Online schema changes for cloudsql DB
ALTER TABLE t MODIFY COLUMN c ENUM(...) ...

If that is only adding a new option on the end and not going past 255 options, that should be a &quot;schema-only&quot; change that happens instantly.  (This was in 5.5, so has been around for 12 years.)
Any other change to the ENUM is likely to require a &quot;copy&quot; of the table (slow).
ALTER TABLE t ADD INDEX (...)

has been efficient (not instant, but did not require copying the table over) since 5.6 (2013).
For other column alterations -- It depends.
That probably does not apply to ADD UNIQUE, and certainly does not apply to any changes to the PRIMARY KEY.
So, based on that info, I would expect Cloud-SQL with 5.7 to handle things &quot;fast&quot;.  (Sorry, but I have no direct knowledge of that port of MySQL.)  I suggest you take it up with the vendor.