PostgreSQL学习篇9.4 序列类型

序列类型：serial、bigserial。PG与Oracle一样有序列，但是MySQL没有。

示例：
postgres=# create table testseq(id serial,name varchar(100));
CREATE TABLE
postgres=# insert into testseq(name) values('dxmy');
INSERT 0 1
postgres=# select * from testseq;
id | name
----+------
1 | dxmy
(1 row)

postgres=# \d testseq;
Table "public.testseq"
Column |          Type          |                      Modifiers
--------+------------------------+------------------------------------------------------
id     | integer                | not null default nextval('testseq_id_seq'::regclass)
name   | character varying(100) |

postgres=#

--看下PG中的sequence
postgres=# create sequence seq_1;
CREATE SEQUENCE
postgres=# \d seq_1;
Sequence "public.seq_1"
Column     |  Type   |        Value
---------------+---------+---------------------
sequence_name | name    | seq_1
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
^
postgres=# select currval('seq_1');
ERROR:  currval of sequence "seq_1" is not yet defined in this session
postgres=# select nextval('seq_1');
nextval
---------
1
(1 row)

postgres=# select nextval('seq_1');
nextval
---------
2
(1 row)

postgres=# select currval('seq_1');
currval
---------
2
(1 row)
postgres=# select * from seq_1;    --这个就比较神奇了
sequence_name | last_value | start_value | increment_by |      max_value      | min_value | cache_value | log_cnt | is_cycled | is_called
---------------+------------+-------------+--------------+---------------------+-----------+-------------+---------+-----------+-----------
seq_1         |          2 |           1 |            1 | 9223372036854775807 |         1 |           1 |      31 | f         | t
(1 row)

postgres=# select setval('seq_1',300);    --可以直接设置sequence的值
setval
--------
300
(1 row)

postgres=# select currval('seq_1');
currval
---------
300
(1 row)

postgres=#