PlanetScale — Vitess serverless MySQL: branches, deploy requests, no FKs

PlanetScale is serverless, MySQL-compatible database-as-a-service built on Vitess — the same horizontal-scaling layer that runs YouTube. You do not ALTER TABLE against production. Schema changes live on an isolated branch (a full copy of the schema), reach production through a deploy request (a pull request for your schema), and apply via Online DDL: Vitess builds a shadow table holding the new schema, replicates rows and in-flight writes old→new with VReplication, then cuts over with no table lock. Because Online DDL and sharding are the platform's center of gravity, you design without foreign keys (FKs are now an opt-in but unsharded-only feature). This skill owns that workflow — not MySQL the engine.

Heuristic for the boundary: would this answer be identical on RDS MySQL? Then it is not this skill.