Index larger than table
Postgres. A frequently-updated `sessions` table (every request bumps `last_seen`) has a btree index on `last_seen`. Lookups on that index have slowly degraded over months from 1 ms to 60 ms, and the index is now larger than the table itself. `pg_stat_user_indexes` shows heavy usage; autovacuum runs. Table row count is stable. CPU/IO fine. The column updated each request is the *indexed* one. Triage why the index keeps bloating and give the durable fix.
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 40 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.