> which made a few dozen records balloon to many thousands of result rows

That doesn't really sound like a place where data is actually conceptually joined. I expect, as it is something commonly attempted, that you were abusing joins to try and work around the n+1 problem. As a corollary to the above, you also shouldn't de-join in application code.

It's a join. A join without any ON or USING clause or any filtering is a Cartesian product which is what's happening here.