问题很简单：如果我已经在一个列类型时间戳没有时区的数据，如果我设置类型为时间戳与时区，postgresql做这个数据是什么？
它保持当前值在本地时间，并将时区设置为本地时间的偏移量：
create table a(t timestamp without time zone,t2 timestamp with time zone);
insert into a(t) values ('2012-03-01'::timestamp);
update a set t2 = t;
select * from a;
t | t2
---------------------+------------------------
2012-03-01 00:00:00 | 2012-03-01 00:00:00-08
alter table a alter column t type timestamp with time zone;
select * from a;
t | t2
------------------------+------------------------
2012-03-01 00:00:00-08 | 2012-03-01 00:00:00-08
根据Alter Table手册：
if @H_403_11@[the USING clause is] omitted,the default conversion is the same as an assignment cast from old data type to new.
根据Date/Time types手册
Conversions between @H_403_11@timestamp without time zone and @H_403_11@timestamp with time zone normally assume that the @H_403_11@timestamp without time zone value should be taken or given as @H_403_11@timezone local time. A different time zone can be specified for the conversion using
AT TIME ZONE.
