It's possible that he is in the wrong context, but we'll assume based on the intro that he is in the correct place.
I'm admittedly a bit of a newbie and certainly a BI developer rather than a DBA, so I accept that I'm going to get a lot of the questions wrong. However, when an explanation starts with 'we'll assume' I feel a bit cheated. I know that assuming something is a certain way is a surefire way to waste a lot of time. I'm sure that everybody has had a conversation along the lines of:
You/them, 'have you checked..?',
You/them, 'it's definitely .....',
You/them, 'yeah, but have you checked it?
Don't get me wrong, I learn something every time, I just feel that answers that make assumptions can stack the deck against getting the answer right, particularly when the assumption can rule in\out a valid answer.
On two occasions I have been asked, "Pray, Mr. Babbage, if you put into the machine wrong figures, will the right answers come out?" ... I am not able rightly to apprehend the kind of confusion of ideas that could provoke such a question.
—Charles Babbage, Passages from the Life of a Philosopher How to post a question to get the most help