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  • PostgreSQL: epoch 新纪元时间的使用

      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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  • 原文地址:https://www.cnblogs.com/kungfupanda/p/4383882.html
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