Thank you @viktorianer. Your description makes sense. If we revise the book in the future, I’ll pay extra attention to this section, as eager loading is of course a very common pattern.
In my experience, I’ve seen poor performance from big lists of values in an IN clause on big tables. I wrote up some thoughts about this in a blog post here: Big Problems From Big IN lists with Ruby on Rails and PostgreSQL | Software Engineer, Author, High Performance PostgreSQL for Rails