User opens the app. Show the nearest 10 coffee shops. Sounds simple until you realize ’nearest’ means computing distance against millions of locations in under 100ms.

Your user is at latitude 37.7749, longitude -122.4194. Your database has 10 million locations. A full table scan comparing every coordinate pair isn’t going to work.

Two users update the same row. Pessimistic locking blocks one until the other finishes. Optimistic locking lets both try and fails the loser. Choosing wrong kills either throughput or correctness.

A follows table with two columns seems trivial. Until you need to query it from both directions, across shards, for millions of users.

OFFSET 50000 makes MySQL scan 50,000 rows just to skip them. Cursor pagination stays fast no matter how deep you go.

You added read replicas to scale reads. Now users update their profile and see the old version. Welcome to replica lag.

ALTER TABLE on a 2M row table locks it for minutes. Your users see errors. Here’s how expand-contract and shadow writes let you migrate without downtime.

Stop staring at EXPLAIN output confused. Learn to read MySQL execution plans like a map and find the root cause of slow queries in seconds, not hours.