hello, For the current iteration of Arrow, can we agree to support int64 UNIX timestamps with a particular resolution (second through nanosecond), as these are reasonably common representations? We can look to expand later if it is needed.
Thanks Wes On Mon, Aug 15, 2016 at 4:12 AM, Wes McKinney <wesmck...@gmail.com> wrote: > Bumping this discussion. As part of finalizing a v1 Arrow spec (for > purposes of moving data between systems, at minimum) we should propose > timestamp metadata and physical memory representation that maximizes > interoperability with other systems. It seems like a fixed decimal > would meet this requirement as UNIX-like timestamps at some resolution > could pass unmodified with appropriate metadata. > > We will also need decimal types in Arrow (at least to accommodate > common database representations and file formats like Parquet), so > this seems like a reasonable potential hierarchy of types: > > Timestamp [logical type] > extends FixedDecimal [logical type] > extends FixedWidth [physical type] > > I did a bit of internet searching but did not find a canonical > reference or implementation of fixed decimals; that would be helpful. > > As an aside: for floating decimal numbers for numerical data we could > utilize an implementation like http://www.bytereef.org/mpdecimal/ > which implements the spec described at > http://speleotrove.com/decimal/decarith.html > > Thanks > Wes > > On Thu, Jul 14, 2016 at 8:18 AM, Alex Samuel <a...@alexsamuel.net> wrote: >> Hi all, >> >> May I suggest that instead of fixed-point decimals, you consider a more >> general fixed-denominator rational representation, for times and other >> purposes? Powers of ten are convenient for humans, but powers of two more >> efficient. For some applications, the efficiency of bit operations over >> divmod is more useful than an exact representation of integral nanoseconds. >> >> std::chrono takes this approach. I'll also humbly point you at my own >> date/time library, https://github.com/alexhsamuel/cron (incomplete but >> basically working), which may provide ideas or useful code. It was intended >> for precisely this sort of application. >> >> Regards, >> Alex >> >> >> On Thu, Jul 14, 2016 at 10:27 AM Uwe Korn <uw...@xhochy.com> wrote: >> >>> I agree with that having a Decimal type for timestamps is a nice >>> definition. Haying your time encoded as seconds or nanoseconds should be >>> the same as having a scale of the respective amount. But I would rather >>> avoid having a separate decimal physical type. Therefore I'd prefer the >>> parquet approach where decimal is only a logical type and backed by >>> either a bytearray, int32 or int64. >>> >>> Thus a more general timestamp could look like: >>> >>> * Decimals are logical types, physical types are the same as defined in >>> Parquet [1] >>> * Base unit for timestamps is seconds, you can get milliseconds and >>> nanoseconds by using a different scale. .(Note that seconds and so on >>> are all powers of ten, thus matching the specification of decimal scale >>> really good). >>> * Timestamp is just another logical type that is referring to Decimal >>> (and optionally may have a timezone) and signalling that we have a Time >>> and not just a "simple" decimal. >>> * For a first iteration, I would assume no timezone or UTC but not >>> include a metadata field. Once we're sure the implementation works, we >>> can add metadata about it. >>> >>> Timedeltas could be addressed in a similar way, just without the need >>> for a timezone. >>> >>> For my usages, I don't have the use-case for a larger than int64 >>> timestamp and would like to have it exactly as such in my computation, >>> thus my preference for the Parquet way. >>> >>> Uwe >>> >>> [1] >>> >>> https://github.com/apache/parquet-format/blob/master/LogicalTypes.md#decimal >>> >>> On 13.07.16 03:06, Julian Hyde wrote: >>> > I'm talking about a fixed decimal type, not floating decimal. (Oracle >>> > numbers are floating decimal. They have a few nice properties, but >>> > they are variable width and can get quite large. I've seen one or two >>> > systems that started with binary flo >> >> >>> * Base unit for timestamps is seconds, you can get milliseconds and >> >> nanoseconds by using a different scale. .(Note that seconds and so on >> >> are all powers of ten, thus matching the specification of decimal scale >> >> really good). >> >> * Timestamp is just another logical type that is referring to Decimal >> >> (and optionally may have a timezone) and signalling that we have a Tim >> >> ating point numbers, which are >>> > much worse for business computing, and then change to Java BigDecimal, >>> > which gives the right answer but are horribly inefficient.) >>> > >>> > A fixed decimal type has virtually zero computational overhead. It >>> > just has a piece of metadata saying something like "every value in >>> > this field is multiplied by 1 million" and leaves it to the client >>> > program to do that multiplying. >>> > >>> > My advice is to create a good fixed decimal type and lean on it heavily. >>> > >>> > Julian >>> > >>> >>>
