Code Room
On-callHard
Question
At 03:40 your Postgres-backed billing service's nightly invoice job, which usually finishes in 8 minutes, is still running after 2 hours and is now blocking the morning batch. The query plan for its main aggregation flipped from a hash join to a nested loop sometime overnight. pg_stat_user_tables shows the invoices table has 40M dead tuples and last_autovacuum is 9 days ago. autovacuum_max_workers is at default and a large data-retention purge ran last night deleting 35M rows. How do you triage and resolve this?
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.
Learn the concepts
Loading whiteboard…
Run or narrate your approach, then ask the coach.