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...