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

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.