![]() ![]() 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.The way FOREIGN KEY constraints are implemented in MySQL (or, rather, in the InnoDB storage engine) interferes with Online DDL operations.There are two major technical reasons why foreign key constraints are not supported: We'll soon cover an example of what a schema looks like with and without foreign key constraints so that this small difference is clear. FOREIGN KEY definition, that is not allowed in your schemas. It is just the enforcement at the database level, the CONSTRAINT. You are encouraged to use the relational model and associate tables by "pointing" rows from one table to another. PlanetScale doesn't support FOREIGN KEY constraints. A constraint also prevents the existence of "orphaned rows" in different methods, as you'll see described soon. Namely, it ensures that a child table can only reference a parent table when the appropriate row exists in the parent table. A table can also refer to itself, as a special case.Ī FOREIGN KEY constraint is a database construct, an implementation that forces the foreign key relationship's integrity (referential integrity). A foreign key typically suggests how you should JOIN tables in most queries. A row in a "parent" table may be referenced by one or more rows in a "child" table. A foreign key is a logical association of rows between two tables, in a parent-child relationship. ![]()
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