[matc]

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

[jeltz]

This is true for NoSQL too. If you change the format of your documents you applications might need to be modified to support both the old and the new format at the same time.