CREATE OR REPLACE FUNCTION public.test(
	)
    RETURNS TABLE(y integer, result text) 
    LANGUAGE 'plpgsql'
    COST 100
    VOLATILE 
    ROWS 1000
AS $BODY$
DECLARE notHasRows BOOL DEFAULT true;
BEGIN
    FOR y, result IN 
		-- здесь есть строки
                -- SELECT s.y, 'hi' result FROM generate_series(1,10,1) AS s(y)
		-- Здесь нет строк
		SELECT s.y, 'hi' result FROM generate_series(1,0,1) AS s(y)
    LOOP
        RETURN NEXT;
    END LOOP;
	
   IF FOUND AND notHasRows THEN
	notHasRows = false;
   END IF;
    
  FOR y, result IN 
		-- здесь есть строки
                -- SELECT s.y, 'hi' result FROM generate_series(1,10,1) AS s(y)
		-- Здесь нет строк
		SELECT s.y, 'hi' result FROM generate_series(1,0,1) AS s(y)
    LOOP
        RETURN NEXT;
    END LOOP;

   IF FOUND AND notHasRows THEN
	notHasRows = false;
   END IF;

    IF notHasRows THEN
 	RAISE EXCEPTION 'нет строк';
    END IF;  
END;
$BODY$;

SELECT test();