N+1 query explosion after deploy
A REST endpoint listing a project's tasks-with-assignees went from 90 ms to 4 s in p95 after a deploy, only for projects with many tasks. The DB's slow query log shows NO single slow query, but `pg_stat_statements` shows a tiny `SELECT ... WHERE user_id = $1` query whose `calls` count exploded ~30x since the deploy. DB CPU is up but no individual query is slow. App traces show hundreds of tiny DB round-trips per request. Triage and fix, and explain why the slow-query log was empty.
What a strong answer looks like
Stop the bleeding first (mitigate), then form hypotheses from real signals. Separate root cause from symptom, communicate status as you go, and close with what prevents a repeat.
0:00 of about 25 min
Which questions mattered is sealed until you submit. Telling you now would just be handing over the edge cases.
Run or narrate your approach, then ask the coach.