My impression is that once you're at Facebook scale, most of your migrations are massive undertakings that need to take into account things like "How many terab

My impression is that once you're at Facebook scale, most of your migrations are massive undertakings that need to take into account things like "How many terabytes more space do we need?", "How do we control the load on our DB nodes while the migration is going on?", "How do we get data from cluster A to cluster B?", "Is adding this index going to take hours and break everything?", and so on. Some of the time you'll be spinning up an entirely new cluster rather than changing the schema of an old one, and when you do migrate an existing cluster there's some five-page document specifying a week-long plan with different phases. Then internally, they work around the inflexible db schemas by using offline batch processing tools or generic systems that can handle arbitrary data, for tasks that would be handled by the one DB in smaller systems.