PlanetScale has announced support for foreign key constraints, a feature that was missing from their product since its launch. The company faced numerous technical challenges in incorporating foreign key support into their system, including limitations in MySQL and the need to integrate it with PlanetScale’s Online DDL and branching features. They had to overcome these challenges in various layers of the product, and the process required careful evaluation and planning. Despite the complexities involved, PlanetScale has successfully implemented foreign key constraint support, although there may be some performance impact when using it with their system.
https://planetscale.com/blog/challenges-of-supporting-foreign-key-constraints