How to select timestamptz with offset with AT TIME ZONE - sql

Question

I've fiddeling around with timestamps in PostgreSQL, reading a interesting Blog about this behavior, but have no clue to solve following problem:
How to select a 'timestamp with time zone' column with AT TIME ZONE from another column, including the offset to UTC in one step?
I've a sample table in PostgreSQL 10:
CREATE TABLE public.test_tz
(
id serial,
in_zone_timestamp timestamp without time zone,
in_zone character varying COLLATE pg_catalog."default",
CONSTRAINT test_tz_pkey PRIMARY KEY (id)
)
WITH (
OIDS = FALSE
)
TABLESPACE pg_default;
PostgreSQL's setting is
timezone=localtime
and the server runs at 'Europe/Berlin'
$ date --> Fri Mar 9 12:15:23 CET 2018
The timestamp column should store with this messy syntax:
INSERT INTO public.test_tz ( in_zone_timestamp, in_zone) VALUES
('2017-02-01 00:00:00' AT TIME ZONE current_setting('TIMEZONE') AT TIME ZONE 'Asia/Singapore', 'Asia/Singapore' );
If someone knows a better way, please don't hold back! The other solution is to use '2017-02-01 00:00:00-08' as the value, but I don't know the offset value.
I want to store the local timestamp like: 'The user hit a key at 2017-02-01 00:00:00 in Singapore'.
If I ask the database: Which time it was here in Europe, when the user in Singapore is hitting a key, I got:
SELECT in_zone_timestamp FROM public.test_tz;
--> '2017-01-31 17:00:00+01'
This seems OK, because Singapore has a offset of +8 hours.
If I want to know, which time it was in Singapore, I use:
SELECT in_zone_timestamp AT TIME ZONE in_zone FROM public.test_tz;
--> '2017-02-01 00:00:00'
That's OK too, but however, it doesn't return the offset to 'UTC', so I can't see that this timestamp is not in my local time!
I try some combinations of AT TIME ZONE or converting to timestamptz, but the results are not what I want. I expected a result like:
--> '2017-02-01 00:00:00+08'
At here, I only see one solution, to manually concat/convert/manipulate the result and add the offset by hand, but is this the only way?
Sorry if I explain this question a little bit too comlicated and hope someone can follow my thoughts.
Thanks in advance

Answer 1

The only way to have PostgreSQL convert a timestamp to a string like 2017-02-01 00:00:00+08 automatically is to change the session time zone:
SET timezone = 'Asia/Singapore';
You can use SET LOCAL to change the setting only for the duration of a transaction.
If you don't want that, you can get the offset with a query like this:
SELECT TIMESTAMP '2000-01-01 00:00:00' AT TIME ZONE 'UTC'
- TIMESTAMP '2000-01-01 00:00:00' AT TIME ZONE 'Asia/Singapore';
?column?
----------
08:00:00
(1 row)

Answer 2

Thanks for clarify, but in my opinion that PostgreSQL has a few shortcomings on this topic.
A 'not so perfect' solution for this problem is really to manually concat the parts needed.
SELECT (test_tz.in_zone_timestamp AT TIME ZONE in_zone) || (SELECT abbrev FROM pg_timezone_names WHERE name = in_zone) FROM public.test_tz;
--> 2017-02-01 00:00:00+08
This provides an acceptable solution, with small imperfections.
In some cases there is the timezone short text in the abbrev-column, like 'CET' or 'PDT' and not the +/- numeric value. This produces results (eg. for 'Europe/Berlin') like:
--> 2017-02-01 02:00:00CET
A better value gives the column 'utc_offset' of pg-timezone-names, but this requires a more complex text manipulation, I don't want at this point.
Second solution can be the manipulation of the output by application and formatting the text to whatever you need.
Hope that helps others to solve such problem without searching the internet for a not available support from the database.
Bye