Migration holding exclusive lock
A deploy at 11:00 ran a schema migration that's now blocking the whole app. The migration does `ALTER TABLE orders ADD COLUMN status_v2 text NOT NULL DEFAULT 'pending'` followed by a `CREATE INDEX` on a 150M-row Postgres table — and it's running inside the deploy's migration step. Since it started, all reads and writes to `orders` hang; `pg_stat_activity` shows the migration holding an `AccessExclusiveLock` on `orders`, and behind it a growing queue of normal app queries all "waiting for lock," including simple SELECTs. The app is effectively down for anything touching orders. The migration has been running 6 minutes with no end in sight. Triage and decide.
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.