production note on session cookies during a Friday deploy

We ran into this around session cookies during a Friday deploy.

I expected a tooling problem. It was an assumption in the schema.

EXPLAIN and a short rollback window saved the afternoon.

Writes slowed first, then a handful of reads timed out.

Dropping the unused index and batching the backfill unblocked the queue.

I now review write paths with the same care as the happy-path query.