"David G. Johnston" <david.g.johnston@gmail.com> writes:
> I don't know that the existing wording is the most clear, but it is correct.
Perhaps it'd be better to write "... we could not have written
SELECT x, y FROM test1 GROUP BY x, because ..." ? The first
half of the example uses "SELECT *", and this bit was meant
to be consistent with that; but if the reader is confused about
how "y" got into the discussion then writing it out explicitly
should help.
regards, tom lane