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)