Re: Default timezone changes in 9.1 - Mailing list pgsql-general

On Sat, Dec 15, 2012 at 11:54 PM, Gavin Flower <GavinFlower@archidevsys.co.nz> wrote:

Please do not top post, see end of email for rest of reply...
(Bottom posting is the convention here, so people can see the context before reading your reply.)

On 16/12/12 16:52, Terence Ferraro wrote:

Sorry about the double post, I thought the list disallowed attachments so I sent it again with a pastebin link instead of an attachment.

This change does not affect the storage at all. If it did, pre-9.1 things would have been a mess. Rather, this allows the system to determine the timezone for localized use at runtime.

Post 9.1, the system determines this via initdb data directory initialization and automatically sets it within postgresql.conf.

In other words, the default now is *not* GMT but rather the system detected timezone at initdb runtime. Removing that statically set configuration option, in this case, *then* assumes GMT.

In other words, it may be 2 AM NZST, but would you really want to walk around and have all of your clocks read 1 PM (GMT)? :)

T.J.

On Sat, Dec 15, 2012 at 10:35 PM, Gavin Flower <GavinFlower@archidevsys.co.nz> wrote:

On 16/12/12 16:07, Terence Ferraro wrote:

We recently began upgrading our clients' servers from 9.0 -> 9.2. After a few deployments and a little digging we noticed that 9.0 -> 9.1 broke the use of no timezone set within postgresql.conf. That is, not setting the option was now defaulting to GMT instead of the system timezone. Unfortunately, this put quite a damper on our "one configuration file to rule them all" setup across servers located throughout various time zones.

I obtained the commit ca4af308c32d03db5fbacb54d6e583ceb904f268 from the git repository and have reversed it against 9.2. Though it didn't apply as smoothly as initially hoped; applying a few of the failed hunks manually allowed me to successfully compile 9.2.1 with these changes reversed.

After some (light) testing, the previous functionality of the system choosing the timezone at runtime seems to be functional again.

I found this functionality invaluable and figured I'd get the patch (http://pastebin.com/5AyaX2RF) posted in case anyone else ever needs this functionality back. It works against 9.2.1; no guarantees on future releases of course and YMMV.

T.J.

I simply do not understand why you would _NOT_ want to store date/times as GMT!

Storing as GMT, allows the times to be easily converted into whatever time zone you are in, also allows for times to be correctly ordered irrespective of time zone.

If I make a phone call from Auckland to New York at 2am NZST on Friday, then my colleague is talking to me at the same time - even though it is still Thursday for them!


Cheers,
Gavin

My machine time is stored in GMT, but displayed in local time.  If I store time as the local time zone, then I could end up with a file created after I had amended it, or having a negative uptime!   Similarly, I want my date/times to be stored in GMT, but displayed in local time.

This is similar to storing money as an integer value of cents then formatting it for output with 2 decimal place.  Storing money as a double, is a common programming error - though, sometimes storing money as a double is actually valid.

The key point is the storage type used should be one most appropriate for processing, and need not be what is displayed.  In a database, someone's name will normally be stored in 2 or fields, but printed out as if it had been stored as a single string.


Cheers,
Gavin

All of your points are correct. The patch I provided changes none of that.

I think what you're missing here is that the timezone option in postgresql.conf does not change the internals of how a timestamp is stored. Rather, without it, by leaving that option blank, you would NOT see your local time when running SELECT CURRENT_TIMESTAMP. Instead, you'd see the GMT time.

Likewise, if I created a configuration file on my local machine, initdb would set the timezone option to "US/Eastern" and if I then distributed that to other servers located in various timezones they would ALL display CURRENT_TIMESTAMP with reference to "US/Eastern" instead of the actual machine timezone.

Previously (and now, with the patch), leaving the timezone option blank would simply determine, at run time, the machine's time zone and display times in that local timezone.

Does that make sense? The timezone parameter has nothing to do with storage types and everything to do with the display of the local time.