Re: Domain based on TIMEZONE WITH TIME ZONE

On 05/10/2018 09:09 AM, Ben Hood wrote:
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>> On 10 May 2018, at 14:41, Adrian Klaver <adrian(dot)klaver(at)aklaver(dot)com> wrote:
>>> OK, so by using TIMESTAMP WITH TIME ZONE, you force all apps to submit timezone qualified timestamps in what language they are written in.
>>
>> Not really:
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>> https://www.postgresql.org/docs/10/static/datatype-datetime.html
>>
>> "For timestamp with time zone, the internally stored value is always in UTC (Universal Coordinated Time, traditionally known as Greenwich Mean Time, GMT). An input value that has an explicit time zone specified is converted to UTC using the appropriate offset for that time zone. If no time zone is stated in the input string, then it is assumed to be in the time zone indicated by the system's TimeZone parameter, and is converted to UTC using the offset for the timezone zone.”
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> Many thanks for this clarification. So therefore you can’t rely on the TIMESTAMP WITH TIME ZONE forcing the app to explicitly specify the offset. This is is because if the app is not specific, then the server will default back to its configured timezone.
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> So to get deterministic timestamps, you could either:
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> a) make sure the server is always configured to run in UTC;
> b) issue SET TIME ZONE ‘UTC’; at the beginning of any application session

Well if you are using a timestamp with timezone field the value is
always going to be stored as UTC. The TimeZone setting just determines
the rotation from the input value to the stored value and the reverse.
My previous point was just that Postgres will not enforce an offset on
input data.

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>>>> After all, it is always converted to UTC
>>>> servside anyway ?
>>> And because of the internal UTC representation, there is no room for ambiguous timezones.
>>
>> Define ambiguous timezone?
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> What I meant to say that is there should be no possibility for an effective timezone to arise implicitly.
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> For example, if you
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> (1) didn’t control the db server config
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> and
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> (2) and you forgot to enforce UTC at a client driver level
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> and
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> (3) didn’t set the offset in the app session
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> Then the only way to know what the effective zone offset will be is to find out what the server default is.
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> Is this plausible?

If you mean find the server default then yes:

test_(aklaver)> select current_setting('TimeZone');
current_setting
-----------------
US/Pacific

https://www.postgresql.org/docs/10/static/functions-admin.html#FUNCTIONS-ADMIN-SET
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--
Adrian Klaver
adrian(dot)klaver(at)aklaver(dot)com