confusion when converting unix timestamp to timestamp with timezone

Question

I have a unix timestamp in seconds and want to convert it to a timestamp with timezone in postgres.

My confusion originates from the fact that the following does not add but removes the time zone:

SELECT to_timestamp(1632610656) at time zone 'Europe/Berlin'

=> "2021-09-26 00:57:36"

If I understand it correctly to_timestamp creates a timestamp with time zone and given this the at time zone 'Europe/Berlin would then remove the time zone? (this feels so strange to me?!).

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If I now want to get the timestamp with time zone in Europe/Berlin, is it correct that I need to do the following?

1. Option

SELECT to_timestamp(1632610656) at time zone 'UTC' at time zone 'Europe/Berlin'
=> "2021-09-25 20:57:36+00"

and NOT

2. Option

SELECT to_timestamp(1632610656) at time zone 'Europe/Berlin' at time zone 'Europe/Berlin'
=> "2021-09-25 22:57:36+00"

They obviously have different results. The first one is the correct one, right?

Answer 1

Different from what your intuition would tell you, a timestamp with time zone does not have a certain time zone associated. Rather, it is an “absolute timestamp”.

If you want a timestamp with time zone to be displayed in a certain time zone, you have to set the timezone parameter in your database session:

SET timezone = 'Europe/Berlin';

SELECT to_timestamp(1632610656);

If you want a timestamp without time zone that shows what a Berlin clock would show at that time, use AT TIME ZONE:

SELECT to_timestamp(1632610656) AT TIME ZONE 'Europe/Berlin';