Normally, I'd do a relatively complicated change of data type on column A like this:

- Create column B of the new data type
- Set B to contain the new values derived from A
- Drop A
- Rename B to A

To guarantee the validity of my data model, I'd like to perform such restructuring within a single transaction. So far, I have not been able to do that, because, depending on the isolation level, either B is not considered to exist yet, or I'm not allowed to rename B ("update conflicts with concurrent update"), or no errors occur but B is empty. Is it possible to perform to do what I ask - I'd love to see how - or is it absolutely required to COMMIT in between?

P.S. Asked on StackOverflow first, so if you prefer, you might as well answer there: http://stackoverflow.com/q/15591799/1163893