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 > > > > > >
