Ecto migration with default value from another column in same table

Hi,

I’ve a few million records, and I’m trying to add a column, uuid, to their table that has a default value of each row’s id field. Is this possible? And if not, would it be possible to give it a default value that increments?

My aim is to have unique values in the new UUID column so I can then add a unique index.

Thanks in advance.