It's possible with hash tables too...

Also, the Unix approach can do something like this by spitting out days at a time and sorting them, and then doing a

    sort -m "$month/*" | uniq -c

on the individual files.

`sort -m` takes basically no memory, and should chew through data fast enough.

Are you pulling that joke every time someone asks something you think you know the answer to? It's kind of contradictory.

I've edited my response, but I think you misunderstand the comic.

Was it the XKCD Ten Thousand comic?

Indeed.