postgresql----serial类型和序列

postgresql序列号（SERIAL）类型包括smallserial（smallint,short）,serial(int)和bigserial(bigint,long long int)，不管是smallserial,serial还是bigserial，其范围都是(1,9223372036854775807)，但是序列号类型其实不是真正的类型，当声明一个字段为序列号类型时其实是创建了一个序列，INSERT时如果没有给该字段赋值会默认获取对应序列的下一个值。

测试表1：

test=# create table tbl_serial(a serial,b varchar(2));
CREATE TABLE
test=# \d tbl_serial
                               Table "public.tbl_serial"
 Column |         Type         |                       Modifiers
--------+----------------------+--------------------------------------------------------
 a      | integer              | not null default nextval(‘tbl_serial_a_seq‘::regclass)
 b      | character varying(2) | 

从结果中看，声明字段a为serial类型，会自动创建一个名为tbl_serial_a_seq的序列，INSERT时缺省为该序列的下一个序列值nextval。

自动创建的序列如下定义：

test=# \d tbl_serial_a_seq
      Sequence "public.tbl_serial_a_seq"
    Column     |  Type   |        Value
---------------+---------+---------------------
 sequence_name | name    | tbl_serial_a_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_serial.a

其实也可以先创建一个序列，然后将表的某字段默认值设为该序列的下一个序列值。

测试表2：

test=# create sequence sql_tbl_serial2_a increment by 1 minvalue 1 no maxvalue start with 1;
CREATE SEQUENCE
test=# create table tbl_serial2(a int not null default nextval(‘sql_tbl_serial2_a‘),b varchar(2));
CREATE TABLE
test=# \d tbl_serial2
                               Table "public.tbl_serial2"
 Column |         Type         |                        Modifiers
--------+----------------------+---------------------------------------------------------
 a      | integer              | not null default nextval(‘sql_tbl_serial2_a‘::regclass)
 b      | character varying(2) | 

test=# \d sql_tbl_serial2_a
      Sequence "public.sql_tbl_serial2_a"
    Column     |  Type   |        Value
---------------+---------+---------------------
 sequence_name | name    | sql_tbl_serial2_a
 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

此方法和第一种直接使用serial类型效果完全相同，但是这里可以自己定义序列名称。

创建序列的语法如下：

test=# \h create sequence
Command:     CREATE SEQUENCE
Description: define a new sequence generator
Syntax:
CREATE [ TEMPORARY | TEMP ] SEQUENCE [ IF NOT EXISTS ] name [ INCREMENT [ BY ] increment ]
    [ MINVALUE minvalue | NO MINVALUE ] [ MAXVALUE maxvalue | NO MAXVALUE ]
    [ START [ WITH ] start ] [ CACHE cache ] [ [ NO ] CYCLE ]
    [ OWNED BY { table_name.column_name | NONE } ]

其实和上面使用\d一个序列时对应的，

INCREMENT BY ： 每次序列增加（或减少）的步长

MINVALUE ： 序列最小值，NO MINVALUE表示没有最小值

MAXVALUE ： 序列最大值，NO MAXVALUE表示没有最大值

START WITH ：以什么序列值开始

CYCLE ： 序列是否循环使用

OWNED BY ： 可以直接指定一个表的字段，也可以不指定。

序列涉及的函数有：

示例1：获取序列tbl_serial_a_seq当前序列值

test=# select currval(‘tbl_serial_a_seq‘);
ERROR:  currval of sequence "tbl_serial_a_seq" is not yet defined in this session
test=#
test=# select nextval(‘tbl_serial_a_seq‘);
 nextval
---------
       1
(1 row)

test=# select currval(‘tbl_serial_a_seq‘);
 currval
---------
       1
(1 row)

示例2：获取最近一次使用nextval的返回值

test=# select nextval(‘sql_tbl_serial2_a‘);
 nextval
---------
       5
(1 row)

test=# select lastval();
 lastval
---------
       5
(1 row)

test=# select nextval(‘tbl_serial_a_seq‘);
 nextval
---------
       3
(1 row)

test=# select lastval();
 lastval
---------
       3
(1 row)

示例3：将序列tbl_serial_a_seq当前值设为100

test=# select setval(‘tbl_serial_a_seq‘,100);
 setval
--------
    100
(1 row)

test=# select currval(‘tbl_serial_a_seq‘);
 currval
---------
     100
(1 row)

示例4：使用is_called标志设置序列值是马上生效(true)还是下次生效(false)

test=# select setval(‘tbl_serial_a_seq‘,200,true);
 setval
--------
    200
(1 row)

test=# select currval(‘tbl_serial_a_seq‘);
 currval
---------
     200
(1 row)

test=# select setval(‘tbl_serial_a_seq‘,300,false);
 setval
--------
    300
(1 row)

test=# select currval(‘tbl_serial_a_seq‘);
 currval
---------
     200
(1 row)

test=# select nextval(‘tbl_serial_a_seq‘);
 nextval
---------
     300
(1 row)

test=# select currval(‘tbl_serial_a_seq‘);
 currval
---------
     300
(1 row)

修改序列和创建序列的语法基本相同，只是用ALTER替换了CREATE，请参考

test=# \h alter sequence
Command:     ALTER SEQUENCE
Description: change the definition of a sequence generator
Syntax:
ALTER SEQUENCE [ IF EXISTS ] 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_name.column_name | NONE } ]

如果一个序列是NO CYCLE，当序列值全部使用完会怎样呢？

test=# create sequence seq_test increment by 2 MINVALUE 1 MAXVALUE 5 START WITH 2 NO CYCLE;
CREATE SEQUENCE
test=# \d seq_test
     Sequence "public.seq_test"
    Column     |  Type   |  Value
---------------+---------+----------
 sequence_name | name    | seq_test
 last_value    | bigint  | 2
 start_value   | bigint  | 2
 increment_by  | bigint  | 2
 max_value     | bigint  | 5
 min_value     | bigint  | 1
 cache_value   | bigint  | 1
 log_cnt       | bigint  | 0
 is_cycled     | boolean | f
 is_called     | boolean | f

test=# select nextval(‘seq_test‘);
 nextval
---------
       2
(1 row)

test=# select nextval(‘seq_test‘);
 nextval
---------
       4
(1 row)

test=# select nextval(‘seq_test‘);
ERROR:  nextval: reached maximum value of sequence "seq_test" (5)

如果序列值用完了当然是错误了！！！如果是CYCLE则会重新从START处开始再次循环。