Postgres 10

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.
Help with an interesting Postgres question: Why isn't an Index Only Scan used on a partition accessed via the parent table?.
CREATE TABLE orders (
    id INT NOT NULL, 
    code INT NOT NULL, 
    service_id INT NOT NULL, 
    status CHARACTER VARYING(50), 
    creation TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
    status_date TIMESTAMP,
    created_user INT
    );
INSERT INTO orders 
(id, code, service_id, status, creation, status_date, created_user) 
VALUES 
(100,2394,558151,'Requested','2019-06-16 11:12','2019-06-18 14:08',1),
(100,2394,558151,'Fulfilled','2018-06-16 11:12','2018-06-18 14:08',1),
(100,2394,558151,'Requested','2019-06-16 11:12','2019-06-18 14:08',1),
(100,2395,558152,'Requested','2019-06-16 11:12','2019-06-18 14:08',1),
(100,2396,558153,'Requested','2019-06-16 11:12','2019-06-18 14:08',1),
(100,2394,558151,'Requested','2019-06-16 11:12','2019-06-18 14:08',1);
6 rows affected

SELECT EXTRACT(YEAR FROM creation) AS year, 
                        orders.code, 
                        COUNT(orders.code) 
                FROM orders
                WHERE status = 'Requested' AND EXTRACT(YEAR FROM creation) = 2019 AND orders.code = 2394
                GROUP BY year, orders.code
                ORDER BY year ASC


year
code
count


2019
2394
3


CREATE OR REPLACE FUNCTION 
yearly_orders(inyear numeric DEFAULT 0, incode INT DEFAULT 0) 
RETURNS TABLE (retyear numeric, retcode bigint, retcout bigint) AS 
$$
DECLARE
    retcount int DEFAULT 0;
    retcode int DEFAULT 0;
    retyear numeric DEFAULT 0;
    row_cnt int DEFAULT 0;
BEGIN
    RETURN QUERY SELECT CAST(EXTRACT(year FROM creation) as numeric) , 
                        CAST(orders.code as bigint) , 
                        COUNT(orders.code) 
                FROM orders
                WHERE status = 'Requested' 
                AND EXTRACT(YEAR FROM creation) = inyear 
                AND orders.code = incode
                GROUP BY EXTRACT(year FROM creation), orders.code
                ORDER BY retyear ASC;
    GET DIAGNOSTICS row_cnt = ROW_COUNT;

   IF row_cnt =  0 THEN
      RAISE SQLSTATE '45000'   -- 5 ASCII chars
      USING MESSAGE = format('No orders found for year %s.',inyear);
   END IF;

END;
$$
LANGUAGE plpgsql;
select * from yearly_orders(2020, 2394);
ERROR:  No orders found for year 2020.
CONTEXT:  PL/pgSQL function yearly_orders(numeric,integer) line 20 at RAISE