Re: Improve timestamp substraction to be DST-aware

On Mon, Dec 2, 2013 at 11:43:06PM +0100, Piotr Marcinczyk wrote:
> > The alternative proposal that's been on the table for awhile (see the
> > preceding entry in the TODO list) is to remove the interval_justify_hours
> > call in timestamp_mi, which would also have the effect of fixing the
> > inconsistency that T1 + (T2 - T1) doesn't necessarily yield T2. And it
> > would do that a lot more straightforwardly, with less risk that there's
> > still corner cases that would misbehave.
> >
> > If it's not the T1 + (T2 - T1) issue that's bothering you, perhaps
> > you should explain exactly what results you're hoping to get by changing
> > this behavior.
> >
>
> In SQL99 "4.7 Datetimes and intervals" I read, that day-time intervals
> (I think, that our interval has this type) should have hours in range
> 0-23. I suggest to remove preceding entry from TODO list, and not treat
> this as alternative. Current behavior is OK.
>
> Regarding this, we have two options: use session TZ, or wait for
> implementation of TZ saved in timestamp field. In my opinion saving TZ
> in field doesn't give serious benefits, and it's probable that it will
> never be implemented. In this case, using session TZ is sufficient.
>
> Can You explain, why do You read this proposal as fuzzy? I believe that
> using session context is normal in many cases. Maybe I should consider
> saving TZ in timestamp once again?

I have remove the TODO item and added an interval subtraction/addition
section to the docs for PG 9.4:

http://www.postgresql.org/docs/devel/static/functions-datetime.html

The paragraphs being with "When adding an interval value to" and
"Subtraction of dates and timestamps can also be complex.". Is there
anything more to add there?

--
Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://enterprisedb.com

+ Everyone has their own god. +