当值位于夏令时边界时，按 TIMESTAMP 字段分组时的 Oracle 奇怪行为

Question

我对带有 TIMESTAMP 和夏令时的 Oracle DB 有一个奇怪的行为。

以下查询在按不同列分组时会产生不同的结果，原因尚不清楚。

当按 my_date_ny_ts 分组时，它返回两个不同的行，而当按 my_date_ny_ts_tz 分组时 - 结果只有一行（对我来说是正确的一行）。

请注意，这些值适用于 11/04/2018 00:00:00 -05:00 和 11/04/2018 00:00:00 -06:00，它们转换为 New_York 时区为 11-04 -2018 01:00 和 02:00 EDT，实际上是 01:00 EDT 和 01:00 EST。

我理解为什么这些值不同，但在将它们转换为 TIMESTAMP without time zone 数据类型（my_date_ny_ts 列）后它们应该相等，因为这种类型不包含任何关于时区和夏令时状态（参见 tdz_ny_ts 值）。只有在我将值转换回 TIMESTAMP WITH TIMEZONE 类型（my_date_ny_ts_tz）之后，它们才变得相等。 我不需要解决方法（已经有了），只是想知道这种行为是 Oracle 错误还是误解：

select count(*), my_date_ny_ts_tz from (

SELECT
 mydate, -- timestamp with timezone
 to_char( mydate, 'TZD') as tdz, -- daylight savings flag - VALUES are NULL because timezone is an offset
 mydate AT TIME ZONE 'America/New_York' AS mydate_ny, -- timestamp with timezone in EST timezone
 to_char( mydate AT TIME ZONE 'America/New_York', 'TZD') as tdz_ny, --timestamp with timezone in EST timezone -  daylight savings flag - RETURNS EDT FOR ONE EST FOR SECOND - RIGHT
cast(mydate AT TIME ZONE 'America/New_York' as timestamp)  as my_date_ny_ts,--cast to timestamp without timezone - GROUP BY RETURNS TWO ROWS - BUG?
to_char( cast(mydate AT TIME ZONE 'America/New_York' as timestamp) , 'TZD') as tdz_ny_ts,--cast to timestamp without timezone -  daylight savings flag - both values are null so why the group by on the prev field doesn't work?
 cast(mydate AT TIME ZONE 'America/New_York' as timestamp) AT TIME ZONE 'America/New_York' as my_date_ny_ts_tz,--cast back to timestamp with timezone in EST timezone - NOW GROUP BY RETURNS ONE ROW
 to_char( cast(mydate AT TIME ZONE 'America/New_York' as timestamp) AT TIME ZONE 'America/New_York', 'TZD') as tdz_ny_ts_tz--daylight savings flag of the prev field - both are EST - RIGHT
 FROM
     (
         SELECT
         to_timestamp_tz('11/04/2018 00:00:00 -05:00','mm/dd/yyyy hh24:mi:ss TZH:TZM') AS mydate
     FROM
         dual
     UNION
     SELECT
         to_timestamp_tz('11/04/2018 00:00:00 -06:00','mm/dd/yyyy hh24:mi:ss TZH:TZM') AS mydate
     FROM
         dual
 )
 ) group by my_date_ny_ts_tz

我的版本如下，但它也发生在 12c 中：

Oracle Database 11g Enterprise Edition Release 11.2.0.4.0 - 64bit Production
PL/SQL Release 11.2.0.4.0 - Production
"CORE   11.2.0.4.0  Production"
TNS for 64-bit Windows: Version 11.2.0.4.0 - Production
NLSRTL Version 11.2.0.4.0 - Production

谢谢

Answer 1

可能是 Oracle 中的错误。您可以将查询缩短为：

SELECT 
    TO_CHAR(CAST(TIMESTAMP '2018-11-04 00:00:00 -05:00' AT TIME ZONE 'America/New_York' AS TIMESTAMP), 'yyyy-mm-dd hh24:mi:ss') AS my_date_ny_ts,
    DUMP(CAST(TIMESTAMP '2018-11-04 00:00:00 -05:00' AT TIME ZONE 'America/New_York' AS TIMESTAMP)) AS my_date_ny_ts_dump
FROM DUAL 
UNION ALL
SELECT
    TO_CHAR(CAST(TIMESTAMP '2018-11-04 00:00:00 -06:00' AT TIME ZONE 'America/New_York' AS TIMESTAMP), 'yyyy-mm-dd hh24:mi:ss'),
    DUMP(CAST(TIMESTAMP '2018-11-04 00:00:00 -06:00' AT TIME ZONE 'America/New_York' AS TIMESTAMP))
FROM DUAL;


+--------------------------------------------------------------------------------------+
|MY_DATE_NY_TS      |MY_DATE_NY_TS_DUMP                                                |
+--------------------------------------------------------------------------------------+
|2018-11-04 01:00:00|Typ=187 Len=20: 226,7,11,4,1,0,0,0,0,0,0,0,252,0,3,0,100,0,11,3   |
|2018-11-04 01:00:00|Typ=187 Len=20: 226,7,11,4,1,0,0,48,0,0,0,0,251,0,3,44,100,0,48,44|
+--------------------------------------------------------------------------------------+

如您所见，时间戳值相同，但 DUMP() 值不同，即，如果您创建 GROUP BY，则会得到两行。

您可以稍微不同地运行它。实际上，我希望得到与上述相同的结果（无论您认为是否正确），但它是不同的：

WITH t AS
    (SELECT 
        CAST(TIMESTAMP '2018-11-04 00:00:00 -05:00' AT TIME ZONE 'America/New_York' AS TIMESTAMP) AS my_date_ny_ts
    FROM DUAL 
    UNION ALL
    SELECT
        CAST(TIMESTAMP '2018-11-04 00:00:00 -06:00' AT TIME ZONE 'America/New_York' AS TIMESTAMP)
    FROM DUAL)
SELECT TO_CHAR(my_date_ny_ts, 'yyyy-mm-dd hh24:mi:ss') AS my_date_ny_ts,
    DUMP(my_date_ny_ts) AS my_date_ny_ts_dump
FROM t;

    +--------------------------------------------------------------------------------------+
|MY_DATE_NY_TS      |MY_DATE_NY_TS_DUMP                                                |
+--------------------------------------------------------------------------------------+
|2018-11-04 01:00:00|Typ=180 Len=7: 120,118,11,4,2,1,1                                 |
|2018-11-04 01:00:00|Typ=180 Len=7: 120,118,11,4,2,1,1                                 |
+--------------------------------------------------------------------------------------+

这对我来说看起来很奇怪。尽管我为两者都制作了CAST(... AS TIMESTAMP)，但一旦我在DUMP 中获得Typ=187 和Typ=180。

看起来 SQL 类型 TIMESTAMP180（请参阅 Oracle Built-In Data Types）与 PL/SQL 类型 TIMESTAMP187（请参阅 PACKAGE SYS.dbms_types）的行为不同 - 但我不知道为什么。

对于解决方法，我建议使用函数SYS_EXTRACT_UTC(...) 而不是CAST(... AS TIMESTAMP)。