+1 On Thu, Sep 29, 2016 at 3:19 PM, Wes McKinney <wesmck...@gmail.com> wrote:
> 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 > >>> > > >>> > >>> >
