PlanetScale has launched support for foreign key constraints in its database service, which was a significant technical challenge to overcome. The company had to address challenges related to branching and deploy requests, query serving, Online DDL, and handling reverts, among others. To tackle these issues, PlanetScale introduced new features such as internal operations tables, rename_table_preserve_foreign_key server variable, and improved locking mechanisms. While the support for foreign key constraints is still limited to unsharded environments, the company expects to expand its capabilities in the future. The launch of foreign key constraint support marks an important milestone for PlanetScale, and it welcomes users to try out the new feature during the open beta period.