How

You do need to use no specific statement on MySQL 5.7. If you would use MySQL 5.6, you have to use the statement ALGORITHM=INPLACE, LOCK=NONE for no downtime.

References

MySQL :: MySQL 5.7 Reference Manual :: 14.13.1 Online DDL Operations

[What is MySQL’s Online DDL Ruby, Internet, and Programming](https://rip.hibariya.org/post/online-ddl/)

Does adding a non-null default column cause downtime in MySQL 5.7? - Roger Lam