用PostgreSQL引发只读列的更新异常

Question

我试图在只读列被修改时引发异常。为此，我在更新之前使用触发器，但是我收到了语法错误。

DROP TRIGGER IF EXISTS check_update_guid_line ON line; 
CREATE TRIGGER check_update_guid_line 
    BEFORE UPDATE ON line FOR EACH ROW 
    WHEN (OLD.guid IS DISTINCT FROM NEW.guid) THEN 
     RAISE EXCEPTION 'you cant modify the guid of a line'); 

ERROR: syntax error at or near "THEN" LINE 29: ...OLD.guid IS DISTINCT FROM NEW.guid) THEN

我想我可以做扩展码了一堆行的过程，但我想知道为什么这个代码不工作，无论如何。

Answer 1

两个步骤分开它，喜欢这里：

CREATE OR REPLACE FUNCTION fn_1() 
RETURNS trigger 
LANGUAGE plpgsql 
COST 1 
AS $function$ 
BEGIN 
    IF NEW.id is distinct from OLD.id then 
    RAISE EXCEPTION '%','whatever it is'; 
    END IF; 
    return NEW; 
END; 
$function$ 
; 

CREATE TRIGGER tgr 
    BEFORE UPDATE ON line 
    FOR EACH ROW 
    EXECUTE PROCEDURE fn_1();