PostgreSQL三种自增列sequence,serial,identity的用法区别

---------------------------------------------------------sequence-------------------------------------------------------------
create sequence myschema.seq_1 INCREMENT BY 1 MINVALUE 1 START WITH 1;
create table myschema.test_seq
(
 id int not null default nextval('myschema.seq_1') primary key,
 name varchar(10)
);
隐式插入
insert into myschema.test_seq (name) values ('aaa');
insert into myschema.test_seq (name) values ('bbb');
insert into myschema.test_seq (name) values ('ccc');
select * from myschema.test_seq;
显式插入
insert into myschema.test_seq (id,name) values (5,'ddd');
select * from test_seq;
再次隐式插入
--可以正常插入
insert into myschema.test_seq (name) values ('eee');
--插入失败，主键重复，因为序列自身是递增的，不会关心表中被显式插入的数据
insert into myschema.test_seq (name) values ('fff');
--重置序列的最大值
select setval('myschema.seq_1',(select max(id) from myschema.test_seq)::BIGINT);
--事务回滚后，序列号并不会回滚
begin;
insert into myschema.test_seq (name) values ('ggg');
rollback;
-- truncate 表之后，序列不受影响
truncate table myschema.test_seq;
--重置序列
ALTER SEQUENCE myschema.seq_1 RESTART WITH 1;
---------------------------------------------------------serial-------------------------------------------------------------
create table myschema.test_serial
(
 id serial primary key,
 name varchar(100)
)
select * from test_serial;
insert into myschema.test_serial(name) values ('aaa');
insert into myschema.test_serial(name) values ('bbb');
insert into myschema.test_serial(name) values ('ccc');
select * from myschema.test_serial;
--显式插入,可以执行
insert into myschema.test_serial(id,name) values (5,'ccc');
--再次隐式插入,第二次会报错，因为隐式插入的话，serial会基于显式插入之前的Id做自增，serial无法意识到当前已经存在的最大值
insert into myschema.test_serial(name) values ('xxx');
insert into myschema.test_serial(name) values ('yyy');
select * from myschema.test_serial;
--truncate table 后serial不会重置
truncate table myschema.test_serial;
insert into myschema.test_serial(name) values ('aaa');
insert into myschema.test_serial(name) values ('bbb');
insert into myschema.test_serial(name) values ('ccc');
select * from myschema.test_serial;
--验证是否会随着事务一起回滚，结论：不会
begin;
insert into myschema.test_serial(name) values ('yyy');
rollback;
--重置serial，需要注意的是重置的Id必须要大于相关表的字段最大Id，否则会产生重号
SELECT SETVAL((SELECT pg_get_serial_sequence('myschema.test_serial', 'id')), 1, false);
---------------------------------------------------------identity-------------------------------------------------------------
drop table myschema.test_identiy_1 
create table myschema.test_identiy_1 
(
 id int generated always as identity (cache 100 START WITH 1 INCREMENT BY 1) primary key , 
 name varchar(100)
);
create table myschema.test_identiy_2
(
 id int generated by default as identity (cache 100 START WITH 1 INCREMENT BY 1) primary key , 
 name varchar(100)
);
insert into myschema.test_identiy_1(name) values ('aaa');
insert into myschema.test_identiy_1(name) values ('bbb');
insert into myschema.test_identiy_1(name) values ('ccc');
insert into myschema.test_identiy_2(name) values ('aaa');
insert into myschema.test_identiy_2(name) values ('bbb');
insert into myschema.test_identiy_2(name) values ('ccc');
select * from myschema.test_identiy_1;
--显式插入值,如果定义为generated always as identity则不允许显式插入，除非增加overriding system value 提示
--一旦提示了overriding system value，可以
insert into myschema.test_identiy_1(id,name) values (5,'ccc');
insert into myschema.test_identiy_1(id,name)overriding system value values (5,'ccc');
select * from myschema.test_identiy_2;
--显式插入值,如果定义为generated by default as identity则允许显式插入，
insert into myschema.test_identiy_2(id,name) values (5,'ccc');
--显式插入后，继续隐式插入,第二次插入会报错，identity已久是不识别表中显式插入后的最大值
insert into myschema.test_identiy_2(name) values ('xxx');
insert into myschema.test_identiy_2(name) values ('yyy');
select * from myschema.test_identiy_2;
总之个identity很扯淡，你定义成always as identity，加上overriding system value可以显式不插入
定义成generated by default as identity也允许显式插入
不管怎么样，既然都允许显式插入，那扯什么淡的来个overriding system value
--truncate后再次插入，自增列不会重置
truncate table myschema.test_identiy_1;
select * from myschema.test_identiy_1;
begin;
insert into myschema.test_identiy_1(name) values ('xxx');
rollback;
--truncate并且RESTART IDENTITY后，会重置自增列
TRUNCATE table myschema.test_identiy_1 RESTART IDENTITY;
select * from myschema.test_identiy_1
--identity自增列的重置表或者更改
ALTER TABLE myschema.test_identiy_1 ALTER COLUMN id RESTART WITH 100;

id name="id">
  generator class="sequence">
  param name="sequence">adminuser/param>
  /generator>
 /id>

/**
 * @hibernate.id generator-class="sequence"
 * @hibernate.generator-param name="sequence" value="adminuser"
 */
 
 private int id;