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

Obviously you have to be able to read and write to the billion row file. The constraint of no external disk means you must restrict yourself to in-place algorithms or make some assumptions about the starting conditions and distribution of data so that you can keep the data sorted as it is added to the file (e.g. use the key as the index and create a large enough file to hold the expected number of keys).

If you must start with an unsorted file and sort it, you can use merge an in-place merge sort operating on very small chunks of the file. Since no constraints are made on the access times of the storage media, it may be very fast.