在MySQL数据库里，在添加字段时可以添加到表的任意位置， 那么在POSTGRESQL里我们是否可以实现呢？
答案是肯定可以的。
PG的语法里并没有类似AFTER, BEFORE的子句。
总结：
第一种方法失败，甚至能导致数据库自动重启。
第二种方法可行，但如果数据量较大，业务停止的时间就会加长。
哪果数据量较小，可以考虑第二种方法。但如果数据量大，停机就没有办法接受了， 可以让业务修改代码来完成需求了。
尝试方法一：
修改pg_attribute中的表字段位置：

postgres=# create table t_col(id int4, name varchar(9));
CREATE TABLE                                          ^
postgres=# insert into t_col values(1,'a');
INSERT 0 1
postgres=# insert into t_col values(2,'b');
INSERT 0 1
postgres=# select attrelid,attname,attnum from pg_attribute where attrelid = (select relfilenode from pg_class where relname = 't_col');
 attrelid | attname  | attnum 
----------+----------+--------
   353681 | tableoid |     -7
   353681 | cmax     |     -6
   353681 | xmax     |     -5
   353681 | cmin     |     -4
   353681 | xmin     |     -3
   353681 | ctid     |     -1
   353681 | id       |      1
   353681 | name     |      2
(8 rows)

postgres=# select ctid, * from t_col;
 ctid  | id | name 
-------+----+------
 (0,1) |  1 | a
 (0,2) |  2 | b
(2 rows)

postgres=# update pg_attribute set attnum=3 where attrelid=353681 and attname='id';
UPDATE 1
postgres=# update pg_attribute set attnum=1 where attrelid=353681 and attname='name';
UPDATE 1
postgres=# update pg_attribute set attnum=2 where attrelid=353681 and attname='id';
UPDATE 1
postgres=# select attrelid,attname,attnum from pg_attribute where attrelid = (select relfilenode from pg_class where relname = 't_col');
 attrelid | attname  | attnum 
----------+----------+--------
   353681 | tableoid |     -7
   353681 | cmax     |     -6
   353681 | xmax     |     -5
   353681 | cmin     |     -4
   353681 | xmin     |     -3
   353681 | ctid     |     -1
   353681 | name     |      1
   353681 | id       |      2
(8 rows)
postgres=# \d t_col
           Table "public.t_col"
 Column |         Type         | Modifiers 
--------+----------------------+-----------
 name   | character varying(9) | 
 id     | integer 

从上面的结果来看，字段顺序已经被调整了过来，尝试读取数据：

postgres=# select * from t_col;
server closed the connection unexpectedly
	This probably means the server terminated abnormally
	before or while processing the request.
The connection to the server was lost. Attempting reset: Failed.
!> 

异常了，数据库被强制重启。

2017-12-05 15:35:11.451 CST [24682-13] [postgres@127.0.0.1]@postgres LOG:  statement: select * from t_col;
2017-12-05 15:35:11.569 CST [5935-18] LOG:  server process (PID 24682) was terminated by signal 11: Segmentation fault
2017-12-05 15:35:11.569 CST [5935-19] DETAIL:  Failed process was running: select * from t_col;
2017-12-05 15:35:11.569 CST [5935-20] LOG:  terminating any other active server processes
2017-12-05 15:35:11.570 CST [23383-2] WARNING:  terminating connection because of crash of another server process
2017-12-05 15:35:11.570 CST [23383-3] DETAIL:  The postmaster has commanded this server process to roll back the current transaction and exit, because another server process exited abnormally and possibly corrupted shared memory.
2017-12-05 15:35:11.570 CST [23383-4] HINT:  In a moment you should be able to reconnect to the database and repeat your command.
2017-12-05 15:35:11.570 CST [25249-1] [postgres@127.0.0.1]@postgres FATAL:  the database system is in recovery mode
2017-12-05 15:35:11.574 CST [5935-21] LOG:  archiver process (PID 23384) exited with exit code 1
2017-12-05 15:35:11.574 CST [23422-1] [repluser@192.168.252.3]@[unknown] WARNING:  terminating connection because of crash of another server process
2017-12-05 15:35:11.574 CST [23422-2] [repluser@192.168.252.3]@[unknown] DETAIL:  The postmaster has commanded this server process to roll back the current transaction and exit, because another server process exited abnormally and possibly corrupted shared memory.
2017-12-05 15:35:11.574 CST [23422-3] [repluser@192.168.252.3]@[unknown] HINT:  In a moment you should be able to reconnect to the database and repeat your command.
2017-12-05 15:35:11.575 CST [5935-22] LOG:  all server processes terminated; reinitializing
2017-12-05 15:35:11.657 CST [25250-1] LOG:  database system was interrupted; last known up at 2017-12-05 15:33:17 CST
2017-12-05 15:35:12.373 CST [25250-2] LOG:  database system was not properly shut down; automatic recovery in progress
2017-12-05 15:35:12.376 CST [25250-3] LOG:  redo starts at 89/FF0DBF80
2017-12-05 15:35:12.376 CST [25250-4] LOG:  record with zero length at 89/FF0DC070
2017-12-05 15:35:12.376 CST [25250-5] LOG:  redo done at 89/FF0DC038
2017-12-05 15:35:12.395 CST [25250-6] LOG:  MultiXact member wraparound protections are now enabled
2017-12-05 15:35:12.397 CST [5935-23] LOG:  database system is ready to accept connections
2017-12-05 15:35:12.397 CST [25254-1] LOG:  autovacuum launcher started

其实t_col这张表已经损坏了，当对这张表操作时，都会出现重启的现象。
尝试把表字段调回去：

postgres=# update pg_attribute set attnum=3 where attrelid=353681 and attname='name';
UPDATE 1
postgres=# update pg_attribute set attnum=1 where attrelid=353681 and attname='id';
UPDATE 1
postgres=# update pg_attribute set attnum=2 where attrelid=353681 and attname='name';
UPDATE 1
postgres=# \d t_col;
           Table "public.t_col"
 Column |         Type         | Modifiers 
--------+----------------------+-----------
 id     | integer              | 
 name   | character varying(9) | 

操作测试：

postgres=# insert into t_col values(3,'c');
INSERT 0 1
postgres=# select * from t_col;
 id | name 
----+------
  1 | a
  2 | b
  3 | c
(3 rows)

尝试方法二：
1. 禁止表写入数据。

postgres=# begin work;
BEGIN
postgres=# lock table t_col in share mode;
LOCK TABLE

2. 在另一个会话中创建要求字段顺序的表，并将原表数据插入到新表中：

postgres=# create table t_col_new(name varchar(9), id int4);
CREATE TABLE
postgres=# insert into t_col_new select name, id from t_col;
INSERT 0 3
postgres=# select * from t_col_new;
 name | id 
------+----
 a    |  1
 b    |  2
 c    |  3
(3 rows)

3. 对表进行RENAME操作：

postgres=# alter table t_col rename to t_col_old;
ALTER TABLE
postgres=# alter table t_col_new rename to t_col;
ALTER TABLE
postgres=# commit work;
COMMIT

4. 数据效验：

postgres=# select * from t_col;
 name | id 
------+----
 a    |  1
 b    |  2
 c    |  3
(3 rows)