A FOREIGN KEY constraint isn't a separate entity. In MySQL, foreign keys are only implemented by the storage engine layer, namely the default and popular InnoDB engine. In other words, if you enforce referential integrity at the application level instead of at the database level, you open the door to all of those benefits. We believe the advantages of Online DDL such as branching, developer-owned schema changes and deployments, non-blocking schema changes, etc., and the advantages of sharding as means of unlimited scaling, outweigh the FOREIGN KEY constraints benefits. This typically happens when you introduce functional partitioning/sharding and/or horizontal sharding. Limited to single MySQL server scope, FOREIGN KEY constraints are impossible to maintain once your data grows and is split over multiple database servers.
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