No, you're probably right - I suppose the database drivers might naively serialize that in a one-to-one representation (to be honest, I've never really looked into the protocols that postgres or Ms sql use for transferring data).
I didn't consider the kind of expansion you're alluding to - on the other hand i can't ever remember seeing an indication that the amount of data returned is an issue with app performance.
But I've certainly seen the number of round-trips result in real issues. And complicated joins, for that matter.
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.
I didn't consider the kind of expansion you're alluding to - on the other hand i can't ever remember seeing an indication that the amount of data returned is an issue with app performance.
But I've certainly seen the number of round-trips result in real issues. And complicated joins, for that matter.