如何添加「刪除級聯」約束？

Question

在PostgreSQL 8中，是否可以在下表中添加「刪除級聯」到兩個外鍵而不丟棄後者？

# \d pref_scores 
     Table "public.pref_scores" 
Column |   Type   | Modifiers 
---------+-----------------------+----------- 
id  | character varying(32) | 
gid  | integer    | 
money | integer    | not null 
quit | boolean    | 
last_ip | inet     | 
Foreign-key constraints: 
    "pref_scores_gid_fkey" FOREIGN KEY (gid) REFERENCES pref_games(gid) 
    "pref_scores_id_fkey" FOREIGN KEY (id) REFERENCES pref_users(id) 

兩個引用的表都低於 - 在這裏：

# \d pref_games 
            Table "public.pref_games" 
    Column |   Type    |      Modifiers 
----------+-----------------------------+---------------------------------------------------------- 
gid  | integer      | not null default nextval('pref_games_gid_seq'::regclass) 
rounds | integer      | not null 
finished | timestamp without time zone | default now() 
Indexes: 
    "pref_games_pkey" PRIMARY KEY, btree (gid) 
Referenced by: 
    TABLE "pref_scores" CONSTRAINT "pref_scores_gid_fkey" FOREIGN KEY (gid) REFERENCES pref_games(gid) 

在這裏：

# \d pref_users 
       Table "public.pref_users" 
    Column |   Type    | Modifiers 
------------+-----------------------------+--------------- 
id   | character varying(32)  | not null 
first_name | character varying(64)  | 
last_name | character varying(64)  | 
female  | boolean      | 
avatar  | character varying(128)  | 
city  | character varying(64)  | 
login  | timestamp without time zone | default now() 
last_ip | inet      | 
logout  | timestamp without time zone | 
vip  | timestamp without time zone | 
mail  | character varying(254)  | 
Indexes: 
    "pref_users_pkey" PRIMARY KEY, btree (id) 
Referenced by: 
    TABLE "pref_cards" CONSTRAINT "pref_cards_id_fkey" FOREIGN KEY (id) REFERENCES pref_users(id) 
    TABLE "pref_catch" CONSTRAINT "pref_catch_id_fkey" FOREIGN KEY (id) REFERENCES pref_users(id) 
    TABLE "pref_chat" CONSTRAINT "pref_chat_id_fkey" FOREIGN KEY (id) REFERENCES pref_users(id) 
    TABLE "pref_game" CONSTRAINT "pref_game_id_fkey" FOREIGN KEY (id) REFERENCES pref_users(id) 
    TABLE "pref_hand" CONSTRAINT "pref_hand_id_fkey" FOREIGN KEY (id) REFERENCES pref_users(id) 
    TABLE "pref_luck" CONSTRAINT "pref_luck_id_fkey" FOREIGN KEY (id) REFERENCES pref_users(id) 
    TABLE "pref_match" CONSTRAINT "pref_match_id_fkey" FOREIGN KEY (id) REFERENCES pref_users(id) 
    TABLE "pref_misere" CONSTRAINT "pref_misere_id_fkey" FOREIGN KEY (id) REFERENCES pref_users(id) 
    TABLE "pref_money" CONSTRAINT "pref_money_id_fkey" FOREIGN KEY (id) REFERENCES pref_users(id) 
    TABLE "pref_pass" CONSTRAINT "pref_pass_id_fkey" FOREIGN KEY (id) REFERENCES pref_users(id) 
    TABLE "pref_payment" CONSTRAINT "pref_payment_id_fkey" FOREIGN KEY (id) REFERENCES pref_users(id) 
    TABLE "pref_rep" CONSTRAINT "pref_rep_author_fkey" FOREIGN KEY (author) REFERENCES pref_users(id) 
    TABLE "pref_rep" CONSTRAINT "pref_rep_id_fkey" FOREIGN KEY (id) REFERENCES pref_users(id) 
    TABLE "pref_scores" CONSTRAINT "pref_scores_id_fkey" FOREIGN KEY (id) REFERENCES pref_users(id) 
    TABLE "pref_status" CONSTRAINT "pref_status_id_fkey" FOREIGN KEY (id) REFERENCES pref_users(id) 

而且也不知是否是有意義的2個index'es添加到舊錶？

UPDATE：謝謝，我也得在郵件列表的建議，我可以在1條語句進行管理，因而沒有必要交易：

ALTER TABLE public.pref_scores 
DROP CONSTRAINT pref_scores_gid_fkey, 
ADD CONSTRAINT pref_scores_gid_fkey 
    FOREIGN KEY (gid) 
    REFERENCES pref_games(gid) 
    ON DELETE CASCADE; 

Answer 1

我敢確定你不能簡單地將on delete cascade添加到現有的外鍵約束。您必須先刪除約束，然後添加正確的版本。在標準的SQL，我認爲這樣做最簡單的方法就是

• 啓動一個事務，
• 刪除外鍵，
• 添加一個外鍵與on delete cascade，終於
• 提交事務

針對要更改的每個外鍵重複該步驟。

但是PostgreSQL有一個非標準的擴展，可以讓你在單個SQL語句中使用多個約束子句。例如

alter table public.pref_scores 
drop constraint pref_scores_gid_fkey, 
add constraint pref_scores_gid_fkey 
    foreign key (gid) 
    references pref_games(gid) 
    on delete cascade; 

如果你不知道你要刪除的外鍵約束的名稱，你可以去查pgAdminIII（只需點擊表名，並期待在DDL，或擴大直到您看到「約束條件」），或者您可以query the information schema。

select * 
from information_schema.key_column_usage 
where position_in_unique_constraint is not null 

Answer 2

用法：

select replace_foreign_key('user_rates_posts', 'post_id', 'ON DELETE CASCADE'); 

功能：

CREATE OR REPLACE FUNCTION 
    replace_foreign_key(f_table VARCHAR, f_column VARCHAR, new_options VARCHAR) 
RETURNS VARCHAR 
AS $$ 
DECLARE constraint_name varchar; 
DECLARE reftable varchar; 
DECLARE refcolumn varchar; 
BEGIN 

SELECT tc.constraint_name, ccu.table_name AS foreign_table_name, ccu.column_name AS foreign_column_name 
FROM 
    information_schema.table_constraints AS tc 
    JOIN information_schema.key_column_usage AS kcu 
     ON tc.constraint_name = kcu.constraint_name 
    JOIN information_schema.constraint_column_usage AS ccu 
     ON ccu.constraint_name = tc.constraint_name 
WHERE constraint_type = 'FOREIGN KEY' 
    AND tc.table_name= f_table AND kcu.column_name= f_column 
INTO constraint_name, reftable, refcolumn; 

EXECUTE 'alter table ' || f_table || ' drop constraint ' || constraint_name || 
', ADD CONSTRAINT ' || constraint_name || ' FOREIGN KEY (' || f_column || ') ' || 
' REFERENCES ' || reftable || '(' || refcolumn || ') ' || new_options || ';'; 

RETURN 'Constraint replaced: ' || constraint_name || ' (' || f_table || '.' || f_column || 
' -> ' || reftable || '.' || refcolumn || '); New options: ' || new_options; 

END; 
$$ LANGUAGE plpgsql; 

注意：此功能不會複製初始外鍵的屬性。 它只需要外國名稱/列名稱，丟棄當前密鑰，並用新的代替。