---------------------------------------------------------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)
);
隱式插入
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;
顯式插入
insert into myschema.test_seq (id,name) values (5,'ddd');
select * from test_seq;
再次隱式插入
--可以正常插入
insert into myschema.test_seq (name) values ('eee');
--插入失敗，主鍵重復(fù)，因?yàn)樾蛄凶陨硎沁f增的，不會關(guān)心表中被顯式插入的數(shù)據(jù)
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);
--事務(wù)回滾后，序列號并不會回滾
begin;
insert into myschema.test_seq (name) values ('ggg');
rollback;
-- truncate 表之后，序列不受影響
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;
--顯式插入,可以執(zhí)行
insert into myschema.test_serial(id,name) values (5,'ccc');
--再次隱式插入,第二次會報錯，因?yàn)殡[式插入的話，serial會基于顯式插入之前的Id做自增，serial無法意識到當(dāng)前已經(jīng)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不會重置
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;
--驗(yàn)證是否會隨著事務(wù)一起回滾，結(jié)論：不會
begin;
insert into myschema.test_serial(name) values ('yyy');
rollback;
--重置serial，需要注意的是重置的Id必須要大于相關(guān)表的字段最大Id，否則會產(chǎn)生重號
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;
--顯式插入值,如果定義為generated always as identity則不允許顯式插入，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;
--顯式插入值,如果定義為generated by default as identity則允許顯式插入，
insert into myschema.test_identiy_2(id,name) values (5,'ccc');
--顯式插入后，繼續(xù)隱式插入,第二次插入會報錯，identity已久是不識別表中顯式插入后的最大值
insert into myschema.test_identiy_2(name) values ('xxx');
insert into myschema.test_identiy_2(name) values ('yyy');
select * from myschema.test_identiy_2;
總之個identity很扯淡，你定義成always as identity，加上overriding system value可以顯式不插入
定義成generated by default as identity也允許顯式插入
不管怎么樣，既然都允許顯式插入，那扯什么淡的來個overriding system value
--truncate后再次插入，自增列不會重置
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，會重置自增列
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;