[uhoh-itsmaciek]

I've worked on a few Postgres drivers and the protocol is basically sending data row-by-row, column-by-column. Sure, it could be more clever, but that's time and cpu and memory spent being clever instead of moving data.

But you're right, there are always trade-offs, and there are some situations where the current AR approach is not ideal, and a single join would be more efficient. However, I suspect this is the way AR does it because it's right enough often enough that it works well as the default approach.