Indexes, rather. They charge by rows read, so if you don't add indexes you will (very quickly) read all the rows in your table on every query. That racks your bill really fast.

The foreign key thing is because the author thought foreign keys add indexes, but here foreign keys are no-ops and don't do anything. Because it's so distributed there's no support for foreign keys.

Nonsense. It wasn't 20k and it was corrected.

link?

Here's the story: https://news.ycombinator.com/item?id=29132572