On 06/18/2018 06:24 AM, Jeremy Finzel wrote:
On Fri, Jun 15, 2018 at 2:57 PM, Adrian Klaver
<adrian.klaver@xxxxxxxxxxx <mailto:adrian.klaver@xxxxxxxxxxx>> wrote:
On 06/15/2018 12:24 PM, Jeremy Finzel wrote:
Hello!
We often prefer to use timestamptz or "timestamp with time zone"
in our environment because of its actually storing "objective
time" with respect to UTC. But in my own work experience, I
have scarcely encountered a case where business users, and
software engineers, do not actually think it means the opposite.
When I say "timestamp with time zone", people think the data is
saved *in* a specific time zone, whereas in reality, the
opposite is true. It is really more like "timestamp UTC" or you
even could say "timestamp at UTC". When you query this of
course, then it shows you the time offset based on your client
timezone setting.
I do believe this is part of the SQL standard, but I have found
that it creates great confusion. I think many devs choose
timestamp instead of timestamptz because they don't really
understand that timestamptz gives you UTC time storage built-in.
That of course means that if you have multiple servers that run
in a different time zone, and you want to replicate that data to
a centralized location, you can easily figure out what objective
time a record changed, for instance, not knowing anything about
what time zone the source system is in.
So it seems to me that "timestamp with time zone" is a misnomer
in a big
It actually is. It is just one timezone though, UTC.
way, and perhaps it's worth at least clarifying the docs about
this, or
https://www.postgresql.org/docs/10/static/datatype-datetime.html#DATATYPE-TIMEZONES
<https://www.postgresql.org/docs/10/static/datatype-datetime.html#DATATYPE-TIMEZONES>
"For timestamp with time zone, the internally stored value is always
in UTC (Universal Coordinated Time, traditionally known as Greenwich
Mean Time, GMT). An input value that has an explicit time zone
specified is converted to UTC using the appropriate offset for that
time zone. If no time zone is stated in the input string, then it is
assumed to be in the time zone indicated by the system's TimeZone
parameter, and is converted to UTC using the offset for the timezone
zone."
How should the above be clarified?
Actually, that is a really good description. But I would say the
problem is it does not have a prominent place on the page, and that many
people reading the docs will make enough assumptions about the data
types before they get down to this part of the page. What is displayed
as nitty-gritty details down the page should be essential reading for
any user of postgres wanting to know how to decide between timestamp and
timestamptz.
There are some descriptions that tend to mislead that perhaps could be
clarified. For example, at the top of the page, timestamptz is
described as "both date and time, with time zone". Given what we all
seem to acknowledge is a misleading description, I think we ought to
either change this to summarize the above very helpful description,
perhaps "both date and time, in UTC" or some such idea.
I like the idea of making that note that is now nested deep in the
example section very prominent at the top of the page, perhaps as one of
these special notes, given how critical timestamps are for nearly any
relational database use.
Thoughts?
Time/dates/timestamps are complicated and there are no end of 'if, and
and buts'. My suggestion would be a note at the top of the page that
says read through this section at least twice and then come back and do
that again. Other gotchas in the section that I have seen in posts to
this list:
"The SQL standard differentiates timestamp without time zone and
timestamp with time zone literals by the presence of a “+” or “-” symbol
and time zone offset after the time. Hence, according to the standard,
TIMESTAMP '2004-10-19 10:23:54'
is a timestamp without time zone, while
TIMESTAMP '2004-10-19 10:23:54+02'
is a timestamp with time zone. PostgreSQL never examines the content of
a literal string before determining its type, and therefore will treat
both of the above as timestamp without time zone.
To ensure that a literal is treated as timestamp with time zone, give it
the correct explicit type:
TIMESTAMP WITH TIME ZONE '2004-10-19 10:23:54+02'
In a literal that has been determined to be timestamp without time zone,
PostgreSQL will silently ignore any time zone indication. That is, the
resulting value is derived from the date/time fields in the input value,
and is not adjusted for time zone."
"One should be wary that the POSIX-style time zone feature can lead to
silently accepting bogus input, since there is no check on the
reasonableness of the zone abbreviations. For example, SET TIMEZONE TO
FOOBAR0 will work, leaving the system effectively using a rather
peculiar abbreviation for UTC. Another issue to keep in mind is that in
POSIX time zone names, positive offsets are used for locations west of
Greenwich. Everywhere else, PostgreSQL follows the ISO-8601 convention
that positive timezone offsets are east of Greenwich."
Thanks,
Jeremy
--
Adrian Klaver
adrian.klaver@xxxxxxxxxxx
