In principle, if there are no aggregate functions, then nodeAgg could return a row immediately upon making any new entry into the hash table. Whether it's worth the code uglification is debatable ... I think it would require a third major pathway through nodeAgg.
Regarding whether it's worth the effort: In each of my three past jobs (all using postgresql) I have met several queries that would fetch a small subset of a large - even huge - input. I think that types of queries are relatively common out there, but if they are executed for e.g. a web-client it is simply a no-go with the current late LIMIT evaluation.
Also, it is my impression that many people use LIMIT to minimize the evaluation time of sub queries from which the outer query only needs a small subset of the sub query output.
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