And if you have enough modifications going on during that time, you'll fill your rollback segment, and the transaction will abort abnormally. (at least, that's how it used to be ... dunno if the latest version still does)

When I've had to run long transactions on an Oracle database where I didn't have enough storage for a second copy, I had to play with the pragma commands to set the isolation level ... unfortunately, it was almost a decade ago, and I don't remember what the command was. serializable is what keeps coming to mind, but I don't think that's right.