[Philip-J-Fry]

>allowing API users to return everything at once can be a problem both for our server (lots of data in RAM when fetching from the DB => OOM, and additional stress on the DB)

You can limit stress on RAM by streaming the data. You should ideally stream rows for any large dataset. Otherwise, like you say you are loading the entire thing into RAM.

[jiggawatts]

Not to mention the latency reduction!

Buffering up the entire data set before encoding it to JSON and sending it is one of the biggest sources of latency in API based software. Streaming can get latencies down to tens of microseconds!