Fix current_timestamp_

[runeksvendsen]

The type of the Postgres expression current_timestamp at time zone 'UTC' is TIMESTAMP WITHOUT TIME ZONE. Consequently, if we attempt to store this in a column of type TIMESTAMP WITH TIME ZONE then Postgres will cast this TIMESTAMP WITHOUT TIME ZONE-value into a TIMESTAMP WITH TIME ZONE using the time zone configured for the Postgres instance in question. This results in current_timestamp at time zone 'UTC' being wrong for all Postgres instances running with a non-UTC time zone (ie. most).

Note that Postgres internally stores a value of type TIMESTAMP WITH TIME ZONE as a UTC timestamp. Ie. it does not store the time zone component anywhere -- a timestamp with a time zone component is simply converted to UTC and then stored. So when current_timestamp returns e.g. 2022-08-08 11:59:35.958693+02, Postgres will correctly convert this to the UTC time 2022-08-08 09:59:35.958693 -- whereas storing 2022-08-08 09:59:35.958693 in the same column would be cast to UTC+2 (2022-08-08 09:59:35.958693+02) and thus be two hours too early.

Example

psql (11.11)
Type "help" for help.

postgres=# CREATE TABLE test_table (time TIMESTAMP WITH TIME ZONE);
CREATE TABLE
postgres=# select current_timestamp;
       current_timestamp       
-------------------------------
 2022-08-09 12:22:31.430855+02
(1 row)

postgres=# select current_timestamp at time zone 'UTC';
         timezone          
---------------------------
 2022-08-09 10:22:36.10837
(1 row)

postgres=# INSERT INTO test_table VALUES (current_timestamp at time zone 'UTC'); 
INSERT 0 1
postgres=# SELECT * FROM test_table;
             time              
-------------------------------
 2022-08-09 10:22:52.618104+02
(1 row)