我就廢話不多說了，大家還是直接看實例吧~
postgres=# create table t1(a int primary key,b text,c date);
CREATE TABLE
postgres=# create table t2(a int primary key,b int references t1(a),c text);
CREATE TABLE
postgres=# insert into t1 (a,b,c) values(1,'aa',now());
INSERT 0 1
postgres=# insert into t1 (a,b,c) values(2,'bb',now());
INSERT 0 1
postgres=# insert into t2 (a,b,c) values (1,1,'aa');
INSERT 0 1
postgres=# insert into t2 (a,b,c) values (2,2,'aa');
INSERT 0 1
postgres=# \d t1
Table "public.t1"
Column | Type | Collation | Nullable | Default
--------+---------+-----------+----------+---------
a | integer | | not null |
b | text | | |
c | date | | |
Indexes:
"t1_pkey" PRIMARY KEY, btree (a)
Referenced by:
TABLE "t2" CONSTRAINT "t2_b_fkey" FOREIGN KEY (b) REFERENCES t1(a)
postgres=# \d t2
Table "public.t2"
Column | Type | Collation | Nullable | Default
--------+---------+-----------+----------+---------
a | integer | | not null |
b | integer | | |
c | text | | |
Indexes:
"t2_pkey" PRIMARY KEY, btree (a)
Foreign-key constraints:
"t2_b_fkey" FOREIGN KEY (b) REFERENCES t1(a)
postgres=#
假設我們想通過腳本向表中加載一些數據。因為我們不知道腳本中加載的順序，我們決定將表t2上的外鍵約束禁用掉，在數據加載之后載開啟外鍵約束：
postgres=# alter table t2 disable trigger all;
ALTER TABLE
postgres=#
這里看起來可能有點奇怪，但是它的確禁用了外鍵約束。如果有其他外鍵約束，當然也是被禁用了。
我們再來看看表t2：
postgres=# \d t2
Table "public.t2"
Column | Type | Collation | Nullable | Default
--------+---------+-----------+----------+---------
a | integer | | not null |
b | integer | | |
c | text | | |
Indexes:
"t2_pkey" PRIMARY KEY, btree (a)
Foreign-key constraints:
"t2_b_fkey" FOREIGN KEY (b) REFERENCES t1(a)
Disabled internal triggers:
"RI_ConstraintTrigger_c_75213" AFTER INSERT ON t2 FROM t1 NOT DEFERRABLE INITIALLY IMMEDIATE FOR EACH ROW EXECUTE PROCEDURE "RI_FKey_check_ins"()
"RI_ConstraintTrigger_c_75214" AFTER UPDATE ON t2 FROM t1 NOT DEFERRABLE INITIALLY IMMEDIATE FOR EACH ROW EXECUTE PROCEDURE "RI_FKey_check_upd"()
postgres=#
關鍵字all將表上的其他內部觸發器也禁用了，需要superser才可以執行成功。
postgres=# create user abce with login password 'abce';
CREATE ROLE
postgres=# \c postgres abce
You are now connected to database "postgres" as user "abce".
postgres=> create table t3 ( a int primary key, b text, c date);
CREATE TABLE
postgres=> create table t4 ( a int primary key, b int references t3(a), c text);
CREATE TABLE
postgres=> alter table t4 disable trigger all;
ERROR: permission denied: "RI_ConstraintTrigger_c_75235" is a system trigger
postgres=>
那作為普通用戶，該如何禁用觸發器呢？
postgres=> alter table t4 disable trigger user;
具體語法為：
DISABLE TRIGGER [ trigger_name | ALL | USER ]
回到t1、t2表。
postgres=# select * from t1;
a | b | c
---+----+------------
1 | aa | 2020-11-04
2 | bb | 2020-11-04
(2 rows)
postgres=# select * from t2;
a | b | c
---+---+----
1 | 1 | aa
2 | 2 | aa
(2 rows)
postgres=# insert into t2 (a,b,c) values (3,3,'cc');
INSERT 0 1
postgres=#
這里插入了一條在t1中不匹配的記錄，但是插入成功了。
postgres=# alter table t2 enable trigger all;
ALTER TABLE
postgres=# \d t2
Table "public.t2"
Column | Type | Collation | Nullable | Default
--------+---------+-----------+----------+---------
a | integer | | not null |
b | integer | | |
c | text | | |
Indexes:
"t2_pkey" PRIMARY KEY, btree (a)
Foreign-key constraints:
"t2_b_fkey" FOREIGN KEY (b) REFERENCES t1(a)
postgres=# alter table t2 validate constraint t2_b_fkey;
ALTER TABLE
postgres=#
是不是很驚訝，PostgreSQL沒有報告不匹配的記錄。為什么呢？
查看一個pg_constraint：
postgres=# select * from pg_constraint where conname='t2_b_fkey' and conrelid='t2'::regclass;
-[ RECORD 1 ]-+----------
conname | t2_b_fkey
connamespace | 2200
contype | f
condeferrable | f
condeferred | f
convalidated | t
conrelid | 75202
contypid | 0
conindid | 75200
conparentid | 0
confrelid | 75194
confupdtype | a
confdeltype | a
confmatchtype | s
conislocal | t
coninhcount | 0
connoinherit | t
conkey | {2}
confkey | {1}
conpfeqop | {96}
conppeqop | {96}
conffeqop | {96}
conexclop |
conbin |
consrc |
postgres=#
convalidated字段的值為t，表明該外鍵約束還是有效的。
哪怕是我們再次將其disable，仍然會顯示是有效的：
postgres=# alter table t2 disable trigger all;
ALTER TABLE
postgres=# select * from pg_constraint where conname='t2_b_fkey' and conrelid='t2'::regclass;
-[ RECORD 1 ]-+----------
conname | t2_b_fkey
connamespace | 2200
contype | f
condeferrable | f
condeferred | f
convalidated | t
conrelid | 75202
contypid | 0
conindid | 75200
conparentid | 0
confrelid | 75194
confupdtype | a
confdeltype | a
confmatchtype | s
conislocal | t
coninhcount | 0
connoinherit | t
conkey | {2}
confkey | {1}
conpfeqop | {96}
conppeqop | {96}
conffeqop | {96}
conexclop |
conbin |
consrc |
postgres=#
這表明當我們開啟(enable)內部觸發器的時候，PostgreSQL不會驗證(validate)約束，因此也不會驗證數據是否會有沖突，因為外鍵約束的狀態始終是有效的。
我們需要做的是先將其變成無效的：
postgres=# alter table t2 alter CONSTRAINT t2_b_fkey not valid;
ERROR: ALTER CONSTRAINT statement constraints cannot be marked NOT VALID
## 需要先將外鍵刪掉，然后重建外鍵約束并將其狀態設置成無效
postgres=# alter table t2 drop constraint t2_b_fkey;
ALTER TABLE
postgres=# delete from t2 where a in (3);
DELETE 1
postgres=# alter table t2 add constraint t2_b_fkey foreign key (b) references t1(a) not valid;
ALTER TABLE
postgres=# \d t2
Table "public.t2"
Column | Type | Collation | Nullable | Default
--------+---------+-----------+----------+---------
a | integer | | not null |
b | integer | | |
c | text | | |
Indexes:
"t2_pkey" PRIMARY KEY, btree (a)
Foreign-key constraints:
"t2_b_fkey" FOREIGN KEY (b) REFERENCES t1(a) NOT VALID
現在，可以看到狀態是無效的了：
postgres=# select * from pg_constraint where conname='t2_b_fkey' and conrelid='t2'::regclass;
-[ RECORD 1 ]-+----------
conname | t2_b_fkey
connamespace | 2200
contype | f
condeferrable | f
condeferred | f
convalidated | f
conrelid | 75202
contypid | 0
conindid | 75200
conparentid | 0
confrelid | 75194
confupdtype | a
confdeltype | a
confmatchtype | s
conislocal | t
coninhcount | 0
connoinherit | t
conkey | {2}
confkey | {1}
conpfeqop | {96}
conppeqop | {96}
conffeqop | {96}
conexclop |
conbin |
consrc |
postgres=#
繼續插入數據：
postgres=# insert into t2(a,b,c) values (3,3,'cc');
ERROR: insert or update on table "t2" violates foreign key constraint "t2_b_fkey"
DETAIL: Key (b)=(3) is not present in table "t1".
postgres=#
是不是更驚訝了？創建了一個無效的約束，只是通知PostgreSQL
不要掃描整個表去驗證所有的行記錄是否有效。對于新插入或更新的行，仍然會檢查是否滿足約束條件，這就是為什么上面插入失敗了。
我們該怎么做呢？
1.刪除所有的外鍵
2.加載數據
3.重新創建外鍵，但是將其狀態設置成無效的，從而避免掃描整個表。之后，新的數據會被驗證了
4.在系統負載低的時候開啟約束驗證(validate the constraints)
另一種方法是：
postgres=# alter table t2 alter constraint t2_b_fkey deferrable;
ALTER TABLE
postgres=# begin;
BEGIN
postgres=# set constraints all deferred;
SET CONSTRAINTS
postgres=# insert into t2 (a,b,c) values (3,3,'cc');
INSERT 0 1
postgres=# insert into t2 (a,b,c) values (4,4,'dd');
INSERT 0 1
postgres=# insert into t1 (a,b,c) values (3,'cc',now());
INSERT 0 1
postgres=# insert into t1 (a,b,c) values (4,'dd',now());
INSERT 0 1
postgres=# commit;
COMMIT
這樣做不好的方面是，在下一次提交時才起作用，因此，你需要將所有的工作放到一個事務中。
本文的關鍵點是，下面的假設將驗證你的數據是錯誤的:
postgres=# alter table t2 disable trigger all;
ALTER TABLE
postgres=# insert into t2 (a,b,c) values (5,5,'ee');
INSERT 0 1
postgres=# alter table t2 enable trigger all;
ALTER TABLE
postgres=#
這只會驗證新的數據，但是并不保證所有的數據都滿足約束：
postgres = # insert into t2 (a,b,c) values (6,6,'ff');
ERROR: insert or update on table "t2" violates foreign key constraint "t2_b_fkey"
DETAIL: Key(b) = (6) is not present in table "t1".
postgres = # select * from t2 where b = 5;
a | b | c
---+---+----
5 | 5 | ee
(1 row)
postgres = # select * from t1 where a = 5;
a | b | c
---+---+---
(0 rows)
最終，還有一種方式來解決，直接修改pg_constraint目錄表。但是并建議用戶這么做！
postgres=# delete from t2 where b = 5;
DELETE 1
postgres=# delete from t2 where b = 5;
DELETE 1
postgres=# alter table t2 disable trigger all;
ALTER TABLE
postgres=# insert into t2 values (5,5,'ee');
INSERT 0 1
postgres=# alter table t2 enable trigger all;
ALTER TABLE
postgres=# update pg_constraint set convalidated = false where conname = 't2_b_fkey' and conrelid = 't2'::regclass;
UPDATE 1
postgres=# alter table t2 validate constraint t2_b_fkey;
ERROR: insert or update on table "t2" violates foreign key constraint "t2_b_fkey"
DETAIL: Key (b)=(5) is not present in table "t1".
postgres=#
以上為個人經驗，希望能給大家一個參考，也希望大家多多支持腳本之家。如有錯誤或未考慮完全的地方，望不吝賜教。
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