> not the best design [...]

So those engineers at Shopify worked hard for months on a more performant system, but they missed the obvious structure? They chose a complex denormalization for no good reason?

It may be true, but I think it's presumptuous to belittle their work when we have only partial information. My guess is that they had good reasons to think that the more obvious ways would not scale.

And from reading your comments in this thread, I believe your structure would fail at their scale. A SQL query that uses 2 sub-queries with "group by" is probably too heavy. From the post, at peaks there would be millions of active shopping carts.

BTW, I suspect most orders are just for 1 or 2 of each item, so the denormalization is not as heavy as it seems.