postgresql创建序列查询序列

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 TABLEtest=# \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 | fOwned 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 SEQUENCEtest=# create table tbl_serial2(a int not null default nextval('sql_tbl_serial2_a'),b varchar(2));CREATE TABLEtest=# \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 SEQUENCEDescription: define a new sequence generatorSyntax: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 sessiontest=# test=# select nextval('tbl_serial_a_seq'); nextval ---------(1 row)test=# select currval('tbl_serial_a_seq'); currval ---------(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 ---------(1 row)test=# select lastval(); lastval ---------(1 row)test=# select nextval('tbl_serial_a_seq'); nextval ---------(1 row)test=# select lastval(); lastval ---------(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 sequenceCommand:     ALTER SEQUENCEDescription: change the definition of a sequence generatorSyntax: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 SEQUENCEtest=# \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 | ftest=# 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。