No problem, you just don't do it all in RAM.. I have written on-disk merge sorts in Perl that worked quite well. I even had to write one once where the dataset I was dealing with was, uncompressed, larger than the hard drive of the machine I was working on! The only painful part was the "processing took several days" bit, but considering that it was for a one-time backfill, that was acceptable. (More painful was the process of iterating through and tracking down discrepancies with the ongoing job I was trying to backfill. Every bug found required redoing large portions of the load from scratch. That was a painful month.)