如何将mySQL中的触发fucntion更改为postgresql触发器

Question

我在MySQL中具有触发功能：

CREATE TRIGGER after_customers_insert
BEFORE INSERT
ON customers FOR EACH ROW
BEGIN
SET @created := (SELECT IFNULL(created_at, NOW()) FROM customers ORDER BY created_at 
DESC LIMIT 1);

IF YEAR(@created) <> YEAR(NEw.created_at) THEN
   SET NEW.Newid :=  1;
ELSE
    SET NEW.Newid := (SELECT IFNULL(MAX(Newid),0) + 1 FROM customers);
END IF;
SET NEW.custom_id = CONCAT('KR_',DATE_FORMAT(now(), '%y'),'_',LPAD(NEW.Newid,3,'0'));
END

我试图在Postgres中转换以下：

CREATE OR REPLACE FUNCTION after_customers_insert()
RETURNS trigger AS
$BODY$
BEGIN
SET @created := (SELECT IFNULL(created_at, NOW()) FROM customers ORDER BY created_at DESC LIMIT 1);

IF YEAR(@created) <> YEAR(NEw.created_at) THEN
   SET NEW.Newid :=  1;
ELSE
    SET NEW.Newid := (SELECT IFNULL(MAX(Newid),0) + 1 FROM customers);
END IF;
SET NEW.custom_id = CONCAT('KR_',DATE_FORMAT(now(), '%y'),'_',LPAD(NEW.Newid,3,'0'));
END
$BODY$
LANGUAGE plpgsql VOLATILE
COST 100;

但没有成功。请有任何帮助吗？