Altering large MySQL tables is a pain. Almost every ALTER TABLE command results in MySQL doing the following: Creation of a temporary copy of the table Doing the modification on the temporary copy ...

Changing storage engine; rebuild table; change file_per_table, ALTER COLUMN OF TABLE, Change auto-increment value, Renaming a MySQL table, ALTER table add INDEX, Changing the type of a primary key ...

If MySQL exits in the middle of an in-place ALTER TABLE operation (ALGORITHM=INPLACE), you may be left with an orphan intermediate table that takes up space on your system.