Postgresql function has unsupported return - database

Question

I'm very new in work with data base, and this is my first time with triggers and functions. I have two tables:
main table "orders"
id | order_status_id | tracking_code
----+-----------------+---------------
12 | 2 | 123456
9 | 2 |
6 | 2 |
7 | 2 |
10 | 2 |
8 | 2 |
11 | 2 |
and another one "order_statuses"
id | status
----+-------------
1 | QUEUE
2 | IN_PROGRESS
3 | IN_DELIVERY
4 | DELIVERED
5 | CANCELED
6 | RETURNED
I want to change value in column order_status_id automatically (from 2 to 3) when I change the value in column tracking_code (default value is null).
I wrote trigger and function:
CREATE TRIGGER change_status
BEFORE UPDATE
OF tracking_code
ON orders
EXECUTE PROCEDURE update_status();
CREATE FUNCTION update_status() RETURNS TRIGGER
AS
$$
DECLARE
passed bigint;
BEGIN
SELECT id INTO passed FROM order_statuses WHERE status = IN_DELIVERY;
NEW.order_status_id = passed;
return NEW;
END;
$$;
but when I try to run this code, I get error:
[2020-09-07 15:22:53] [42804] ERROR: function "update_status" in FROM has unsupported return type trigger
[2020-09-07 15:22:53] Где: PL/pgSQL function inline_code_block line 5 at OPEN
I tried to implement a lot of answers for the same question from stack overflow, but nothing helps.
can someone tell me what I do wrong and if is my code "good" in general?!

Answer 1

Your trigger definition is OK, except for one thing; the documentation says:
FOR EACH ROW
FOR EACH STATEMENT
This specifies whether the trigger function should be fired once for every row affected by the trigger event, or just once per SQL statement. If neither is specified, FOR EACH STATEMENT is the default.
So you probably want to specify FOR EACH ROW.
The error message, however, has a different cause:
There is a DO statement somewhere in your code (“inline_code_block”) that calls update_status directly. That doesn't work. You can use trigger functions only in trigger definitions.