一、簡介
序列對象（也叫序列生成器）就是用CREATE SEQUENCE 創(chuàng)建的特殊的單行表。一個序列對象通常用于為行或者表生成唯一的標(biāo)識符。
二、創(chuàng)建序列
方法一：直接在表中指定字段類型為serial 類型
david=# create table tbl_xulie (
david(# id serial,
david(# name text);
NOTICE: CREATE TABLE will create implicit sequence "tbl_xulie_id_seq" for serial column "tbl_xulie.id"
CREATE TABLE
david=#
方法二：先創(chuàng)建序列名稱，然后在新建的表中列屬性指定序列就可以了，該列需int 類型
創(chuàng)建序列的語法：
CREATE [ TEMPORARY | TEMP ] SEQUENCE name [ INCREMENT [ BY ] increment ]
[ MINVALUE minvalue | NO MINVALUE ] [ MAXVALUE maxvalue | NO MAXVALUE ]
[ START [ WITH ] start ] [ CACHE cache ] [ [ NO ] CYCLE ]
[ OWNED BY { table.column | NONE } ]
實例：
david=# create sequence tbl_xulie2_id_seq increment by 1 minvalue 1 no maxvalue start with 1;
CREATE SEQUENCE
david=#
david=# create table tbl_xulie2 (
david(# id int4 not null default nextval('tbl_xulie2_id_seq'),
david(# name text);
CREATE TABLE
david=#
三、查看序列
david=# \d tbl_xulie
Table "public.tbl_xulie"
Column | Type | Modifiers
--------+---------+--------------------------------------------------------
id | integer | not null default nextval('tbl_xulie_id_seq'::regclass)
name | text |
david=# \d tbl_xulie2
Table "public.tbl_xulie2"
Column | Type | Modifiers
--------+---------+---------------------------------------------------------
id | integer | not null default nextval('tbl_xulie2_id_seq'::regclass)
name | text |
david=#
查看序列屬性
david=# \d tbl_xulie_id_seq
Sequence "public.tbl_xulie_id_seq"
Column | Type | Value
---------------+---------+---------------------
sequence_name | name | tbl_xulie_id_seq
last_value | bigint | 1
start_value | bigint | 1
increment_by | bigint | 1
max_value | bigint | 9223372036854775807
min_value | bigint | 1
cache_value | bigint | 1
log_cnt | bigint | 0
is_cycled | boolean | f
is_called | boolean | f
Owned by: public.tbl_xulie.id
david=# select * from tbl_xulie2_id_seq;
sequence_name | last_value | start_value | increment_by | max_value | min_value | cache_value | log_cnt | is_cycled | is_called
-------------------+------------+-------------+--------------+---------------------+-----------+-------------+---------+-----------+-----------
tbl_xulie2_id_seq | 1 | 1 | 1 | 9223372036854775807 | 1 | 1 | 0 | f | f
(1 row)
四、序列應(yīng)用
4.1 在INSERT 命令中使用序列
david=# insert into tbl_xulie values (nextval('tbl_xulie_id_seq'), 'David');
INSERT 0 1
david=# insert into tbl_xulie values (nextval('tbl_xulie_id_seq'), 'Sandy');
INSERT 0 1
david=# select * from tbl_xulie;
id | name
----+-------
1 | David
2 | Sandy
(2 rows)
4.2 數(shù)據(jù)遷移后更新序列
david=# truncate tbl_xulie;
TRUNCATE TABLE
david=#
david=# insert into tbl_xulie values (nextval('tbl_xulie_id_seq'), 'Sandy');
INSERT 0 1
david=# insert into tbl_xulie values (nextval('tbl_xulie_id_seq'), 'David');
INSERT 0 1
david=# insert into tbl_xulie values (nextval('tbl_xulie_id_seq'), 'Eagle');
INSERT 0 1
david=# insert into tbl_xulie values (nextval('tbl_xulie_id_seq'), 'Miles');
INSERT 0 1
david=# insert into tbl_xulie values (nextval('tbl_xulie_id_seq'), 'Simon');
INSERT 0 1
david=# insert into tbl_xulie values (nextval('tbl_xulie_id_seq'), 'Rock');
INSERT 0 1
david=# insert into tbl_xulie values (nextval('tbl_xulie_id_seq'), 'Peter');
INSERT 0 1
david=# insert into tbl_xulie values (nextval('tbl_xulie_id_seq'), 'Sally');
INSERT 0 1
david=# insert into tbl_xulie values (nextval('tbl_xulie_id_seq'), 'Nicole');
INSERT 0 1
david=# insert into tbl_xulie values (nextval('tbl_xulie_id_seq'), 'Monica');
INSERT 0 1
david=# insert into tbl_xulie values (nextval('tbl_xulie_id_seq'), 'Renee');
INSERT 0 1
david=# select * from tbl_xulie;
id | name
----+--------
15 | Sandy
16 | David
17 | Eagle
18 | Miles
19 | Simon
20 | Rock
21 | Peter
22 | Sally
23 | Nicole
24 | Monica
25 | Renee
(11 rows)
david=# copy tbl_xulie to '/tmp/tbl_xulie.sql';
COPY 11
david=# truncate tbl_xulie;
TRUNCATE TABLE
david=# alter sequence tbl_xulie_id_seq restart with 100;
ALTER SEQUENCE
david=# select currval('tbl_xulie_id_seq');
currval
david=# select nextval('tbl_xulie_id_seq');
nextval
david=# select nextval('tbl_xulie_id_seq');
nextval
---------
101
(1 row)
david=# begin;
BEGIN
david=# copy tbl_xulie from '/tmp/tbl_xulie.sql';
COPY 11
david=# select setval('tbl_xulie_id_seq', max(id)) from tbl_xulie;
setval
david=# end;
COMMIT
david=# insert into tbl_xulie values (nextval('tbl_xulie_id_seq'), 'Flash');
INSERT 0 1
david=# select * from tbl_xulie;
id | name
----+--------
15 | Sandy
16 | David
17 | Eagle
18 | Miles
19 | Simon
20 | Rock
21 | Peter
22 | Sally
23 | Nicole
24 | Monica
25 | Renee
26 | Flash
(12 rows)
david=# select nextval('tbl_xulie_id_seq');
nextval
五、序列函數(shù)
下面序列函數(shù)，為我們從序列對象中獲取最新的序列值提供了簡單和并發(fā)讀取安全的方法。
5.1 查看下一個序列值
david=# select nextval('tbl_xulie_id_seq');
nextval
david=# select nextval('tbl_xulie_id_seq');
nextval
5.2 查看序列最近使用值
david=# select nextval('tbl_xulie_id_seq');
nextval
david=# select currval('tbl_xulie_id_seq');
currval
david=# select currval('tbl_xulie_id_seq');
currval
5.3 重置序列
方法一：使用序列函數(shù)
a. setval(regclass, bigint)
david=# truncate tbl_xulie;
TRUNCATE TABLE
david=# select setval('tbl_xulie_id_seq', 1);
setval
david=# insert into tbl_xulie values (nextval('tbl_xulie_id_seq'), 'Sandy');
INSERT 0 1
david=# insert into tbl_xulie values (nextval('tbl_xulie_id_seq'), 'David');
INSERT 0 1
david=# select * from tbl_xulie;
id | name
----+-------
2 | Sandy
3 | David
(2 rows)
david=# select currval('tbl_xulie_id_seq');
currval
david=# select nextval('tbl_xulie_id_seq');
nextval
---------
4
(1 row)
b. setval(regclass, bigint, boolean)
b.1 setval(regclass, bigint, true)
david=# truncate tbl_xulie;
TRUNCATE TABLE
david=# select setval('tbl_xulie_id_seq', 1, true);
setval
david=# insert into tbl_xulie values (nextval('tbl_xulie_id_seq'), 'Sandy');
INSERT 0 1
david=# insert into tbl_xulie values (nextval('tbl_xulie_id_seq'), 'David');
INSERT 0 1
david=# select * from tbl_xulie;
id | name
----+-------
2 | Sandy
3 | David
(2 rows)
效果同a. setval(regclass, bigint)
b.2 setval(regclass, bigint, false)
david=# truncate tbl_xulie;
TRUNCATE TABLE
david=# select setval('tbl_xulie_id_seq', 1, false);
setval
david=# insert into tbl_xulie values (nextval('tbl_xulie_id_seq'), 'Sandy');
INSERT 0 1
david=# insert into tbl_xulie values (nextval('tbl_xulie_id_seq'), 'David');
INSERT 0 1
david=# select * from tbl_xulie;
id | name
----+-------
1 | Sandy
2 | David
(2 rows)
方法二：修改序列
修改序列的語法：
ALTER SEQUENCE name [ INCREMENT [ BY ] increment ]
[ MINVALUE minvalue | NO MINVALUE ] [ MAXVALUE maxvalue | NO MAXVALUE ]
[ START [ WITH ] start ]
[ RESTART [ [ WITH ] restart ] ]
[ CACHE cache ] [ [ NO ] CYCLE ]
[ OWNED BY { table.column | NONE } ]
ALTER SEQUENCE name OWNER TO new_owner
ALTER SEQUENCE name RENAME TO new_name
ALTER SEQUENCE name SET SCHEMA new_schema
實例：
david=# truncate tbl_xulie;
TRUNCATE TABLE
david=# alter sequence tbl_xulie_id_seq restart with 0;
ERROR: RESTART value (0) cannot be less than MINVALUE (1)
david=# alter sequence tbl_xulie_id_seq restart with 1;
ALTER SEQUENCE
david=# insert into tbl_xulie values (nextval('tbl_xulie_id_seq'), 'David');
INSERT 0 1
david=# insert into tbl_xulie values (nextval('tbl_xulie_id_seq'), 'Sandy');
INSERT 0 1
david=# select * from tbl_xulie;
id | name
----+-------
1 | David
2 | Sandy
(2 rows)
david=# select nextval('tbl_xulie_id_seq');
nextval
六、刪除序列
語法：
DROP SEQUENCE [ IF EXISTS ] name [, ...] [ CASCADE | RESTRICT ]
當(dāng)有表字段使用到PG序列時，不能直接刪除。
david=# drop sequence tbl_xulie2_id_seq;
ERROR: cannot drop sequence tbl_xulie2_id_seq because other objects depend on it
DETAIL: default for table tbl_xulie2 column id depends on sequence tbl_xulie2_id_seq
HINT: Use DROP ... CASCADE to drop the dependent objects too.
david=# drop table tbl_xulie2;
DROP TABLE
david=# drop sequence tbl_xulie2_id_seq;
DROP SEQUENCE
david=#
說明：對于序列是由建表時指定serial 創(chuàng)建的，刪除該表的同時，對應(yīng)的序列也會被刪除。
七、其他說明
a.currval取得的是當(dāng)前會話的序列值，在當(dāng)前會話中該值不會因為其他會話取了nextval而變化。會變化的是全局的last_value值,并且當(dāng)前會話中如果沒有讀過nextval值時直接讀currval是會報錯的。
b.對于序列是由建表時指定serial時創(chuàng)建時，刪除該表的同時，對應(yīng)的序列也會被刪除。
c.表主鍵數(shù)據(jù)可以用跟表相關(guān)的序列，也可以用其他序列，但不推薦，只是PG默認(rèn)它沒錯。
d.為使多用戶并發(fā)下同一個序列取值不會重復(fù)，nextval是不會rollback的，不過可以使用setval重置
如果一個序列對象是帶著缺省參數(shù)創(chuàng)建的，那么對它調(diào)用 nextval 將返回從1 開始的后續(xù)的數(shù)值。 其它的行為可以通過使用 CREATE SEQUENCE 命令里的 特殊參數(shù)獲??；參閱其命令參考頁獲取更多信息。
e.為了避免從同一個序列獲取數(shù)值的當(dāng)前事務(wù)被阻塞， nextval 操作決不會回滾；也就是說，一旦一個數(shù)值已經(jīng)被抓走， 那么就認(rèn)為它已經(jīng)用過了，即使調(diào)用 nextval 的事務(wù)后面又退出了也一樣。這就意味著退出的事務(wù)可能在序列賦予的數(shù)值中留下"空洞"。 setval 操作也決不回滾。
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