alter table - unnecessary "copy to tmp table" ?

[Dean Richardson]

"copy to tmp table" !

When I make changes to an enum column (to add or remove a value in the enum string) or drop an index, or add an index ... why does MySQL make a copy of the entire data table before performing the alter operations ... or have I misunderstood what "copy to tmp table" means?

As the actual data in the table isn't being altered I would think (it appears that in the case of enum it's the .frm file that needs altering, and in the case of the drop/.add index it appears it's the MYI file) it doesn't make sense to me to copy the whole MYD file before the alter operation ... especially when the table is into GB size.

What's happening with the "copy to tmp table" ?

Thanks,
Dean