新纪元时间 Epoch 是以 1970-01-01 00:00:00 UTC 为标准的时间，将目标时间与 1970-01-01 00:00:00
时间的差值以秒来计算 ，单位是秒，可以是负值; 有些应用会将时间存储成epoch 时间形式，以提高读取效率，
下面演示下 pg 中 epoch 时间的使用换算方法。

--1 将 time stamp 时间转换成 epoch 时间
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 将epoch 时间转换成  time stamp  时间
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 手册上关于 epoch 的解释
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
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