浅谈PostgreSQL的timestamp类型
一 PostgreSQL 中的时间类型如下
Name Storage Size Description Low Value High Value Resolution
timestamp [ (p) ] [ without time zone ] 8 bytes both date and time (no time zone) 4713 BC 294276 AD 1 microsecond / 14 digits
timestamp [ (p) ] with time zone 8 bytes both date and time, with time zone 4713 BC 294276 AD 1 microsecond / 14 digits
date 4 bytes date (no time of day) 4713 BC 5874897 AD 1 day
time [ (p) ][ without time zone ] 8 bytes time of day (no date) 00:00:0024:00:00 1 microsecond / 14 digits
time [ (p) ]with time zone 12 bytes times of day only, with time zone 00:00:00+1459 24:00:00-1459 1 microsecond / 14 digits
interval [ fields ] [ (p) ] 12 bytes time interval -178000000 years178000000 years 1 microsecond / 14 digits
备注：这里不准备详细介绍各种类型，请注意上面表格中的[ (p) ] ，这个是什么呢?这个是指时间的精度，
time, timestamp, 和 interval 类型都可以指定精度，精度的取值范围是 0 到 6， 下面通过具体实验来体验下精度。
二 current_timestamp 实验
--2.1 查询 current_timestamp
skytf=> select current_timestamp;
now
-------------------------------
2012-06-07 14:00:02.412827+08
(1 row)
备注： current_timestamp 函数返回时间类型为 timestamp with time zone，故返回结果后面包括时区 +08 ，以及精度 412827，那么如何去掉精度和时区呢?
--2.2 去掉精度
skytf=> select current_timestamp(0);
timestamptz
------------------------
2012-06-07 14:07:17+08
(1 row)
--2.3 去掉时区
skytf=> select current_timestamp(0)::timestamp without time zone;
timestamp
---------------------
2012-06-07 14:07:49
(1 row)
select to_timestamp('2015/9/18 13:57:05','yyyy/mm/dd hh24:mi:ss')::timestamp without time zone;
--2015-09-18 13:57:05
--2.4 也可以用 cast 函数类型转换
skytf=> select cast (current_timestamp(0) as timestamp without time zone);
timestamp
---------------------
2012-06-07 14:14:55
(1 row)
--2.5 了解 [p] 的含义
skytf=> select current_timestamp(2)::timestamp without time zone;
timestamp
------------------------
2012-06-07 14:15:42.64
(1 row)
skytf=> select current_timestamp(6)::timestamp without time zone;
timestamp
----------------------------
2012-06-07 14:15:46.281422
(1 row)
备注：可见 [p] 是指时间类型小数点后面的精度，如果 p 指定 2，则精度为2，如果 p 指定 6则精度为 6;
所以在定义表的时候就应该事先定义 timestamp 时间类型的精度。
三 创建表测试，定义时间类型精度为0
skytf=> create table test_p (id int4 primary key, create_time timestamp(0) without time zone);
NOTICE: CREATE TABLE / PRIMARY KEY will create implicit index "test_p_pkey" for table "test_p"
CREATE TABLE
skytf=> \d test_p
Table "skytf.test_p"
Column | Type | Modifiers
-------------+--------------------------------+-----------
id | integer | not null
create_time | timestamp(0) without time zone |
Indexes:
"test_p_pkey" PRIMARY KEY, btree (id)
skytf=> select current_timestamp;
now
-------------------------------
2012-06-07 14:18:31.683105+08
(1 row)
skytf=> insert into test_p values (1,current_timestamp);
INSERT 0 1
skytf=> select * from test_p;
id | create_time
----+---------------------
1 | 2012-06-07 14:19:02
(1 row)