This is an issue that is pretty common when auto generating files. For instance when generating receipt PDF it could feel natural to store them in folders by a

This is an issue that is pretty common when auto generating files. For instance when generating receipt PDF it could feel natural to store them in folders by account ID. Except there will be a bunch of accounts generating 20 or 30 receipts a day, which isn't much on the face of it. But within months it becomes a pain to list receipts across accounts, within a year or two even individual account receipts become a nightmare to list, and fixing the situation requires a few tricks to avoid all the tools that assume directory listing cost nothing.