But the standard says that dropping system versioning should automatically drop all historical records (2 under Part 2: Foundation, 11.30 <drop system versioning clause>). That actually makes sense though: when you do DML we automatically update the start/end columns, but we don't save copies of the previous data (and incidentally the end column will always be the max value.)
This is what I was referring to when I mentioned a side-table. deleting history would be an O(1) operation. Any other misunderstandings are all mine.
