[mkonecny]

Are you sure? This was the case in mysql 5.5 but 5.6 and up will not take the table offline anymore.

> Several ALTER TABLE operations can be performed without copying the table, without blocking inserts, updates, and deletes to the table, or both. These enhancements are known collectively as online DDL. See Section 14.13, “InnoDB and Online DDL” for details.

https://dev.mysql.com/doc/refman/5.6/en/mysql-nutshell.html

[verelo]

No, but it would take so much CPU and OI, it might as well. I doubt the application would be in great shape while this operation took place.