Database Scaling & Consistency Quiz

Sample questions

1. Database CPU is at 85%. pg_stat_statements shows a 14 s report (4 calls/day) and a 38 ms list query (2.4M calls/day). Which do you optimise first?
2. In EXPLAIN ANALYZE, a node estimated 50 rows but returned 2,000,000. What does that most likely mean?
3. Which index best serves WHERE account_id = ? ORDER BY issued_at DESC LIMIT 50?
4. Only 0.5% of orders are 'pending', and a job queries them by created_at. What index is a good fit in Postgres?
5. An admin page listing 100 orders runs 201 queries. What's the classic cause?