Let's say that I start a new Transaction. I pull the Autoincremented ID that I need for the transaction from the future new record. And then I lock that record so another transaction can't write to it. So in the mean time, another user has created a new transaction therefor creating a new record. His ID is 100. The ID I have locked is 99 and then I decide to cancel my transaction. So now I have records 95 96 97 98 100 Will the next transaction use record 99 or 101?