I definitely agree that there may be some unexpected consequences of using the primary key in this way. It certainly won't behave like a timestamp and could distort the function outputs.
Would it be possible to substitute the integer count of seconds since epoch (or some other arbitrary base timestamp) and use that instead? Then you'd still have an immutable function to satisfy the index, but also you'd preserve valuable information about the relative time intervals between posts.
Yeah. If you wanted to delay making posts visible but create them in advance, they wouldn't have the same ranking as though they were just posted. You'd have to create them at the exact time you'd want them to start being displayed in order for the desired ranking to be correct.
The example was a social media site like Instagram, not a blog, which doesn't allow future posting. If you have different design constraints than the given example, obviously the implementation will need to adjusted to the new requirements.
