最近学习PGSQL。来比较一下他和MYSQL自增字段的不同点。
1、自增序列。MYSQL从最后一个ID自增。
测试数据。

1, I love this girl.
2, I hate this girl.
3, She is my girl.
4, She is your girl.
MYSQL：
mysql> create database test;
Query OK, 1 row affected (0.10 sec)

mysql> use test
Database changed
mysql> create table t(id int not null auto_increment primary key,
    -> username char(20) not null);
Query OK, 0 rows affected (0.02 sec)
mysql> load data infile '/tmp/test.sql' into table t fields terminated by ',';
Query OK, 4 rows affected (0.00 sec)
Records: 4 Deleted: 0 Skipped: 0 Warnings: 0

mysql> select * from t;
+----+-------------------+

| id | username |
+----+-------------------+

| 1 | I love this girl. |
| 2 | I hate this girl. |
| 3 | She is my girl. |
| 4 | She is your girl. |
+----+-------------------+

4 rows in set (0.00 sec)

mysql> insert into t values (6,'This is inserted');
Query OK, 1 row affected (0.00 sec)

mysql> insert into t(username) values('This is last');
Query OK, 1 row affected (0.00 sec)

mysql> select * from t;
+----+-------------------+

| id | username |
+----+-------------------+

| 1 | I love this girl. |
| 2 | I hate this girl. |
| 3 | She is my girl. |
| 4 | She is your girl. |
| 6 | This is inserted |
| 7 | This is last |
+----+-------------------+


mysql> truncate table t;
Query OK, 0 rows affected (0.00 sec)

mysql> insert into t(username) values('This is last');
Query OK, 1 row affected (0.00 sec)

mysql> insert into t(username) values('This is last');
Query OK, 1 row affected (0.00 sec)

mysql> insert into t(username) values('This is last');
Query OK, 1 row affected (0.00 sec)

mysql> select * from t;
+----+--------------+

| id | username |
+----+--------------+

| 1 | This is last |
| 2 | This is last |
| 3 | This is last |
+----+--------------+

3 rows in set (0.00 sec)

PGSQL从1开始逐个尝试。

[root@localhost ~]# psql -Upostgres -hlocalhost
。。。
postgres=# create database test;
CREATE DATABASE
postgres=# \c test
You are now connected to database "test".
test=# create table t(id serial not null,username char(20) not null);
NOTICE: CREATE TABLE will create implicit sequence "t_id_seq" for serial column "t.id"
CREATE TABLE
test=# \d t;
                             Table "public.t"
  Column | Type | Modifiers
----------+---------------+------------------------------------------------

 id | integer | not null default nextval('t_id_seq'::regclass)
 username | character(20) | not null
test=# copy t from '/tmp/test.sql' with csv;
COPY 4
test=# select * from t;
 id | username
----+----------------------

  1 | I love this girl.
  2 | I hate this girl.
  3 | She is my girl.
  4 | She is your girl.
(4 rows)

test=# insert into t values (6,'This is inserted');
INSERT 0 1
test=# insert into t(username) values('This is last');
ID1重复
ERROR: duplicate key violates unique constraint "t_pkey"
test=# insert into t(username) values('This is last');
ID2重复
ERROR: duplicate key violates unique constraint "t_pkey"
test=# insert into t(username) values('This is last');
。。。
ID5没有。插入
INSERT 0 1
test=# insert into t(username) values('This is last');
ID6又重复
ERROR: duplicate key violates unique constraint "t_pkey"
test=# insert into t(username) values('This is last');
...
INSERT 0 1
test=# insert into t(username) values('This is last');
INSERT 0 1
test=# insert into t(username) values('This is last');
INSERT 0 1
test=# select * from t;
 id | username
----+----------------------

  1 | I love this girl.
  2 | I hate this girl.
  3 | She is my girl.
  4 | She is your girl.
  6 | This is inserted
  5 | This is last
  7 | This is last
  8 | This is last
  9 | This is last
(9 rows)
看一下DELETE操作。
test=# delete from t;
DELETE 9
test=# insert into t(username) values('This is last');
INSERT 0 1
test=# insert into t(username) values('This is last');
INSERT 0 1
test=# insert into t(username) values('This is last');
INSERT 0 1
test=# select * from t;
 id | username
----+----------------------

 10 | This is last
 11 | This is last
 12 | This is last
(3 rows)
这个和MYSQL一样的。
TRUNCATE虽然和MYSQL一样可以快速清空表数据。可是ID还是从最后一个开始增加的，如果想从１开始的话，就得用setval函数来设置。
test=# truncate table t;
TRUNCATE TABLE
test=# insert into t(username) values('This is last');
INSERT 0 1
test=# insert into t(username) values('This is last');
INSERT 0 1
test=# insert into t(username) values('This is last');
INSERT 0 1
test=# select * from t;
 id | username
----+----------------------

 13 | This is last
 14 | This is last
 15 | This is last
(3 rows)
至于怎么从1重新开始。还在学习中。。。


2、得到刚刚插入的自增ID。

 在MYSQL里面：
mysql> truncate table t;
Query OK, 0 rows affected (0.00 sec)

mysql> insert into t(username) values('This is last');
Query OK, 1 row affected (0.00 sec)

mysql> select last_insert_id();
+------------------+

| last_insert_id() |
+------------------+

| 1 |
+------------------+

1 row in set (0.00 sec)
在POSTGRESQL里面：

test=# drop table t
test-# ;
DROP TABLE
test=# create table t(id serial not null primary key,username char(20) not null);
NOTICE: CREATE TABLE will create implicit sequence "t_id_seq" for serial column "t.id"
NOTICE: CREATE TABLE / PRIMARY KEY will create implicit index "t_pkey" for table "t"
CREATE TABLE
test=# \d t
                             Table "public.t"
  Column | Type | Modifiers
----------+---------------+------------------------------------------------

 id | integer | not null default nextval('t_id_seq'::regclass)
 username | character(20) | not null
Indexes:
    "t_pkey" PRIMARY KEY, btree (id)

test=# insert into t(username) values('This is test name');
INSERT 0 1
test=# select * from t;
 id | username
----+----------------------

  1 | This is test name
(1 row)

test=# select currval('t_id_seq');
 currval
---------

       1
(1 row)

test=#
3、设置自增ID的开始值。
MYSQL:

mysql> alter table t auto_increment = 3;
Query OK, 1 row affected (0.01 sec)
Records: 1 Duplicates: 0 Warnings: 0

mysql> insert into t(username) values('This is last');
Query OK, 1 row affected (0.00 sec)

mysql> select * from t;
+----+--------------+

| id | username |
+----+--------------+

| 1 | This is last |
| 3 | This is last |
+----+--------------+

2 rows in set (0.00 sec)
POSTGRESQL:

 t_girl=# select setval('t_id_seq',1,false);
 setval
--------

      1
(1 row)

Time: 19.554 ms
t_girl=# insert into t(username) values('wangwei'),('×××');
INSERT 0 2
Time: 1.882 ms
t_girl=# select * from t;
 id | username
----+----------------------

  1 | wangwei
  2 | ×××
(2 rows)

Time: 0.598 ms