关于时区：PostgreSQL改变没有时区的类型时间戳 – >带时区

PostgreSQL alter type timestamp without time zone -> with time zone

问题很简单：如果我已经在没有时区的列类型时间戳中有数据，如果我将类型设置为带时区的时间戳，postgresql对这些数据做了什么？

它将当前值保持在本地时间，并将时区设置为本地时间的偏移量：

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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 [the USING clause is] omitted, the default conversion is the same as an assignment cast from old data type to new.

根据日期/时间类型的手册

Conversions between timestamp without time zone and timestamp with time zone normally assume that the timestamp without time zone value should be taken or given as timezone local time. A different time zone can be specified for the conversion using AT TIME ZONE.