Hi Wes, Yes, I agree that we should do that, but then we have a problem of what to do in the other direction, i.e. when we use the new logical types API to read a TIMESTAMP_MILLIS or TIMESTAMP_MICROS, how should we set the UTC normalized flag? Tim has started a discussion about that, suggesting to use three states that I just answered.
Br, Zoltan On Wed, Jul 10, 2019 at 3:52 PM Wes McKinney <[email protected]> wrote: > Thank for the comments. > > So in summary I think that we need to set the TIMESTAMP_* converted > types to maintain forward compatibility and stay consistent with what > we were doing in the C++ library prior to the introduction of the > LogicalType metadata. > > On Wed, Jul 10, 2019 at 8:20 AM Zoltan Ivanfi <[email protected]> > wrote: > > > > Hi Wes, > > > > Both of the semantics are deterministic in one aspect and indeterministic > > in another. Timestamps of instant semantic will always refer to the same > > instant, but their user-facing representation (how they get displayed) > > depends on the user's time zone. Timestamps of local semantics always > have > > the same user-facing representation but the instant they refer to is > > undefined (or ambigous, depending on your point of view). > > > > My understanding is that Spark uses instant semantics, i.e., timestamps > are > > stored normalized to UTC and are displayed adjusted to the user's local > > time zone. > > > > Br, > > > > Zoltan > > > > On Tue, Jul 9, 2019 at 7:04 PM Wes McKinney <[email protected]> wrote: > > > > > Thanks Zoltan. > > > > > > This is definitely a tricky issue. > > > > > > Spark's application of localtime semantics to timestamp data has been > > > a source of issues for many people. Personally I don't find that > > > behavior to be particularly helpful since depending on the session > > > time zone, you will get different results on data not marked as > > > UTC-normalized. > > > > > > In pandas, as contrast, we apply UTC semantics to > > > naive/not-explicitly-normalized data so at least the code produces > > > deterministic results on all environments. That seems strictly better > > > to me -- if you want a localized interpretation of naive data, you > > > must opt into this rather than having it automatically selected based > > > on your locale. The instances of people shooting their toes off due to > > > time zones are practically non-existent, whereas I'm hearing about > > > Spark gotchas all the time. > > > > > > On Tue, Jul 9, 2019 at 11:34 AM Zoltan Ivanfi <[email protected] > > > > > wrote: > > > > > > > > Hi Wes, > > > > > > > > The rules for TIMESTAMP forward-compatibility were created based on > the > > > > assumption that TIMESTAMP_MILLIS and TIMESTAMP_MICROS have only been > used > > > > in the instant aka. UTC-normalized semantics so far. This assumption > was > > > > supported by two sources: > > > > > > > > 1. The specification: parquet-format defined TIMESTAMP_MILLIS and > > > > TIMESTAMP_MICROS as the number of milli/microseconds elapsed since > the > > > Unix > > > > epoch, an instant specified in UTC, from which it follows that they > have > > > > instant semantics (because timestamps of local semantics do not > > > correspond > > > > to a single instant). > > > > > > > > 2. Anecdotal knowledge: We were not aware of any software component > that > > > > used these types differently from the specification. > > > > > > > > Based on your e-mail, we were wrong on #2. > > > > > > > > From this false premise it followed that TIMESTAMPs with local > semantics > > > > were a new type and did not need to be annotated with the old types > to > > > > maintain compatibility. In fact, annotating them with the old types > were > > > > considered to be harmful, since it would have mislead older readers > into > > > > thinking that they can read TIMESTAMPs with local semantics, when in > > > > reality they would have misinterpreted them as TIMESTAMPs with > instant > > > > semantics. This would have lead to a difference of several hours, > > > > corresponding to the time zone offset. > > > > > > > > In the light of your e-mail, this misinterpretation of timestamps may > > > > already be happening, since if Arrow annotates local timestamps with > > > > TIMESTAMP_MILLIS or TIMESTMAP_MICROS, Spark probably misinterprets > them > > > as > > > > timestamps with instant semantics, leading to a difference of several > > > hours. > > > > > > > > Based on this, I think it would make sense from Arrow's point of > view to > > > > annotate both semantics with the old types, since that is its > historical > > > > behaviour and keeping it up is needed for maintaining compatibilty. > I'm > > > not > > > > so sure about the Java library though, since as far as I know, these > > > types > > > > were never used in the local sense there (although I may be wrong > again). > > > > Were we to decide that Arrow and parquet-mr should behave > differently in > > > > this aspect though, it may be tricky to convey this distinction in > the > > > > specification. I would be interested in hearing your and other > > > developers' > > > > opinions on this. > > > > > > > > Thanks, > > > > > > > > Zoltan > > > > > > > > On Tue, Jul 9, 2019 at 5:39 PM Wes McKinney <[email protected]> > wrote: > > > > > > > > > hi folks, > > > > > > > > > > We have just recently implemented the new LogicalType unions in the > > > > > Parquet C++ library and we have run into a forward compatibility > > > > > problem with reader versions prior to this implementation. > > > > > > > > > > To recap the issue, prior to the introduction of LogicalType, the > > > > > Parquet format had no explicit notion of time zones or UTC > > > > > normalization. The new TimestampType provides a flag to indicate > > > > > UTC-normalization > > > > > > > > > > struct TimestampType { > > > > > 1: required bool isAdjustedToUTC > > > > > 2: required TimeUnit unit > > > > > } > > > > > > > > > > When using this new type, the ConvertedType field must also be set > for > > > > > forward compatibility (so that old readers can still understand the > > > > > data), but parquet.thrift says > > > > > > > > > > // use ConvertedType TIMESTAMP_MICROS for > TIMESTAMP(isAdjustedToUTC = > > > > > true, unit = MICROS) > > > > > // use ConvertedType TIMESTAMP_MILLIS for > TIMESTAMP(isAdjustedToUTC = > > > > > true, unit = MILLIS) > > > > > 8: TimestampType TIMESTAMP > > > > > > > > > > In Apache Arrow, we have 2 varieties of timestamps: > > > > > > > > > > * Timestamp without time zone (no UTC normalization indicated) > > > > > * Timestamp with time zone (values UTC-normalized) > > > > > > > > > > Prior to the introduction of LogicalType, we would set either > > > > > TIMESTAMP_MILLIS or TIMESTAMP_MICROS unconditional on UTC > > > > > normalization. So when reading the data back, any notion of having > had > > > > > a time zone is lost (it could be stored in schema metadata if > > > > > desired). > > > > > > > > > > I believe that setting the TIMESTAMP_* ConvertedType _only_ when > > > > > isAdjustedToUTC is true creates a forward compatibility break in > this > > > > > regard. This was reported to us shortly after releasing Apache > Arrow > > > > > 0.14.0: > > > > > > > > > > https://issues.apache.org/jira/browse/ARROW-5878 > > > > > > > > > > We are discussing setting the ConvertedType unconditionally in > > > > > > > > > > https://github.com/apache/arrow/pull/4825 > > > > > > > > > > This might need to be a setting that is toggled when data is coming > > > > > from Arrow, but I wonder if the text in parquet.thrift is the > intended > > > > > forward compatibility interpretation, and if not should we amend. > > > > > > > > > > Thanks, > > > > > Wes > > > > > > > > >
