在PostgreSQL 8中,是否可以ON DELETE CASCADES
在下表中添加两个外键而不删除后者?
# \d scores
Table "public.scores"
Column | Type | Modifiers
---------+-----------------------+-----------
id | character varying(32) |
gid | integer |
money | integer | not null
quit | boolean |
last_ip | inet |
Foreign-key constraints:
"scores_gid_fkey" FOREIGN KEY (gid) REFERENCES games(gid)
"scores_id_fkey" FOREIGN KEY (id) REFERENCES users(id)
这两个引用的表都在下面-这里:
# \d games
Table "public.games"
Column | Type | Modifiers
----------+-----------------------------+----------------------------------------------------------
gid | integer | not null default nextval('games_gid_seq'::regclass)
rounds | integer | not null
finished | timestamp without time zone | default now()
Indexes:
"games_pkey" PRIMARY KEY, btree (gid)
Referenced by:
TABLE "scores" CONSTRAINT "scores_gid_fkey" FOREIGN KEY (gid) REFERENCES games(gid)
和这里:
# \d users
Table "public.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:
"users_pkey" PRIMARY KEY, btree (id)
Referenced by:
TABLE "cards" CONSTRAINT "cards_id_fkey" FOREIGN KEY (id) REFERENCES users(id)
TABLE "catch" CONSTRAINT "catch_id_fkey" FOREIGN KEY (id) REFERENCES users(id)
TABLE "chat" CONSTRAINT "chat_id_fkey" FOREIGN KEY (id) REFERENCES users(id)
TABLE "game" CONSTRAINT "game_id_fkey" FOREIGN KEY (id) REFERENCES users(id)
TABLE "hand" CONSTRAINT "hand_id_fkey" FOREIGN KEY (id) REFERENCES users(id)
TABLE "luck" CONSTRAINT "luck_id_fkey" FOREIGN KEY (id) REFERENCES users(id)
TABLE "match" CONSTRAINT "match_id_fkey" FOREIGN KEY (id) REFERENCES users(id)
TABLE "misere" CONSTRAINT "misere_id_fkey" FOREIGN KEY (id) REFERENCES users(id)
TABLE "money" CONSTRAINT "money_id_fkey" FOREIGN KEY (id) REFERENCES users(id)
TABLE "pass" CONSTRAINT "pass_id_fkey" FOREIGN KEY (id) REFERENCES users(id)
TABLE "payment" CONSTRAINT "payment_id_fkey" FOREIGN KEY (id) REFERENCES users(id)
TABLE "rep" CONSTRAINT "rep_author_fkey" FOREIGN KEY (author) REFERENCES users(id)
TABLE "rep" CONSTRAINT "rep_id_fkey" FOREIGN KEY (id) REFERENCES users(id)
TABLE "scores" CONSTRAINT "scores_id_fkey" FOREIGN KEY (id) REFERENCES users(id)
TABLE "status" CONSTRAINT "status_id_fkey" FOREIGN KEY (id) REFERENCES users(id)
而且我不知道在前面的表中添加2个索引是否有意义?
更新:谢谢,我在邮件列表中也得到了建议,我可以在1条语句中进行管理,因此无需显式启动事务:
ALTER TABLE public.scores
DROP CONSTRAINT scores_gid_fkey,
ADD CONSTRAINT scores_gid_fkey
FOREIGN KEY (gid)
REFERENCES games(gid)
ON DELETE CASCADE;
谢谢!我实际上注意到删除操作要花很长时间,但不知道那是原因
—
Alexander Farber 2012年
当外键索引不值得使用时,会是哪种情况?
—
Alexander Farber 2012年
我把你的发现纳入了我的答案。(该声明也是一笔交易。)
—
Mike Sherrill'Cat Recall'
@AlexanderFarber:什么时候您想在FK的引用列上省略索引?当存在另一个不完全匹配的索引时,该匹配就足够好了(例如,对于频繁的相似性搜索,您可能会有一个trigram索引,对于FK删除也可以)。删除很少,可以安排在下班时间进行。当表具有引用值的频繁更新时。当引用表很小但经常更新时。异常经常发生,以至于PostgreSQL社区更喜欢对其进行控制而不是使其自动化。
—
kgrittn
pref_scores.gid
)。如果您在被引用的表上删除很多行,那么删除这些表将需要很长时间。一些数据库会在引用列上自动创建索引;PostgreSQL由您自己决定,因为在某些情况下它不值得。