> ALTER takes ages to do because of the ACID constraints of MySQL. No, I don't think it has much to with ACID. Rather, they made a simple single implementation

> ALTER takes ages to do because of the ACID constraints of MySQL. No, I don't think it has much to with ACID. Rather, they made a simple single implementation of ALTER TABLE that copies the whole table out on any change whatsoever. Add a column? Recreate the table. Add a table comment? Recreate the table. Drop an index? Recreate the table. They could have identified cases where in principle the table & metadata could be modified in-place, but that would be a lot harder than a simple copy. It would probably necessitate changes to the legacy architecture, which in turn would require a host of other changes.