用PostgreSQL引發只讀列的更新異常

Question

我試圖在只讀列被修改時引發異常。爲此，我在更新之前使用觸發器，但是我收到了語法錯誤。

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

我想我可以做擴展碼了一堆行的過程，但我想知道爲什麼這個代碼不工作，無論如何。

Answer 1

兩個步驟分開它，喜歡這裏：

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();