-- SELECT * FROM orders WHERE YEAR(placed_at) = 2024;
-- After:
SELECT order_id, customer_id, total_inr, placed_at
FROM orders
WHERE placed_at >= '2024-01-01' AND placed_at < '2025-01-01';What happened?
- Range on placed_at uses index on placed_at.
- YEAR(placed_at) function prevents index use on column.
- Selecting only needed columns reduces I/O.
Practice next
- Capture slow query from log.
- EXPLAIN before rewrite.
- Apply sargable date range + column list.
- Rewrite IN (subquery) to JOIN — compare plans.
- Add functional index on (YEAR(placed_at)) only if range rewrite impossible.
Remember
Write WHERE that indexes can seek. Fetch only needed columns. Measure with EXPLAIN ANALYZE.
DataFlow finance export
Monthly export rewritten from YEAR() filter to range — runtime 40s to 3s.
Outcome: Job finishes before business hours.
Interview prep for this lesson
Practice these questions aloud after reading—each links to a full structured answer.
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