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

Another question to have asked, is "What is the nature of the rows?" If the number of distinct values is low enough, then the answer might be a pigeon hole sort.

For example, say the file to be sorted only contained rows that held a number between 0 and 100 inclusive. Create an array of 101 unsigned 32 bit or 64 bit integers with a value of 0. As you read a row, use it to index the array and increament the count of that element. Once the file is read, start at 0, read the the number of zeros read and spit out that many, go to 1, repeat. Expand the array size as needed to handle the set of numbers coming through. Of course there are limits, say the values that can be seen span from -2e9 to +2e9. That's going to require 4e9 bins, which is not going to fit in 640K of RAM.

If instead the rows are strings, but you are still looking at a small enough set of distinct value, then use an associative array or hash table to hold the counts.