2 files, with a million rows each, and dozens of fields each. Suggests that we are likely to be throwing around a few million objects, many of which are going to take up a few hundred bytes.

Even with modern-day computers, this is starting to push what I feel comfortable with assuming is available.

As for not having to lose data you intend to keep, I agree that it is possible to do so. I was just pointing out that your code wouldn't in the circumstance described by the original question.