PostgreSQL 约束延迟触发

PG只支持以下几种约束延迟触发（并不是不触发，而是再事务结束的时候触发）

UNIQUE、PRIMARY KEY、REFERENCES（外键）和 EXCLUDE

测试例子:

sql 复制代码

CREATE TABLE logs (
    id INTEGER,
    log_time TIMESTAMP,
    message TEXT,
    CONSTRAINT pk_logs 
        PRIMARY KEY (id) 
        DEFERRABLE
);

BEGIN;
SET CONSTRAINTS pk_logs DEFERRED;

-- 可以暂时插入重复ID，最后再修复
INSERT INTO logs (id, log_time, message) VALUES
(1, '2024-01-01 10:00:00', 'Message 1'),
(1, '2024-01-01 10:01:00', 'Message 2'), -- 暂时允许重复
(2, '2024-01-01 10:02:00', 'Message 3');

-- 修复重复的ID
UPDATE logs SET id = 3 WHERE id = 1 AND message = 'Message 2';

COMMIT;  -- 提交时才检查主键唯一性


建表的时候指定DEFERRABLE是可以延迟的意思，是延迟的前提条件，默认是 INITIALLY IMMEDIATE

这时候需要在开始事务之后使用
SET CONSTRAINTS DEFERRED语句来确定是否要真的延迟

当然也可以指定INITIALLY DEFERRED 这样在事务中就不需要执行
SET CONSTRAINTS DEFERRED 语句了

如下图所测试

指定INITIALLY DEFERRED

sql 复制代码

CREATE TABLE logs (
    id INTEGER,
    log_time TIMESTAMP,
    message TEXT,
    CONSTRAINT pk_logs 
        PRIMARY KEY (id) 
        DEFERRABLE INITIALLY DEFERRED
);

begin;

INSERT INTO logs (id, log_time, message) VALUES
(1, '2024-01-01 10:00:00', 'Message 1'),
(1, '2024-01-01 10:01:00', 'Message 2'), 
(2, '2024-01-01 10:02:00', 'Message 3');

-- 修复重复的ID
UPDATE logs SET id = 3 WHERE id = 1 AND message = 'Message 2';

COMMIT;  -- 提交时才检查主键唯一性

修改约束

当然也可以修改约束延迟模式

但是需要先删除掉约束，重建的时候指定

sql 复制代码

postgres=# alter table logs drop constraint pk_logs;
ALTER TABLE
postgres=# alter table logs add constraint pk_logs primary key(id) deferrable initially deferred;
ALTER TABLE

当然oracle和达梦默认就是延迟模式

如下所示：

sql 复制代码

Connected to:
Oracle Database 19c Enterprise Edition Release 19.0.0.0.0 - Production
Version 19.18.0.0.0

SQL> create table tab(id numeric primary key);

Table created.

SQL> insert into tab values (1);

1 row created.

SQL> insert into tab values(2);

1 row created.

SQL> commit;

Commit complete.

SQL> update tab set id=id+1;

2 rows updated.

SQL> commit;

Commit complete.

SQL>

达梦

sql 复制代码

SQL> create table tab(id int);
executed successfully
used time: 4.234(ms). Execute id is 1702.
SQL> alter table tab add constraint tab_pkey primary key(id);
executed successfully
used time: 12.044(ms). Execute id is 1703.
SQL> insert into tab values (1),(2);
affect rows 2

used time: 1.255(ms). Execute id is 1704.
SQL> commit;
executed successfully
used time: 1.605(ms). Execute id is 1705.
SQL> select * from v$version;

LINEID     BANNER
---------- ---------------------------------
1          DM Database Server 64 V8
2          DB Version: 0x7000c
3          03134284194-20240703-234060-20108
4          Msg Version: 12
5          Gsu level(5) cnt: 0

used time: 0.336(ms). Execute id is 1706.
SQL> update tab set id=id+1;
affect rows 2

used time: 1.135(ms). Execute id is 1707.
SQL> commit;
executed successfully
used time: 1.596(ms). Execute id is 1708.
SQL>