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The problem with MySQL foreign key constraints in Online Schema Changes

This post explains the inherent problem of running online schema changes in MySQL, on tables participating in a foreign key relationship. We’ll lay some ground rules and facts, sketch a simplified schema, and dive into an online schema change operation. Our discussion applies to pt-online-schema-change, gh-ost, and Vitess based migrations, or any other online schema change tool that works with a shadow/ghost table like the Facebook tools. Why Online Schema Change? Online schema change tools come...

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