ALTER TABLE MODIFY ENUM takes a long time. Why?

[Margaret MacDonald]

I'm doing a MODIFY on an enum column (I typoed one of the 30 tokens). Since an enum is merely a way to name a set of integers, I thought the ALTER would finish in a flash, since all it would have to do is change the list of tokens to which the underlying integers map. But it seems to be fingering through the whole 2.3M-record table for some reason. Why would it do that, does anyone know? Surely it doesn't store the string value, does it?

Thanks for any insights.