Postgres Pro
Downloads
Home   >   mailing lists

Thread: Inconsistent time zone output for JSON at UTC

Inconsistent time zone output for JSON at UTC

From
Gregory Jensen
Date:
Hello,

My problem is that depending on the scenario, a query returning JSON will inconsistently return time zone information for timestamps.

Create the database and some test data:
create table test_tz (id serial, created_at timestamp);
insert into test_tz (created_at) values (NOW());


When the server time zone setting is UTC - timezone = 'UTC'

select to_json(created_at::timestamptz) from test_tz;
              to_json              
------------------------------------
 "2022-05-31T10:20:07.133799+00:00"
(1 row)

When the server time zone setting has an offset (BST for me) - timezone = 'Europe/London'

select to_json(created_at::timestamptz) from test_tz;
              to_json              
------------------------------------
 "2022-05-31T10:20:07.133799+01:00"
(1 row)

BUT with the server set to something other than UTC and with my various attempts to get a UTC time zone back with my timestamp in JSON - the time zone is being dropped.

select to_json(created_at::timestamptz at time zone 'UTC') from test_tz;
           to_json            
------------------------------
 "2022-05-31T09:20:07.133799"
(1 row)

select to_json(created_at::timestamptz at time zone '+00:00') from test_tz;
           to_json            
------------------------------
 "2022-05-31T09:20:07.133799"
(1 row)

select to_json(created_at::timestamptz at time zone '00:00') from test_tz;
           to_json            
------------------------------
 "2022-05-31T09:20:07.133799"
(1 row)

This inconsistency is causing problems when trying to parse results in Golang. The version with the time zone is what's expected. if this was consistent i could just tweak the expected date format in my program but at the moment i don't seem to be able to write a query than can cope with different database configurations.

i've not been able to find from the documentation whether this is expected behaviour and i should just configure my postgresql servers consistently or is it something inconsistent that could use a fix.

Thanks,
G

Re: Inconsistent time zone output for JSON at UTC

From
"David G. Johnston"
Date:
On Tuesday, May 31, 2022, Gregory Jensen <GreatDJonfire@hotmail.co.uk> wrote:

When the server time zone setting is UTC - timezone = 'UTC'

select to_json(created_at::timestamptz) from test_tz;
              to_json              
------------------------------------
 "2022-05-31T10:20:07.133799+00:00"
(1 row)

This is the solution.  The timezone setting isn't a “server setting”, it is a session-local setting with an initial value based upon server configuration.  Change it for the session to UTC if you for some reason must output +00.

The at time zone operator outputs a timestamp without timezone and that is why those variants don’t produce any time zone offset.

David J.

Re: Inconsistent time zone output for JSON at UTC

From
Gregory Jensen
Date:
Thanks David,

The explanation of 'at time zone' explains what I'd missed there. Much appreciated!
From: David G. Johnston <david.g.johnston@gmail.com>
Sent: 31 May 2022 13:00
To: Gregory Jensen <GreatDJonfire@hotmail.co.uk>
Cc: pgsql-bugs@lists.postgresql.org <pgsql-bugs@lists.postgresql.org>
Subject: Re: Inconsistent time zone output for JSON at UTC
 
On Tuesday, May 31, 2022, Gregory Jensen <GreatDJonfire@hotmail.co.uk> wrote:

When the server time zone setting is UTC - timezone = 'UTC'

select to_json(created_at::timestamptz) from test_tz;
              to_json              
------------------------------------
 "2022-05-31T10:20:07.133799+00:00"
(1 row)

This is the solution.  The timezone setting isn't a “server setting”, it is a session-local setting with an initial value based upon server configuration.  Change it for the session to UTC if you for some reason must output +00.

The at time zone operator outputs a timestamp without timezone and that is why those variants don’t produce any time zone offset.

David J.

Re: Inconsistent time zone output for JSON at UTC

From
Tom Lane
Date:
"David G. Johnston" <david.g.johnston@gmail.com> writes:
> This is the solution.  The timezone setting isn't a “server setting”, it is
> a session-local setting with an initial value based upon server
> configuration.  Change it for the session to UTC if you for some reason
> must output +00.
> The at time zone operator outputs a timestamp without timezone and that is
> why those variants don’t produce any time zone offset.

The use of JSON has nothing whatever to do with this; you're just getting
an equivalent of the string representation of the timestamp.  It is
modified to fit some ISO format spec or other, but it's the same data:

regression=# show timezone;
     TimeZone
------------------
 America/New_York
(1 row)

regression=# select now(), to_json(now());
              now              |              to_json
-------------------------------+------------------------------------
 2022-05-31 10:22:00.413512-04 | "2022-05-31T10:22:00.413512-04:00"
(1 row)

regression=# set timezone to 'UTC';
SET
regression=# select now(), to_json(now());
              now              |              to_json
-------------------------------+------------------------------------
 2022-05-31 14:22:03.559057+00 | "2022-05-31T14:22:03.559057+00:00"
(1 row)

If you use timestamp-without-time-zone, you get something like

regression=# select localtimestamp, to_json(localtimestamp);
       localtimestamp       |           to_json
----------------------------+------------------------------
 2022-05-31 14:24:34.927072 | "2022-05-31T14:24:34.927072"
(1 row)

            regards, tom lane