PostgreSQL的SERIAL与MySQL的AUTO_INCREMENT？

我有这个MySQL的片段：

CREATE TABLE seq_test (
  id INT PRIMARY KEY AUTO_INCREMENT,
  name TEXT
);

INSERT INTO seq_test(id, name) VALUES (1, 'one');
INSERT INTO seq_test(name) VALUES ('two');

当我尝试在Postgresql中编写它时：

CREATE TABLE seq_test (
  id SERIAL PRIMARY KEY,
  name TEXT
);

INSERT INTO seq_test(id, name) VALUES (1, 'one');
INSERT INTO seq_test(name) VALUES ('two');

我收到以下错误：

[23505] ERROR: duplicate key value violates unique constraint "seq_test_pkey"
  Detail: Key (id)=(1) already exists.

这是因为在Postgresql中,插入一个不会增加下一个插入的id.如何创建表以使其符合MysqL行为？

这是一个人为的例子,但是我将一个从MysqL迁移到Postgresql的大型代码库,部分代码(我无法控制)使用两种样式(即有和没有id),它们在MysqL中工作但是在Postgresql中不起作用.

一个丑陋的黑客就是总是做SELECT setval(‘my_table_id_seq’,(SELECT count(*)FROM my_table),TRUE)……

解决方法:

对此没有任何不解决的解决方案：您要么坚持SERIAL功能,要么自己处理.

但是你让我感兴趣,而且我已经想出了这个(我希望有点少hacky)解决方案：创建一个触发器来完成所有肮脏的工作.

触发功能

(添加通知,以便我们可以看到发生了什么)：

CREATE OR REPLACE FUNCTION update_seq_val_seq_test()
  RETURNS TRIGGER AS $$
BEGIN
  RAISE NOTICE 'id is %', NEW.id;
  IF NEW.id > currval('seq_test_id_seq' :: REGCLASS)
  THEN
    RAISE NOTICE 'curval is %', currval('seq_test_id_seq' :: REGCLASS);
    PERFORM setval('seq_test_id_seq' :: REGCLASS, (NEW.id) :: BIGINT);
    RAISE NOTICE 'new curval is %', currval('seq_test_id_seq' :: REGCLASS); END IF;
  RETURN NULL;
END;
$$
LANGUAGE 'plpgsql' COST 1;

设置触发器

CREATE TRIGGER seq_test_update_serial
  AFTER INSERT ON seq_test
  FOR EACH ROW EXECUTE PROCEDURE update_seq_val_seq_test();

拉动扳机

Fast’n’dirty测试

tests2=# insert into seq_test (name) values ('first');
NOTICE:  id is 30
INSERT 0 1
tests2=# select * from seq_test;
 id | name  
----+-------
 30 | first
(1 row)

tests2=# select currval('seq_test_id_seq'::regclass);
 currval 
---------
      30
(1 row)

tests2=# insert into seq_test (id, name) values (31, 'thirty one');
NOTICE:  id is 31
NOTICE:  curval is 30
NOTICE:  new curval is 31
INSERT 0 1
tests2=# select currval('seq_test_id_seq'::regclass);
 currval 
---------
      31
(1 row)

tests2=# select * from seq_test;
 id |    name    
----+------------
 30 | first
 31 | thirty one
(2 rows)

tests2=# insert into seq_test (name) values ('thirty dunno what');
NOTICE:  id is 32
INSERT 0 1
tests2=# insert into seq_test (id, name) values (21, 'back to the future');
NOTICE:  id is 21
INSERT 0 1
tests2=# select currval('seq_test_id_seq'::regclass);
 currval 
---------
      32
(1 row)

tests2=# select * from seq_test;
 id |        name        
----+--------------------
 30 | first
 31 | thirty one
 32 | thirty dunno what
 21 | back to the future
(4 rows)

tests2=# insert into seq_test (name) values ('thirty dunno what++');
NOTICE:  id is 33
INSERT 0 1
tests2=# select * from seq_test;
 id |        name         
----+---------------------
 30 | first
 31 | thirty one
 32 | thirty dunno what
 21 | back to the future
 33 | thirty dunno what++
(5 rows)

所以,现在Postgres正在处理这个案例,就像你想要的那样,但是有很多东西要检查你：如何使用批量插入,回滚,这个触发器如何影响性能,以及更多的乐趣.