tajo-dev mailing list archives

Site index · List index
Message view « Date » · « Thread »
Top « Date » · « Thread »
From Hyunsik Choi <hyun...@apache.org>
Subject Re: Timezone issues
Date Tue, 25 Nov 2014 08:54:11 GMT
I'm happy to work with you and fix a major nuisance :) Later, I'll
share the timezone related problem with you when I found additional
bugs.

Warm regards,
Hyunsik

On Tue, Nov 25, 2014 at 4:17 PM, Jaewoong Jung <jungjw@gmail.com> wrote:
> Everything is very clear now thanks to your explanation. :)
>
> Okay, then I'll fix the issue by making DateDatum timezone-neutral and
> TimeDatum UTC-based. Also, I'll play with PostgreSQL to understand its
> timezone model better.
>
> Meanwhile, please feel free to assign timezone-related bugs to me as
> you see fit.
>
> On Mon, Nov 24, 2014 at 5:52 PM, Hyunsik Choi <hyunsik@apache.org> wrote:
>> Thank you all guys for your comments.
>>
>> Jaewoong,
>>
>> I leave inline comments. If my answers are not enough for your
>> question or I misunderstood your question, please feel free to ask
>> additional questions.
>>
>> Best regards,
>> Hyunsik
>>
>> On Fri, Nov 21, 2014 at 12:50 AM, Jaewoong Jung <jungjw@gmail.com> wrote:
>>> There's another issue that hopefully Hyunsik would be able to clarify,
>>> and it's very crucial to handling timezones in these data types.
>>>
>>> Q: So, let's say (and I agree) TimeDatum represents an instant, so can
>>> be timezoned. Then, is it a UTC time or a local time?
>>>
>>
>> TimeDatum is UTC or should be UTC if some parts are not.
>>
>>> Let me explain why this question is important.
>>>
>>> DateDatum represents an instant. And, it is implicitly timezoned to
>>> the user local time. Which means, if I use '11/20/2014', it's
>>> '11/20/2014 0:00:00 PST' and is equivalent to a TimestampDatum for
>>> '11/20/2014 8:00:00 UTC'. (BTW, the compareTo implementation for
>>> DateDatum is broken in this regard. I'll file a separate issue for
>>> that.)
>>
>> I agree with your proposal. I also think that DateDatum does not need
>> to be timezoned. We should keep it as DateDatum instead of
>> TimestampDatum. Thank you for nice finding!
>>
>>> What about TimeDatum? It is currently timezoned to UTC, (surprise,
>>> anyone?) if I understood the code correctly. When we add a TimeDatum
>>> to a DateDatum, we convert the TimeDatum to the user local time, which
>>> implies TimeDatum is UTC-based. (Also, the comment next to the line
>>> explicitly mentions it.)
>>
>> You are right. TimeDatum represents UTC time.
>>
>> FYI, I'd like to describe additional background. There are only two
>> entry points to take time or timestamp values. One is records in input
>> tables, and another is SQL statements. Currently, Input table uses the
>> system global timezone specified in TajoConf (tajo-site.xml file).
>> Later, we will add one table property to allow users to specify
>> timezone for each table. For SQL statement (e.g., SELECT time
>> '03:00:00'), we will use client timezone. Also, we will provide some
>> expression to allow users to specify timezone for time or timestamp in
>> SQL statements.
>>
>> Consequently, only two entry points have to deal with timezone for
>> Timestamp and Time. Other parts in Tajo should deal with all values in
>> UTC.
>>
>>>
>>> So, here's the problem.
>>>
>>> Why do they have different timezones? They're incomplete as an instant
>>> when used alone and are complementary to each other. This is an
>>> important concept. To understand it, you have to think about why
>>> adding DateDatum and TimeDatum is allowed in the code. Originally,
>>> instants can't be added. (And, that's why I thought TimeDatum is not
>>> an instant.) You can't add (say) 11/20/2014 to 11/23/2019. Subtracting
>>> an instant from an instant makes sense and results in a period, but
>>> they can't be added.
>>>
>>> However, in the case of DateDatum and TimeDatum, additions are allowed
>>> because they're complementary to each other, and what the code does
>>> **conceptually** is concatenate the two.
>>
>> Great insight! So far, I haven't thought it.
>>
>>> Therefore, because they're intended to be used together, I'd argue
>>> they shouldn't have different timezones. Also, if they have different
>>> timezones, additions can't have a simple correct answer. What's the
>>> correct answer of 11/20/2014 (PST) + 8:00:00 (UTC)? There's no clear
>>> answer because they can't be simply concatenated.
>>>
>>> (FWIW, the current Tajo code thinks the answer is 11/20/2014 8:00:00
>>> in UTC. How many of you got it right?)
>>
>> You are definitely right :) If they have different timezone, the
>> problem becomes very complicated. Nobody wants it :) As I mentioned,
>> Timezone problem of Timestamp and Time data types should be addressed
>> in two entry points and client. We need to keep the processing
>> approach simple.
>>
>>> This can cause a lot of confusion to users. When they use a date
>>> alone, it is interpreted as a local time. But, as soon as they add a
>>> time to it, it is silently converted to UTC in a way which is very
>>> unexpected to many users.
>>>
>>> Why am I emphasizing it is unexpected? Look at the comparison below.
>>>
>>> What's the answer to 11/20/2014 (DateDatum) + 8 hours (IntervalDatum)?
>>> It's 11/20/2014 8:00 in a local time (PST on my machine). How about
>>> 11/20/2014 (DateDatum) + 8:00:00 (TimeDatum)? It's 11/20/2014 8:00 in
>>> UTC as I wrote above. How many users would be able to expect this?
>>
>> They are definitely a bug. We follow PostgreSQL in all aspects, and
>> the following results come from the PostgreSQL. The results of two
>> operations are the same.
>>
>> hyunsik=> SELECT date '11/20/2014' + time '08:00';
>>       ?column?
>> ---------------------
>>  2014-11-20 08:00:00
>> (1 row)
>>
>> hyunsik=> select date '11/20/2014' + interval '8 hrs';
>>       ?column?
>> ---------------------
>>  2014-11-20 08:00:00
>> (1 row)
>>
>>
>>>
>>> So, coming back to my original question. What timezone should a
>>> TimeDatum have? UTC or local time? It's currently UTC. But, I believe
>>> it should be changed to the local time zone.
>>
>> TimeDatum is UTC.
>>
>> In sum, we should keep both TimeDatum and TimestampDatum UTC values.
>> Then, we should address timezone offsets in two entry points and
>> client side.
>>
>>>
>>> (Sorry for the long email. But, I think it's critical to get this
>>> right and build consensus over it so that we can provide a consistent
>>> behavior going forward. The actual fix will be really simple, though.)
>>>
>>> On Thu, Nov 20, 2014 at 2:44 AM, Hyunsik Choi <hyunsik.choi@gmail.com>
wrote:
>>>> Those parts have poor documentation.
>>>>
>>>> I agree with your investigation. I also could find many misuse of timezone
>>>> in many parts. We should make them clear and fix them in this chance.
>>>>
>>>> I just got off the plane, and I'm still on the road. So, I'll give more
>>>> comments tomorrow.
>>>>
>>>> - hyunsik
>>>>
>>>> On Thu, Nov 20, 2014 at 5:24 PM, Jaewoong Jung <jungjw@gmail.com> wrote:
>>>>
>>>>> Wow, this seemingly trivial issue has surprisingly many problems involved.
>>>>>
>>>>> The most critical one, though, is TimeMeta class. Presumably because
>>>>> it is poorly documented, it is being used for two different purposes
>>>>> in Tajo code base. Some code treats it as a date time representation,
>>>>> which I believe is the original intention, but some treat it as the
>>>>> human-readable equivalent of TimeDatum by completely ignoring
>>>>> date-related fields.
>>>>>
>>>>> For example, DateTimeUtil.date2j(long julianDate, TimeMeta tm), which
>>>>> converts a julian timestamp to a TimeMeta value, doesn't touch
>>>>> dayOfMonth, monthOfYear, or years values and just puts all values for
>>>>> hour and above units in hours field.
>>>>>
>>>>> There are other minor problems like incorrect comments and absent
>>>>> default values, but the most critical one is misuse of TimeMeta.
>>>>>
>>>>> I'll try to break up my patches so that each has a clear and
>>>>> easy-to-understand goal. Sending this heads-up to let you know
>>>>> there'll be more issues filed and patches sent than you might expect.
>>>>>
>>>>> On Wed, Nov 19, 2014 at 10:56 PM, Jaewoong Jung <jungjw@gmail.com>
wrote:
>>>>> > Yeah, after some more research, I found that TImeDatum is a somewhat
>>>>> > ambiguous data type.
>>>>> >
>>>>> > Its original purpose is to represent a time of a day, i.e. hh:mm:ss
>>>>> > part of an instant. So, it could be viewed as instant data though
some
>>>>> > may argue it's incomplete to fully represent an instant. Anyway,
given
>>>>> > that TimestampDatum has limitation in terms of the time range it
can
>>>>> > represent, and given that Tajo doesn't have DateTime data type,
>>>>> > clients should be allowed to use it with a timezone.
>>>>> >
>>>>> > I'll change the direction and try to address the issue by fixing
the
>>>>> > underflow error.
>>>>> >
>>>>> > Thanks for the input. :)
>>>>> >
>>>>> > On Wed, Nov 19, 2014 at 10:24 PM, Jong-young Park <eminency@gmail.com>
>>>>> wrote:
>>>>> >> Hi, Jaewoong.
>>>>> >>
>>>>> >> To express time period value, IntervalDatum is existing as I
know.
>>>>> >>
>>>>> >> So I think it is right that TimeDatum is for some time point.
>>>>> >>
>>>>> >> And TimestampDatum seems it is doing both roles of DateDatum
and
>>>>> TimeDatum.
>>>>> >>
>>>>> >> Regards,
>>>>> >> Jongyoung.
>>>>> >>
>>>>> >> On Wed Nov 19 2014 at 오후 5:23:40 Jaewoong Jung <jungjw@gmail.com>
>>>>> wrote:
>>>>> >>
>>>>> >>> It turns out, TAJO-1191 is slightly more complicated than
I thought.
>>>>> >>> (https://issues.apache.org/jira/browse/TAJO-1191)
>>>>> >>>
>>>>> >>> Basically, it's about whether TimeDatum may have a timezone
tied with
>>>>> >>> it. I **believe** TimeDatum is originally designed to hold
a time
>>>>> >>> period value, not an instant. (TimestampDatum seems to be
the
>>>>> >>> canonical container for instants.) So, it doesn't make sense
to apply
>>>>> >>> any timezones to TimeDatum values, but it's being done in
a few
>>>>> >>> places. And, that's why the test is failing on my machines.
>>>>> >>>
>>>>> >>> I'm going to try to fix it by removing all timezone-related
references
>>>>> >>> around the class, but I want to check my assumption with
you before I
>>>>> >>> proceed.
>>>>> >>>
>>>>> >>> What do you think about it?
>>>>> >>>
>>>>>

Mime
View raw message