Re: Returning timestamp with timezone at specified timezone irrespective of client timezone

On 9/27/20 2:16 PM, aNullValue (Drew Stemen) wrote:
> Hello,
>
> I've attempted to obtain help with this problem from several other
> places, but numerous individuals recommended I ask this mailing list.
>
> What I need is for the ability to return a timestamp with timezone,
> using the UTC offset that corresponds to a column-defined timezone,
> irrespective of the client/session configured timezone.
>
> I have three columns in a table:
> Timezone: 'US/Eastern'
> Date: 2020-10-31
> Time: 08:00
>
> The output I'm able to find includes these possibilities:
> '2020-10-31 08:00:00'
> '2020-10-31 12:00:00+00'
>
> Whereas what I actually need is:
> '2020-10-31 08:00:00-05'
>
> Using the postgresql session-level timezone configuration won't work
> because I need multiple timezones to be handled in a single set.
>
> Example code follows. I'm not using to_char in the examples as I likely
> would in the production code, but I haven't found any way that it could
> be helpful here regardless.
>
> ---------------------------------------------------
>
> SET TIME ZONE 'UTC';
>
> CREATE TABLE loc
> (
>     id serial not null,
> timezone text not null,
>     loc_date date NOT NULL,
>     loc_time text NOT NULL,
>     CONSTRAINT loc_pkey PRIMARY KEY (id),
>     CONSTRAINT loc_loc_time_check CHECK (loc_time ~
> '(^(2[0-3]|[01][0-9]|[0-9]):?([0-5][0-9]):?([0-5][0-9])?$)|(^(1[012]|0[1-9]|[1-9]):[0-5][0-9]
> [AaPp][Mm]$)'::text)
> )
> ;
>
> INSERT INTO loc (timezone, loc_date, loc_time) VALUES
> ('US/Eastern', '2020-10-31', '08:00'),
> ('US/Eastern', '2020-11-03', '08:00'),
> ('US/Central', '2020-10-31', '08:00'),
> ('US/Central', '2020-11-03', '08:00');
>
> SELECT *
> , timezone(l.timezone, l.loc_date + l.loc_time::time without time zone)
> tswtz
> , (l.loc_date + l.loc_time::time without time zone) tswotz
> FROM loc l
> ORDER BY timezone, loc_date, loc_time
> ;
>
> ---------------------------------------------------
>
> id |  timezone  |  loc_date  | loc_time |         tswtz          |
> tswotz
> ----+------------+------------+----------+------------------------+---------------------
>   7 | US/Central | 2020-10-31 | 08:00    | 2020-10-31 13:00:00+00 |
> 2020-10-31 08:00:00
>   8 | US/Central | 2020-11-03 | 08:00    | 2020-11-03 14:00:00+00 |
> 2020-11-03 08:00:00
>   5 | US/Eastern | 2020-10-31 | 08:00    | 2020-10-31 12:00:00+00 |
> 2020-10-31 08:00:00
>   6 | US/Eastern | 2020-11-03 | 08:00    | 2020-11-03 13:00:00+00 |
> 2020-11-03 08:00:00
> (4 rows)
>
> What I actually need is, in example id=7, '2020-10-31 08:00:00-05'.
>
> Is this even possible? Several people have proposed that I write a
> custom function to do this on a per-row basis, which... I suppose I can
> do... I'm just blown away that this isn't something that just works "out
> of the box".
>

Something like?:

select '2020-10-31' || ' 08:00 ' || utc_offset from pg_timezone_names
where name = 'US/Eastern';
?column?
----------------------------
2020-10-31 08:00 -04:00:00

--
Adrian Klaver
adrian(dot)klaver(at)aklaver(dot)com