Message-ID: <501C0884.1040106@shopzeus.com>
Date: Fri, 03 Aug 2012 19:21:08 +0200
From: Laszlo Nagy <gandalf@shopzeus.com>
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To: Steve Crawford <scrawford@pinpointresearch.com>,
	pgsql-performance@postgresql.org
Subject: Re: [ADMIN] Messed up time zones
References: <501B8C6F.6040100@shopzeus.com> <501BAA84.9010306@shopzeus.com>
	<501BAE33.4090102@shopzeus.com> <16399.1344003584@sss.pgh.pa.us>
	<501BED07.5070603@shopzeus.com>
	<501BFA4F.5010709@pinpointresearch.com>
In-Reply-To: <501BFA4F.5010709@pinpointresearch.com>
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> All the above are the exact same point in time merely stated as 
> relevant to each location. Note that given a timestamp with time zone 
> and a zone, PostgreSQL returns a timestamp without time zone (you know 
> the zone since you specified it). 
Yes, I know the zone. But I don't know the offset from UTC.

Example:

template1=> set timezone to 'UTC';
SET
template1=> select ('2011-10-30 01:00:00'::timestamptz) at time zone 
'Europe/Budapest';
       timezone
---------------------
  2011-10-30 02:00:00    -- Is it winter or summer time?
(1 row)

template1=> select ('2011-10-30 00:00:00'::timestamptz) at time zone 
'Europe/Budapest';
       timezone
---------------------
  2011-10-30 02:00:00    -- Is it winter or summer time? What is the 
offset from UTC here? Can you tell me when it was in UTC?
(1 row)

template1=>

What is more:

template1=> select (('2011-10-30 00:00:00'::timestamptz) at time zone 
'Europe/Budapest') is distinct from (('2011-10-30 
01:00:00'::timestamptz) at time zone 'Europe/Budapest');
  ?column?
----------
  f
(1 row)

template1=>

Yeah, we know what time zone it is in, but we don't know when it was, 
thanks a lot. :-( It would be unambiguous to store the UTC offset along 
with the value. But it is not how it was implemented.