Re: BUG #16797: EXTRACT(EPOCH FROM timestamp) is not using local timezone

+pgsql-bugs

Wondering then, when local timezone is set to anything other than UTC, why
does:
'01/01/1970 00:00:00'::timestamp =
'01/01/1970 00:00:00'::timestamptz

To compare these datetime values, postgres is making an implicit cast of
some kind - and if they are equal then their epoch values should be equal
as well.  Thus, to be consistent with the extract epoch from timestamp
method chosen in 9.2, these should not be equal or perhaps a type-mismatch
error.

Personally, I prefered the previous behavior with the implicit cast to
timestamptz when asked to convert timestamp for extracting epoch or other
with timezone related purposes - just seems more consistent and expected to
me.  And yes - I agree being type explicit is the better route here, vs
relying on implicit behaviours that could change - I'll do that, this one
just bit me from some two decade old code being moved to a new postgres
instance.

kind regards,
-dana

On Wed, Dec 30, 2020 at 2:01 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:

> PG Bug reporting form <noreply@postgresql.org> writes:
> > EXTRACT(EPOCH FROM timestamp) should be using the local timezone
>
> No, type timestamp is explicitly *not* timezone aware.  If you use
> timestamptz (a/k/a timestamp with time zone) you will get the
> answer you want.
>
> > -- Expected results (seen from PostgreSQL 9.1.11):
>
> > # SET TIME ZONE 'EST5EDT'; select extract(epoch from ('01/01/1970
> > 00:00:00'::timestamp));
>
> This was a bug, cf 9.2.0 release notes [1]:
>
>     Make EXTRACT(EPOCH FROM timestamp without time zone) measure the epoch
>     from local midnight, not UTC midnight (Tom Lane)
>
>     This change reverts an ill-considered change made in release 7.3.
>     Measuring from UTC midnight was inconsistent because it made the
>     result dependent on the timezone setting, which computations for
>     timestamp without time zone should not be. The previous behavior
>     remains available by casting the input value to timestamp with time
>     zone.
>
>                         regards, tom lane
>
> [1] https://www.postgresql.org/docs/release/9.2.0/
>