The fact that ALTER TABLE locks on most databases is not the primary reason changing a schema is hard. The primary reason is that a schema change breaks the app

The fact that ALTER TABLE locks on most databases is not the primary reason changing a schema is hard. The primary reason is that a schema change breaks the application [1]. Migrations are programs, but your existing live application is also a program. Changing the schema breaks your live application to prepare for the new version of the application. [1] - http://chronicdb.com/blogs/change_is_not_the_enemy http://chronicdb.com/blogs/change_is_not_the_enemy