My query took 2 minutes. I added ONE line. Now it's 0.3 seconds. 🚀 Me: "Why is this so slow?" Senior Dev: "Do you have an index?" Me: "A what now?" The Problem: SELECT * FROM orders WHERE customer_id = 12345; -- ⏱️ 127 seconds The Fix: CREATE INDEX idx_customer ON orders(customer_id); -- ⏱️ 0.3 seconds The Rule: Index columns in your WHERE clause. That's it. 400x faster. What obvious thing did you learn embarrassingly late? 👇 #SQL #DataEngineeringtamil #DataEngineering