PostgreSQL timestamp with timezone apply offset when reading

Question

First of all I know there are a lot threads about this and I could use the solutions there, f.e. write a python function which converts datetime object to the timezone I need and add this as a filter to jinja2.

But I wanted to skip this step.

Usually PostgreSQL saves datetime objects in UTC.

What I did is: I changed the default timezone which postgreSQL uses to save datetime objects to: Europe/Berlin

ALTER DATABASE postgres SET timezone TO 'Europe/Berlin';

This worked, SHOW timezone; tells me its Europe/Berlin.

But what Postgres does, it still saves the datetime as UTC, but with a +1 offset for Europe/Berlin, which is actually not a problem.

I assumed that when reading the datetime object, Postgres would apply the offset and return the Europe/Berlin time, but it doesn't. It still returns the UTC time, so whats the point in changing the timezone?

I still need to write this filter and manually convert it.

I think there should be a simple way to apply the offset on DB level, or am I wrong?

EDIT

Some information, which might help, the model:

class User(UserMixin, Base):
    __tablename__ = 'users'
    date_added = Column(DateTime(timezone=True), nullable=False)

How it looks in the DB:

And here is an example how I load it in jinja2/html:

{{ tenant.date_added.strftime('%d.%m.%Y um %H:%M Uhr') }}

Answer 1

First, you are right to handle this completely inside the database. When dealing with time zones, either use timestamp with time zone throughout and let the database handle conversions and such, or use timestamp without time zone throughout, store UTC timestamps and let the application deal with time zone handling. Mixing these two is usually not a good idea.

PostgreSQL internally stores a timestamp with time zone as a timestamp in UTC. Upon conversion to a string , e.g. when data are sent to the client, the data is converted to the session time zone, which is configured by the timezone parameter.

Setting timezone with ALTER DATABASE will change the setting for all future sessions, that is, you have to disconnect and reconnect for the new setting to take effect.

However, every session is free (and encouraged) to change timezone using the SET command or the set_config function, and this determines to which time zone the timestamps will be converted. Use SHOW timezone to see your current setting.

If you store a timestamp without an explicit time zone (like 2019-02-01 12:00:00), it will be interpreted in your current session time zone. So if that is Europe/Berlin, the UTC timestamp stored internally will be 2019-02-01 11:00:00 UTC. Now if you SELECT the value, it will be displayed as 2019-02-01 12:00:00+01, because that is your current time zone offset. If you change timezone, the same value will be displayed differently.