关于日期：Oracle中SYS_EXTRACT_UTC()的反转是什么？


What is the inverse of SYS_EXTRACT_UTC() in Oracle?

这个问题

设original_date为date。 如何从SYS_EXTRACT_UTC(cast(original_date as timestamp ))的结果中专门通过内部Oracle函数返回其值，即无论数据库的任何时区设置如何。

背景

一些愚蠢的程序将日期值写入UTC中的一个表中，即

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cast(SYS_EXTRACT_UTC(SYSTIMESTAMP) as date)

在此表的日期类型列中存储而不是SYSDATE。

在所有其他表中，简单地将SYSDATE存储在此类列中。 我的任务是将这些值一起使用，所以我想要回到SYS_EXTRACT_UTC()函数的效果。 只有当我手动指定我的时区时，我才能解决这个问题

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cast( FROM_TZ(cast(my_utc_date as TIMESTAMP), 'UTC')  AT TIME ZONE 'Europe/Budapest' as date)

但是，如果我使用DBTIMEZONE而不是'Europe/Budapest'，那么我得到错误的结果，可能是因为DBTIMEZONE忽略了夏令时的夏令时

例如，当to_char(SYSDATE，'YYYY-MM-DD HH24：MI：SS')='2016-05-19 13:45:12'时，程序存储

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 cast(SYS_EXTRACT_UTC(cast(SYSDATE as timestamp)) as date)

我的测试查询是：

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SELECT  
        original_date,
        stored_utc_date,
        cast( FROM_TZ(cast(stored_utc_date as TIMESTAMP), 'UTC')  AT TIME ZONE 'Europe/Budapest' as date) as reverted_good,
        cast( FROM_TZ(cast(stored_utc_date as TIMESTAMP), 'UTC')  AT TIME ZONE DBTIMEZONE as date) as reverted_wrong
from (
        select original_date, cast( SYS_EXTRACT_UTC(cast(original_date as timestamp )) as date) stored_utc_date
        from (select to_date('2016-05-19 13:45:12','YYYY-MM-DD HH24:MI:SS') original_date from dual)
    )

其结果是：

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ORIGINAL_DATE       STORED_UTC_DATE     REVERTED_GOOD       REVERTED_WRONG    
------------------- ------------------- ------------------- -------------------
2016-05-19 13:45:12 2016-05-19 11:45:12 2016-05-19 13:45:12 2016-05-19 12:45:12
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您可以获取systimestamp时区区域并使用它：

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FROM_TZ(cast(stored_utc_date as TIMESTAMP), 'UTC') AT TIME ZONE to_char(systimestamp, 'TZR')

使用您的测试数据(但将布达佩斯改为伦敦，因为这是我的本地区域)：

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SELECT  
        original_date,
        stored_utc_date,
        cast(FROM_TZ(cast(stored_utc_date as TIMESTAMP), 'UTC') AT TIME ZONE 'Europe/London' as date) as reverted_good,
        cast(FROM_TZ(cast(stored_utc_date as TIMESTAMP), 'UTC') AT TIME ZONE DBTIMEZONE as date) as reverted_wrong,
        cast(FROM_TZ(cast(stored_utc_date as TIMESTAMP), 'UTC') AT TIME ZONE TO_CHAR(systimestamp, 'TZR') as date) as reverted_right
from (
        select original_date, cast( SYS_EXTRACT_UTC(cast(original_date as timestamp )) as date) stored_utc_date
        from (select to_date('2016-05-19 13:45:12','YYYY-MM-DD HH24:MI:SS') original_date from dual)
    )
/

ORIGINAL_DATE       STORED_UTC_DATE     REVERTED_GOOD       REVERTED_WRONG      REVERTED_RIGHT    
------------------- ------------------- ------------------- ------------------- -------------------
2016-05-19 13:45:12 2016-05-19 12:45:12 2016-05-19 13:45:12 2016-05-19 12:45:12 2016-05-19 13:45:12

除了...不能一直工作，因为TZR被报告为偏移量(因为它基于操作系统TZ)，并且您无法从偏移量中猜出区域。如果原始日期是在冬季，而您在夏季运行，反之亦然，那么恢复日期将是一小时。因此，有效地恢复日期的一半总是错误的 - 但是哪一半将取决于您何时运行查询。

看起来你可以通过使用DBTIMEZONE作为本地时区来解决这个问题：

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cast(FROM_TZ(cast(stored_utc_date as TIMESTAMP), 'UTC') AT TIME ZONE DBTIMEZONE
  as timestamp with local time zone

您的测试查询再次：

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SELECT  
        original_date,
        stored_utc_date,
        cast(FROM_TZ(cast(stored_utc_date as TIMESTAMP), 'UTC') AT TIME ZONE 'Europe/London' as date) as reverted_good,
        cast(FROM_TZ(cast(stored_utc_date as TIMESTAMP), 'UTC') AT TIME ZONE DBTIMEZONE as date) as reverted_wrong,
        cast(cast(FROM_TZ(cast(stored_utc_date as TIMESTAMP), 'UTC') AT TIME ZONE DBTIMEZONE as timestamp with local time zone) as date) as reverted_right
from (
        select original_date, cast( SYS_EXTRACT_UTC(cast(original_date as timestamp )) as date) stored_utc_date
        from (select to_date('2016-05-19 13:45:12','YYYY-MM-DD HH24:MI:SS') original_date from dual)
    )
/

ORIGINAL_DATE       STORED_UTC_DATE     REVERTED_GOOD       REVERTED_WRONG      REVERTED_RIGHT    
------------------- ------------------- ------------------- ------------------- -------------------
2016-05-19 13:45:12 2016-05-19 12:45:12 2016-05-19 13:45:12 2016-05-19 12:45:12 2016-05-19 13:45:12

更广泛的测试查询，包含全年的日期：

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with t as (
  select from_tz(cast(add_months(trunc(sysdate, 'MM'), 1-level) as timestamp), 'Europe/London')
    as original_systimestamp
  from dual
  connect by level <= 12
)
select original_systimestamp,
  cast(cast(from_tz(sys_extract_utc(original_systimestamp), 'UTC')
    at time zone dbtimezone as timestamp with local time zone) as date) as good_date,
  sys_extract_utc(original_systimestamp) as utc_timestamp,
  from_tz(sys_extract_utc(original_systimestamp), 'UTC')
    at time zone to_char(systimestamp, 'TZR') as at_systimezone,
  from_tz(sys_extract_utc(original_systimestamp), 'UTC')
    at time zone dbtimezone as at_dbtimezone,
  cast(from_tz(sys_extract_utc(original_systimestamp), 'UTC')
    at time zone dbtimezone as timestamp with local time zone) as at_local_dbtimezone
from t
order by original_systimestamp;

ORIGINAL_SYSTIMESTAMP               GOOD_DATE           UTC_TIMESTAMP         AT_SYSTIMEZONE               AT_DBTIMEZONE                AT_LOCAL_DBTIMEZONE        
----------------------------------- ------------------- --------------------- ---------------------------- ---------------------------- ----------------------------
2015-06-01 00:00:00.0 Europe/London 2015-06-01 00:00:00 2015-05-31 23:00:00.0 2015-06-01 00:00:00.0 +01:00 2015-05-31 23:00:00.0 +00:00 2015-06-01 00:00:00.0      
2015-07-01 00:00:00.0 Europe/London 2015-07-01 00:00:00 2015-06-30 23:00:00.0 2015-07-01 00:00:00.0 +01:00 2015-06-30 23:00:00.0 +00:00 2015-07-01 00:00:00.0      
2015-08-01 00:00:00.0 Europe/London 2015-08-01 00:00:00 2015-07-31 23:00:00.0 2015-08-01 00:00:00.0 +01:00 2015-07-31 23:00:00.0 +00:00 2015-08-01 00:00:00.0      
2015-09-01 00:00:00.0 Europe/London 2015-09-01 00:00:00 2015-08-31 23:00:00.0 2015-09-01 00:00:00.0 +01:00 2015-08-31 23:00:00.0 +00:00 2015-09-01 00:00:00.0      
2015-10-01 00:00:00.0 Europe/London 2015-10-01 00:00:00 2015-09-30 23:00:00.0 2015-10-01 00:00:00.0 +01:00 2015-09-30 23:00:00.0 +00:00 2015-10-01 00:00:00.0      
2015-11-01 00:00:00.0 Europe/London 2015-11-01 00:00:00 2015-11-01 00:00:00.0 2015-11-01 01:00:00.0 +01:00 2015-11-01 00:00:00.0 +00:00 2015-11-01 00:00:00.0      
2015-12-01 00:00:00.0 Europe/London 2015-12-01 00:00:00 2015-12-01 00:00:00.0 2015-12-01 01:00:00.0 +01:00 2015-12-01 00:00:00.0 +00:00 2015-12-01 00:00:00.0      
2016-01-01 00:00:00.0 Europe/London 2016-01-01 00:00:00 2016-01-01 00:00:00.0 2016-01-01 01:00:00.0 +01:00 2016-01-01 00:00:00.0 +00:00 2016-01-01 00:00:00.0      
2016-02-01 00:00:00.0 Europe/London 2016-02-01 00:00:00 2016-02-01 00:00:00.0 2016-02-01 01:00:00.0 +01:00 2016-02-01 00:00:00.0 +00:00 2016-02-01 00:00:00.0      
2016-03-01 00:00:00.0 Europe/London 2016-03-01 00:00:00 2016-03-01 00:00:00.0 2016-03-01 01:00:00.0 +01:00 2016-03-01 00:00:00.0 +00:00 2016-03-01 00:00:00.0      
2016-04-01 00:00:00.0 Europe/London 2016-04-01 00:00:00 2016-03-31 23:00:00.0 2016-04-01 00:00:00.0 +01:00 2016-03-31 23:00:00.0 +00:00 2016-04-01 00:00:00.0      
2016-05-01 00:00:00.0 Europe/London 2016-05-01 00:00:00 2016-04-30 23:00:00.0 2016-05-01 00:00:00.0 +01:00 2016-04-30 23:00:00.0 +00:00 2016-05-01 00:00:00.0

但即使只有会话时区与数据库服务器的区域匹配才有效;如果我将会话时区设置为欧洲/伦敦以外的其他地方，那么它就会消失。而且你依赖于能够设置会话时区，问题中的第一个查询与硬编码的区域并不是真的更糟糕......

值得注意的是，DBTIMEZONE并不一定告诉你任何有用的东西; Oracle建议将其设置为UTC。因此，如果您不能使用它，并且无法真正使用从systimestamp中提取的TZR(因为它实际上是一个偏移量，并且您无法将其转换为某个区域，因此无法知道DST信息)，我不会我们认为有一种方法可以从数据库中保存的任何东西中找出原始系统时间。您似乎需要在某个时刻提供服务器时区区域 - 通过设置会话时区或在原始查询中。

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