PostgreSQL Oracle 兼容性之 - NEW_TIME , SYS_EXTRACT_UTC

背景

如何将一个时区的本地时间，转换为另一个时区的本地时间？

在Oracle中可以使用NEW_TIME这个函数实现这个目的。

NEW_TIME(ts, tz1, tz2)

NEW_TIME returns the date and time in time zone timezone2 when date and time in time zone timezone1 are date.

The arguments timezone1 and timezone2 can be any of these text strings:

AST, ADT: Atlantic Standard or Daylight Time
BST, BDT: Bering Standard or Daylight Time
CST, CDT: Central Standard or Daylight Time
EST, EDT: Eastern Standard or Daylight Time
GMT: Greenwich Mean Time
HST, HDT: Alaska-Hawaii Standard Time or Daylight Time.
MST, MDT: Mountain Standard or Daylight Time
NST: Newfoundland Standard Time
PST, PDT: Pacific Standard or Daylight Time
YST, YDT: Yukon Standard or Daylight Time

例子

ALTER SESSION SET NLS_DATE_FORMAT =
   'DD-MON-YYYY HH24:MI:SS';

SELECT NEW_TIME(TO_DATE(
   '11-10-99 01:23:45', 'MM-DD-YY HH24:MI:SS'),
   'AST', 'PST') "New Date and Time" FROM DUAL;

New Date and Time
--------------------
09-NOV-1999 21:23:45

另外Oracle还有一个函数SYS_EXTRACT_UTC，是将一个timestamptz转换成UTC时区的本地时间

https://docs.oracle.com/cd/B12037_01/server.101/b10759/functions152.htm

SELECT SYS_EXTRACT_UTC(TIMESTAMP '2000-03-28 11:30:00.00 -08:00')
   FROM DUAL;

SYS_EXTRACT_UTC(TIMESTAMP'2000-03-2811:30:00.00-08:00')
-----------------------------------------------------------------
28-MAR-00 07.30.00 PM

PostgreSQL new_time

了解功能之后，PG就很容易实现new_time的函数了。

CREATE OR REPLACE FUNCTION public.new_time(ts timestamp without time zone, tz1 text, tz2 text)
 RETURNS timestamp without time zone
 LANGUAGE plpgsql
 STRICT
AS $function$
declare 
  res timestamp;
begin
  execute 'set local timezone to '''|| tz2||''''; 
  -- raise notice '%', current_setting('timezone');
  select (timestamptz(ts || tz1))::timestamp into res;
  return res;
end;
$function$;

PostgreSQL的时区列表。

postgres=# select * from pg_timezone_names ;
               name               | abbrev | utc_offset | is_dst 
----------------------------------+--------+------------+--------
 PST8PDT                          | PDT    | -07:00:00  | t
 Eire                             | IST    | 01:00:00   | t
 Antarctica/DumontDUrville        | DDUT   | 10:00:00   | f
 Antarctica/Syowa                 | SYOT   | 03:00:00   | f
 Antarctica/McMurdo               | NZST   | 12:00:00   | f
 Antarctica/Rothera               | ROTT   | -03:00:00  | f
 Antarctica/Casey                 | AWST   | 08:00:00   | f
 Antarctica/Davis                 | DAVT   | 07:00:00   | f
 Antarctica/Macquarie             | MIST   | 11:00:00   | f
 Antarctica/South_Pole            | NZST   | 12:00:00   | f
 Antarctica/Troll                 | CEST   | 02:00:00   | t
 Antarctica/Mawson                | MAWT   | 05:00:00   | f
 Antarctica/Palmer                | CLST   | -03:00:00  | t
 Antarctica/Vostok                | VOST   | 06:00:00   | f
 HST                              | HST    | -10:00:00  | f
 Iceland                          | GMT    | 00:00:00   | f
 CST6CDT                          | CDT    | -05:00:00  | t
 Kwajalein                        | MHT    | 12:00:00   | f
 GMT                              | GMT    | 00:00:00   | f
 Australia/Broken_Hill            | ACST   | 09:30:00   | f
 Australia/Canberra               | AEST   | 10:00:00   | f
 Australia/Currie                 | AEST   | 10:00:00   | f
 Australia/Yancowinna             | ACST   | 09:30:00   | f
 Australia/Brisbane               | AEST   | 10:00:00   | f
 Australia/West                   | AWST   | 08:00:00   | f
......
 Atlantic/Madeira                 | WEST   | 01:00:00   | t
 Egypt                            | EET    | 02:00:00   | f
 Canada/Central                   | CDT    | -05:00:00  | t
 Canada/Eastern                   | EDT    | -04:00:00  | t
 Canada/East-Saskatchewan         | CST    | -06:00:00  | f
 Canada/Saskatchewan              | CST    | -06:00:00  | f
 Canada/Pacific                   | PDT    | -07:00:00  | t
 Canada/Yukon                     | PDT    | -07:00:00  | t
 Canada/Atlantic                  | ADT    | -03:00:00  | t
 Canada/Newfoundland              | NDT    | -02:30:00  | t
 Canada/Mountain                  | MDT    | -06:00:00  | t
(589 rows)

验证对应oracle的例子

https://www.postgresql.org/docs/7.2/static/timezones.html
PST	-08:00	Pacific Standard Time
AST	-04:00	Atlantic Standard Time (Canada)
  
最好直接使用时区  
  
postgres=# select new_time('1999-10-11 01:23:45', '-04', '-08');
NOTICE:  <-08>+08
      new_time       
---------------------
 1999-10-10 21:23:45
(1 row)

PostgreSQL SYS_EXTRACT_UTC

自定义函数如下

CREATE OR REPLACE FUNCTION public.SYS_EXTRACT_UTC(ts timestamp with time zone)
 RETURNS timestamp without time zone
 LANGUAGE plpgsql
 STRICT
AS $function$
declare 
  res timestamp;
begin
  set local timezone to UTC;
  select ts::timestamp into res;
  return res;
end;
$function$;

验证

postgres=# select SYS_EXTRACT_UTC('2000-03-28 11:30:00.00-08:00');
   sys_extract_utc   
---------------------
 2000-03-28 19:30:00
(1 row)

参考

https://www.postgresql.org/docs/7.2/static/timezones.html

https://docs.oracle.com/cd/B19306_01/server.102/b14200/functions092.htm

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