PostgreSQL: epoch 新紀元時間的使用

  新紀元時間 Epoch 是以 1970-01-01 00:00:00 UTC 為標准的時間，將目標時間與 1970-01-01 00:00:00
時間的差值以秒來計算 ，單位是秒，可以是負值; 有些應用會將時間存儲成epoch 時間形式，以提高讀取效率，
下面演示下 pg 中 epoch 時間的使用換算方法。

--1 將 time stamp 時間轉換成 epoch 時間
francs=> select extract(epoch from timestamp without time zone '1970-01-01 01:00:00');
 date_part 
-----------
      3600
(1 row)

francs=> select extract(epoch from timestamp without time zone '1970-01-01 02:00:00');
 date_part 
-----------
      7200
(1 row)

francs=> select extract(epoch from interval '+1 hours');
 date_part 
-----------
      3600
(1 row)

francs=> select extract(epoch from interval '-1 hours');
 date_part 
-----------
     -3600
(1 row)

 

--2 將epoch 時間轉換成  time stamp  時間
francs=> select timestamp without time zone 'epoch' + 3600 * interval '1 second';
      ?column?       
---------------------
 1970-01-01 01:00:00
(1 row)

francs=> select timestamp without time zone 'epoch' + 7200 * interval '1 second';
      ?column?       
---------------------
 1970-01-01 02:00:00
(1 row)

--3 手冊上關於 epoch 的解釋
      For date and timestamp values, the number of seconds since 1970-01-01 00:00:00 UTC (can be negative); 
for interval values, the total number of seconds in the interval 

 

epoch

For date and timestamp values, the number of seconds since 1970-01-01 00:00:00 UTC (can be negative); for interval values, the total number of seconds in the interval

SELECT EXTRACT(EPOCH FROM TIMESTAMP WITH TIME ZONE '2001-02-16 20:38:40.12-08');
Result: 982384720.12

SELECT EXTRACT(EPOCH FROM INTERVAL '5 days 3 hours');
Result: 442800

Here is how you can convert an epoch value back to a time stamp:

SELECT TIMESTAMP WITH TIME ZONE 'epoch' + 982384720.12 * INTERVAL '1 second';

http://www.postgresql.org/docs/9.1/static/functions-datetime.html