Missing index on growing table
An orders API (Ruby on Rails, Postgres) degrades gradually over two weeks — nobody noticed a single bad deploy. p99 on GET /orders crept from 120ms to 3s. The slow-query log now shows the order-history query (filter by `customer_id`, order by `created_at`) doing a sequential scan over the orders table, which has grown from 2M to 30M rows after a successful growth quarter. `EXPLAIN ANALYZE` confirms a Seq Scan + sort. The table has a primary key on `id` but no other indexes. Triage and mitigate.
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 30 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.