Hmmm. PostgreSQL treats output rowsets as their own thing, so this works differently in a subquery than in a top-level query. Not that that's desireable, it's just a consequence of an implementation that gives a lot more flexibility in other areas (table functions, for example, where Postgres goes well beyond the standard). So looking at the standard I agree that having your example work is superior to not having it work. I'd appreciate a footnote for that particular item for Postgres, though, since the most common forms of GROUP BY dependencies do work.
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