To be clear, this is not a Vitess/PlanetScale-specific opinion or choice. Foreign key constraints are a bit of a controversial topic in large-scale MySQL enviro

To be clear, this is not a Vitess/PlanetScale-specific opinion or choice. Foreign key constraints are a bit of a controversial topic in large-scale MySQL environments in general, which is the greater context in which this design decision was made by the Vitess team. PlanetScale's (and Vitess') non-blocking schema changes rely on open source tools for MySQL like pt-online-schema-change and gh-ost, which are widely used in production environments everywhere, and neither of them are too comfortable supporting FK's, though pt-osc does accommodate them to some extent (https://www.percona.com/doc/percona-toolkit/3.0/pt-online-schema-change.html https://www.percona.com/doc/percona-toolkit/3.0/pt-online-sc...). gh-ost's lack of support was discussed on HN previously here: https://news.ycombinator.com/item?id=16983620 https://news.ycombinator.com/item?id=16983620 A good collection of resources on why they're considered problematic and many companies designing large-scale MySQL schemas tend to drop them can also be found here: https://federico-razzoli.com/foreign-key-bugs-in-mysql-and-mariadb https://federico-razzoli.com/foreign-key-bugs-in-mysql-and-m...