Scenario: Database queries are slow, causing application timeouts.
Investigation steps:
Check slow query log
Run EXPLAIN on slow queries
Check for missing indexes
Check connection pool usage
Check replication lag (if read replicas)
Common causes:
- Missing or unused indexes
- Lock contention
- Connection pool exhaustion
- Query returning too many rows
- Replication lag on read replicas
Quick wins:
- Add missing indexes
- Increase connection pool
- Add query timeouts
- Cache frequent queries
Interview tip: Know the difference between read and write bottlenecks. Different solutions apply.