How to sort millions of rows of data in a file with less/meagre memory

Question

(From here)

I attended an interview last week and this question was asked:

How do you sort a billion rows of data in a file with only 640KB of memory in

Answer 1

You can find the discussion on a similar problem in Jon Bentley Programming Pearls Column. 1. Here Bentley deals with a problem of sorting millions of area codes which are guaranteed to be unique by using a bitset data-structure.