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深入解讀PostgreSQL中的序列及其相關函數的用法

一、簡介

序列對象（也叫序列生成器）就是用CREATE SEQUENCE 創建的特殊的單行表。一個序列對象通常用于為行或者表生成唯一的標識符。

二、創建序列

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 數據遷移后更新序列

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

---------
   25
(1 row)

david=# select nextval('tbl_xulie_id_seq');
 nextval

---------
   100
(1 row)

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

--------
   25
(1 row)

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

---------
   27
(1 row)

五、序列函數

下面序列函數，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

---------
    3
(1 row)

david=# select nextval('tbl_xulie_id_seq');
 nextval

---------
    4
(1 row)

5.2 查看序列最近使用值

david=# select nextval('tbl_xulie_id_seq');
 nextval

---------
    4
(1 row)

david=# select currval('tbl_xulie_id_seq');
 currval

---------
    4
(1 row)

david=# select currval('tbl_xulie_id_seq');
 currval

---------
    4
(1 row)

5.3 重置序列

方法一：使用序列函數

a. setval(regclass, bigint)

david=# truncate tbl_xulie;
TRUNCATE TABLE
david=# select setval('tbl_xulie_id_seq', 1);
 setval

--------
   1
(1 row)

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

---------
    3
(1 row)

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

--------
   1
(1 row)

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

--------
   1
(1 row)

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

---------
    3
(1 row)

六、刪除序列

語法：


DROP SEQUENCE [ IF EXISTS ] name [, ...] [ CASCADE | RESTRICT ]

當有表字段使用到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 創建的，刪除該表的同時，對應的序列也會被刪除。

七、其他說明
a.currval取得的是當前會話的序列值，在當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時創建時，刪除該表的同時，對應的序列也會被刪除。
c.表主鍵數據可以用跟表相關的序列，也可以用其他序列，但不推薦，只是PG默認它沒錯。
d.為使多用戶并發下同一個序列取值不會重復，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創建的，那么對它調用 nextval 將返回從1 開始的后續的數值。其它的行為可以通過使用 CREATE SEQUENCE 命令里的特殊參數獲取；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值的當前事務被阻塞， nextval 操作決不會回滾；也就是說，一旦一個數值已經被抓走，那么就認為它已經用過了，即使調用 nextval 的事務后面又退出了也一樣。這就意味著退出的事務可能在序列賦予的數值中留下"空洞"。 setval 操作也決不回滾。

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