Looking more at the Postgres spec and storage details, I'd be supportive of having a COMPLEX interval type which could be a packed type (possibly using the same 16-byte storage layout as Postgres -- depending on whether this complex interval needs granularity smaller than seconds, more analysis needed). I think that adding an entry to the IntervalUnit enum doesn't pose any forward compatibility problems (because implementations _should_ be able to recognized an unsupported unit?). It isn't wonderful to have 3 different types of intervals (ideally just SIMPLE and COMPLEX), but it seems like something we can live with. If we're going to do this, we should try to do it properly now since I would probably object at adding a 4th interval type if the need for it came up in the future.
On Fri, Apr 2, 2021 at 5:37 PM Micah Kornfield <emkornfi...@gmail.com> wrote: > > > > > However it seems a little unfortunate that there is now way to represent a > > "common" interval like "1 week and 1 hour" with native arrow types > > > I might have misunderstood,but at least in postgres, I thought this boils > down to "0 months, 7 days, 3600 seconds". Since months is 0, this seems > like it fits squarely in the existing interval type Days_Mills. > > I thought what can't be represented today is "1 Year 1 Hour". It seems > like none of the proposals so far cover weeks as an explicit type? > > On Fri, Apr 2, 2021 at 2:42 PM Andrew Lamb <al...@influxdata.com> wrote: > > > I think it is plausible that we use Arrow structs to create a synthetic > > interval type for DataFusion (I don't have a compelling usecase to store > > the intervals themselves, or to expose them outside of DataFusion). > > > > However it seems a little unfortunate that there is now way to represent a > > "common" interval like "1 week and 1 hour" with native arrow types > > > > > > > > On Fri, Apr 2, 2021 at 4:38 PM Micah Kornfield <emkornfi...@gmail.com> > > wrote: > > > >> The real usecase I have is "postgres compatibility" > >> > >> > >> Yeah, I'm a little conflicted on this. A broader analysis might be > >> necessary and I'd welcome others thoughts, but at what point should we > >> mostly consider the type system closed? Should we be aiming for full > >> parity with ANSI SQL/Postgres SQL or something else? > >> > >> > >>> I have no known need for the actual postgres timestamp internal > >>> representation. > >> > >> > >> I suppose there is an edge case that the seconds range is larger for > >> microseconds compared to nanoseconds with the simple representation. But > >> that seems minor. > >> > >> On Fri, Apr 2, 2021 at 1:25 PM Andrew Lamb <al...@influxdata.com> wrote: > >> > >>> The real usecase I have is "postgres compatibility" - in the sense that > >>> we can write SQL queries / expressions that use postgres interval type [1] > >>> and corresponding expressions with the full postgres interval range. I > >>> have > >>> no known need for the actual postgres timestamp internal representation. > >>> > >>> A more vague usecase I would like (but do not strictly need) is for > >>> expressing intervals up to the same precision as IOx's underlying > >>> timestamps (which are stored as nanosecond precision). > >>> > >>> [1] > >>> https://www.postgresql.org/docs/current/datatype-datetime.html#DATATYPE-INTERVAL-INPUT > >>> > >>> "Internally interval values are stored as months, days, and seconds. > >>> This is done because the number of days in a month varies, and a day can > >>> have 23 or 25 hours if a daylight savings time adjustment is involved. The > >>> months and days fields are integers while the seconds field can store > >>> fractions. Because intervals are usually created from constant strings or > >>> timestamp subtraction, this storage method works well in most cases, > >>> but can cause unexpected results: > >>> " > >>> > >>> On Fri, Apr 2, 2021 at 3:53 PM Micah Kornfield <emkornfi...@gmail.com> > >>> wrote: > >>> > >>>> Andrew is the use-case you have simply postgres compatibility or is it > >>>> more > >>>> extensive? > >>>> > >>>> One potential problem with combining Month and Day fields, is that the > >>>> type > >>>> no longer has a defined sort order (the existing Day-Millisecond type > >>>> without assumptions, in particular because I don't think today there is > >>>> an > >>>> explicit constraint on the bounds for the millisecond component). > >>>> > >>>> -Micah > >>>> > >>>> > >>>> > >>>> On Wed, Mar 31, 2021 at 9:03 AM Antoine Pitrou <anto...@python.org> > >>>> wrote: > >>>> > >>>> > > >>>> > Le 31/03/2021 à 17:55, Micah Kornfield a écrit : > >>>> > > Thanks for the feedback. A couple of points here and some responses > >>>> > below. > >>>> > > > >>>> > > * One other question is whether the Nanoseconds should actually be > >>>> > > configurable (i.e. use milliseconds or microseconds). I would lean > >>>> > towards > >>>> > > no. > >>>> > > >>>> > Same for me. > >>>> > > >>>> > > * I'm also still not 100% convinced we need this as a first class > >>>> type in > >>>> > > arrow or if we should be looking more closely at the Struct (in the > >>>> Arrow > >>>> > > sense) based implementation. In the future where alternative > >>>> encodings > >>>> > are > >>>> > > supported, this could allow for much smaller footprints for this > >>>> type. > >>>> > > >>>> > Having a "packed" first class type allows for better locality when > >>>> > accessing data. It doesn't sound very likely that you'd access only > >>>> one > >>>> > component of the interval. > >>>> > > >>>> > But I have no idea how important this is, and temporal datetypes are > >>>> > generally cumbersome to add support for (conversions, arithmetic, > >>>> etc.), > >>>> > so it would be nice to avoid adding too many of them :-) > >>>> > > >>>> > Regards > >>>> > > >>>> > Antoine. > >>>> > > >>>> > > >>>> > > >>>> > > > >>>> > > The 3 > >>>> > >> field implementation doesn't seem to have any way to represent > >>>> integral > >>>> > >> days, so I am also not sure about that one. > >>>> > > > >>>> > > > >>>> > > Sorry this was an email gaffe. I intended Month (32 bit int), Day > >>>> (32 > >>>> > bit > >>>> > > int), Nanosecond (64 bit int). > >>>> > > > >>>> > > OTOH I don't really understand the point of supporting "the most > >>>> > >> reasonable ranges for Year, Month and Nanoseconds independently". > >>>> What > >>>> > >> does it bring to encode more than one month in the nanoseconds > >>>> field? > >>>> > > > >>>> > > > >>>> > > I'm happy with simplicity. In the past there has been some > >>>> reference to > >>>> > > people wanting to store very large timestamps (fall out of > >>>> Nanoseconds > >>>> > max > >>>> > > representable value) but we've concluded that this wasn't something > >>>> that > >>>> > we > >>>> > > wanted to really support. > >>>> > > > >>>> > > > >>>> > > > >>>> > > > >>>> > > > >>>> > > > >>>> > > On Wed, Mar 31, 2021 at 4:49 AM Antoine Pitrou <anto...@python.org> > >>>> > wrote: > >>>> > > > >>>> > >> > >>>> > >> I would favour the following characteristics : > >>>> > >> - support for nanoseconds (especially as other Arrow temporal types > >>>> > >> support it) > >>>> > >> - easy to handle (which excludes the ZetaSQL representtaion IMHO) > >>>> > >> > >>>> > >> OTOH I don't really understand the point of supporting "the most > >>>> > >> reasonable ranges for Year, Month and Nanoseconds independently". > >>>> What > >>>> > >> does it bring to encode more than one month in the nanoseconds > >>>> field? > >>>> > >> You can already use the Duration type for that. > >>>> > >> > >>>> > >> Regards > >>>> > >> > >>>> > >> Antoine. > >>>> > >> > >>>> > >> > >>>> > >> Le 31/03/2021 à 05:48, Micah Kornfield a écrit : > >>>> > >>> To follow-up on this conversation I did some analysis on interval > >>>> > types: > >>>> > >>> > >>>> > >>> > >>>> > >> > >>>> > > >>>> https://docs.google.com/document/d/1i1E_fdQ_xODZcAhsV11Pfq27O50k679OYHXFJpm9NS0/edit > >>>> > >> Please feel free to add more details/systems I missed. > >>>> > >>> > >>>> > >>> Given the disparate requirements of different systems I think the > >>>> > >> following might make sense for official types (if there isn't > >>>> > consensus, I > >>>> > >> might try to contributation extension Array implementations for > >>>> them to > >>>> > >> Java and C++/Python separately). > >>>> > >>> > >>>> > >>> 1. 3 fields: Year (32 bit), Month (32 bit), Nanoseconds (64 bit) > >>>> all > >>>> > >> signed. > >>>> > >>> 2. Postgres representation (Downside is it doesn't support > >>>> > Nanoseconds, > >>>> > >> only microseconds). > >>>> > >>> 3. ZetaSQL implementation (Requires some bit manipulation) but > >>>> > supports > >>>> > >> the most reasonable ranges for Year, Month and Nanoseconds > >>>> > independently. > >>>> > >>> > >>>> > >>> Thoughts? > >>>> > >>> > >>>> > >>> Micah > >>>> > >>> > >>>> > >>> On 2021/02/18 04:30:55 Micah Kornfield wrote: > >>>> > >>>>> > >>>> > >>>>> I didn’t find any page/documentation on how to do RFC in Arrow > >>>> > >> protocol, > >>>> > >>>>> so can anyone point me to it or PR with email will be enough? > >>>> > >>>> > >>>> > >>>> That is enough to start discussion. Before formal acceptance and > >>>> > >> merging > >>>> > >>>> of the PR there needs to be a Java and C++ implementations for > >>>> the > >>>> > type > >>>> > >>>> that pass integration tests. At the time this guideline was > >>>> > instituted > >>>> > >>>> Java and C++ were considered the "reference" implementations (I > >>>> think > >>>> > >> they > >>>> > >>>> still have the most complete integration test coverage). > >>>> > >>>> > >>>> > >>>> My understanding is that the current modelling of intervals > >>>> mimics SQL > >>>> > >>>> standards (e.g. SQL Server [1]). So it would also be good to > >>>> step > >>>> > back > >>>> > >> and > >>>> > >>>> understand what problem DF is trying to solve and how it differs > >>>> from > >>>> > >> other > >>>> > >>>> SQL implementations. I'd be hesitant to accept COMPLEX as a new > >>>> type > >>>> > >>>> without a much deeper analysis into calendar representations > >>>> within > >>>> > >> Arrow > >>>> > >>>> and how they relate to other existing systems (e.g. Hive and some > >>>> > >>>> assortment of existing SQL databases). For instance the current > >>>> > >> modelling > >>>> > >>>> of timestamps does not lend itself to constructing a COMPLEX > >>>> interval > >>>> > >> type > >>>> > >>>> particularly well. (Duration was introduced for this reason). > >>>> > >>>> > >>>> > >>>> I think both Wes's suggestion of FixedSizeBinary and Andrew's of > >>>> > >> composing > >>>> > >>>> the with a struct are good stop-gaps. These obviously have > >>>> different > >>>> > >>>> trade-offs. Ultimately, it would be good to define common > >>>> extension > >>>> > >> types > >>>> > >>>> that can represent this use-case if there really is demand for > >>>> it (if > >>>> > it > >>>> > >>>> doesn't become a top level type). > >>>> > >>>> > >>>> > >>>> [1] > >>>> > >>>> > >>>> > >> > >>>> > > >>>> https://docs.microsoft.com/en-us/sql/odbc/reference/appendixes/interval-data-types?view=sql-server-ver15 > >>>> > >>>> > >>>> > >>>> -Micah > >>>> > >>>> > >>>> > >>>> On Wed, Feb 17, 2021 at 2:05 PM Andrew Lamb < > >>>> al...@influxdata.com> > >>>> > >> wrote: > >>>> > >>>> > >>>> > >>>>> That is a great suggestion Wes, thank you. > >>>> > >>>>> > >>>> > >>>>> I wonder if we could get away with a 128 bit representation > >>>> that is > >>>> > the > >>>> > >>>>> concatenation of the two existing interval types > >>>> > (YearMonth)(DayTime). > >>>> > >> Or > >>>> > >>>>> maybe even define a `struct` type with those fields that is > >>>> used by > >>>> > >>>>> DataFusion. > >>>> > >>>>> > >>>> > >>>>> Basically, given our reading of the Arrow spec[1], it is > >>>> currently > >>>> > not > >>>> > >>>>> possible to precisely represent an interval that has both > >>>> monthly and > >>>> > >>>>> sub-montly granularity. > >>>> > >>>>> > >>>> > >>>>> As Dmtry says, if you have an interval seemingly simple like 1 > >>>> > month, > >>>> > >> 1 > >>>> > >>>>> day > >>>> > >>>>> > >>>> > >>>>> Using IntervalUnit(YEAR_MONTH) can't represent the 1 day > >>>> > >>>>> Using IntervalUnit(DAY_TIME) can't represent the month as > >>>> different > >>>> > >> months > >>>> > >>>>> have different numbers of days > >>>> > >>>>> > >>>> > >>>>> [1] > >>>> > >>>>> > >>>> > >> > >>>> https://github.com/apache/arrow/blob/master/format/Schema.fbs#L249-L260 > >>>> > >>>>> > >>>> > >>>>> > >>>> > >>>>> On Wed, Feb 17, 2021 at 5:01 PM Wes McKinney < > >>>> wesmck...@gmail.com> > >>>> > >> wrote: > >>>> > >>>>> > >>>> > >>>>>> On Wed, Feb 17, 2021 at 3:46 PM <t...@dmtry.me> wrote: > >>>> > >>>>>>> > >>>> > >>>>>>>> It's unclear to me that this needs to be introduced into the > >>>> > >>>>> top-level > >>>> > >>>>>>> > >>>> > >>>>>>> Similar thing to columnar format, How to store interval like 1 > >>>> > month > >>>> > >> 1 > >>>> > >>>>>> day 1 hour? It’s not possible to do it without converting 1 > >>>> month to > >>>> > >> 30 > >>>> > >>>>>> days, which is a bad way. > >>>> > >>>>>>> > >>>> > >>>>>> > >>>> > >>>>>> Presumably you can represent a complex interval in a fixed > >>>> number of > >>>> > >>>>>> bytes, and then embed the data in a FixedSizeBinary type. You > >>>> can > >>>> > >>>>>> adorn this type with extension type metadata so that > >>>> DataFusion can > >>>> > >>>>>> then apply Interval semantics to it. This could also serve as > >>>> an > >>>> > >>>>>> interim strategy for you to proceed with implementation while > >>>> > >>>>>> proposing a top-level type to the Arrow format (which may or > >>>> may not > >>>> > >>>>>> be accepting) so you aren't blocked on acceptance of changes > >>>> into > >>>> > >>>>>> Schema.fbs. > >>>> > >>>>>> > >>>> > >>>>>>>> On 17 Feb 2021, at 21:02, Wes McKinney <wesmck...@gmail.com> > >>>> > wrote: > >>>> > >>>>>>>> > >>>> > >>>>>>>> It's unclear to me that this needs to be introduced into the > >>>> > >>>>> top-level > >>>> > >>>>>>>> columnar format without more analysis — have you considered > >>>> > >>>>>>>> implementing this for DataFusion as an extension type for > >>>> the time > >>>> > >>>>>>>> being? > >>>> > >>>>>>>> > >>>> > >>>>>>>> On Wed, Feb 17, 2021 at 11:59 AM t...@dmtry.me <mailto: > >>>> > >> t...@dmtry.me > >>>> > >>>>>> > >>>> > >>>>>> <t...@dmtry.me <mailto:t...@dmtry.me>> wrote: > >>>> > >>>>>>>>> > >>>> > >>>>>>>>> Hi, > >>>> > >>>>>>>>> > >>>> > >>>>>>>>> For now, There are only two types of IntervalUnit inside > >>>> Arrow: > >>>> > >>>>>>>>> > >>>> > >>>>>>>>> - YearMonth - month stored as int32 > >>>> > >>>>>>>>> - DayTime - days as int32 and time in milliseconds as in32. > >>>> > Total > >>>> > >>>>>> (64 bites) > >>>> > >>>>>>>>> > >>>> > >>>>>>>>> Since DF is using Arrow, It’s not possible to store > >>>> “Complex” > >>>> > >>>>>> intervals such 1 MONTH 1 DAY 1 HOUR. > >>>> > >>>>>>>>> I think, the best way to understand the problem will be to > >>>> read a > >>>> > >>>>>> comment from DF codebase: > >>>> > >>>>>> > >>>> > >>>>> > >>>> > >> > >>>> > > >>>> https://github.com/apache/arrow/blob/bca7d2fe84ccd8fc1129cb4d85448eb0779c52c3/rust/datafusion/src/sql/planner.rs#L1148 > >>>> > >>>>>>>>> > >>>> > >>>>>>>>> // Interval is tricky thing > >>>> > >>>>>>>>> // 1 day is not 24 hours because timezones, 1 year > >>>> != > >>>> > >>>>> 365/364! > >>>> > >>>>>> 30 days != 1 month > >>>> > >>>>>>>>> // The true way to store and calculate intervals > >>>> is to > >>>> > >> store > >>>> > >>>>>> it as it defined > >>>> > >>>>>>>>> // Due the fact that Arrow supports only two types > >>>> > >> YearMonth > >>>> > >>>>>> (month) and DayTime (day, time) > >>>> > >>>>>>>>> // It's not possible to store complex intervals > >>>> > >>>>>>>>> // It's possible to do select (NOW() + INTERVAL '1 > >>>> > year') + > >>>> > >>>>>> INTERVAL '1 day'; as workaround > >>>> > >>>>>>>>> if result_month != 0 && (result_days != 0 || > >>>> > result_millis > >>>> > >> != > >>>> > >>>>>> 0) { > >>>> > >>>>>>>>> return > >>>> Err(DataFusionError::NotImplemented(format!( > >>>> > >>>>>>>>> "DF does not support intervals that have > >>>> both a > >>>> > >>>>>> Year/Month part as well as Days/Hours/Mins/Seconds: {:?}. > >>>> Hint: try > >>>> > >>>>>> breaking the interval into two parts, one with Year/Month and > >>>> the > >>>> > >> other > >>>> > >>>>>> with Days/Hours/Mins/Seconds - e.g. (NOW() + INTERVAL '1 > >>>> year') + > >>>> > >>>>> INTERVAL > >>>> > >>>>>> '1 day'", > >>>> > >>>>>>>>> value > >>>> > >>>>>>>>> ))); > >>>> > >>>>>>>>> } > >>>> > >>>>>>>>> > >>>> > >>>>>>>>> > >>>> > >>>>>>>>> > >>>> > >>>>>>>>> I prepared a PR > >>>> https://github.com/apache/arrow/pull/9516/files > >>>> > < > >>>> > >>>>>> https://github.com/apache/arrow/pull/9516/files> < > >>>> > >>>>>> https://github.com/apache/arrow/pull/9516/files < > >>>> > >>>>>> https://github.com/apache/arrow/pull/9516/files>> that > >>>> introduce a > >>>> > >> new > >>>> > >>>>>> type for IntervalUnit called Complex, that store both > >>>> YearMonth and > >>>> > >>>>> DayTime > >>>> > >>>>>> to support complex interval. > >>>> > >>>>>>>>> I didn’t find any page/documentation on how to do RFC in > >>>> Arrow > >>>> > >>>>>> protocol, so can anyone point me to it or PR with email will be > >>>> > >> enough? > >>>> > >>>>>>>>> > >>>> > >>>>>>>>> Thanks. > >>>> > >>>>>>> > >>>> > >>>>>> > >>>> > >>>>> > >>>> > >>>> > >>>> > >> > >>>> > > > >>>> > > >>>> > >>>
