Aggregate functions with FROM clause and ROW_COUNT diagnostics

[Alexey Dokuchaev <danfe@xxxxxx>]

Hi,

I'm seeing somewhat confusing results here with 9.6.8, and cannot find
the answer in the docs or google.

I'm returning JSON array (or any array, it does not make a difference)
from my plpgsql function like this:

    OUT retcode int,
    OUT result json)
    . . .
    result := json_agg(_) FROM (
      SELECT foo, bar, baz ...
      FROM t1, t2, t3 WHERE ...) AS _;		-- this works fine

    GET DIAGNOSTICS retcode = ROW_COUNT;	-- always returns 1

I'd expected `retcode' to contain the number of SELECT'ed rows, but it
is something else (always 1).  Apparently, aggregation functions like
json_agg()/array_agg() mangle the ROW_COUNT from the inner SELECT (the
one I'm interested in).

Is this expected and correct behavior?  Is it possible to obtain the
first ROW_COUNT (after SELECT) without performing it twice?  Thanks,

./danfe