Query plan regression post-load
MySQL 8 (InnoDB). A reporting endpoint that has run in ~120 ms for months suddenly p99'd to 9 s starting at 02:10. No deploy went out. The slow query is a 3-table join filtered on a recently-bulk-loaded `orders` partition. `EXPLAIN` now shows a full table scan + nested loop where it used to use a covering index, and `rows` estimates are off by ~1000x. A large overnight bulk import finished at 02:05. Triage and mitigate, then make it not recur.
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 35 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.