I was traveling today but I can do a hangout about this next week.
On Thu, Apr 28, 2016 at 7:53 PM, Jacques Nadeau <jacq...@apache.org> wrote: > Let's do a quick hangout on this. I'd like to better understand as I'm not > sure we're all talking about the same thing. > > On Thu, Apr 28, 2016 at 5:30 PM, Micah Kornfield <emkornfi...@gmail.com> > wrote: > >> I'm -1 on making a new primitive type in the memory layout spec [1]. >> >> +1 on clarifying [2], to indicate it is expected that the "Values >> array" for Utf8 and Binary types should never contain null elements. >> >> [1] https://github.com/apache/arrow/blob/master/format/Layout.md >> [2] https://github.com/apache/arrow/blob/master/format/Message.fbs >> >> On Thu, Apr 28, 2016 at 3:08 PM, Wes McKinney <w...@cloudera.com> wrote: >> > Bumping this conversation. >> > >> > I'm +0 on making VARBINARY and String (identical VARBINARY but with a >> > UTF8 guarantee) primitive types in the spec. Let me know what others >> > think. >> > >> > Thanks >> > >> > On Fri, Apr 22, 2016 at 6:30 PM, Wes McKinney <w...@cloudera.com> wrote: >> >> On Fri, Apr 22, 2016 at 6:06 PM, Jacques Nadeau <jacq...@apache.org> >> wrote: >> >>> On Fri, Apr 22, 2016 at 2:42 PM, Wes McKinney <w...@cloudera.com> >> wrote: >> >>> >> >>>> On Fri, Apr 22, 2016 at 4:56 PM, Micah Kornfield < >> emkornfi...@gmail.com> >> >>>> wrote: >> >>>> > I like the current scheme of making String (UTF8) a primitive type >> in >> >>>> > regards to RPC but not modeling it as a special Array type. I think >> >>>> > the key is formally describing how logical types map to physical >> types >> >>>> > either is the Flatbuffer schema or in a separate document. >> >>>> > >> >>>> > I think there are two use-cases here: >> >>>> > 1. Reconstructing Array's off the wire. >> >>>> > 2. Writing algorithms/builders to deal with specific logical types >> >>>> > built on Arrays. >> >>>> > >> >>>> > For case 1, I think it is simpler to not special case string types >> as >> >>>> > primitives. Understanding that a logical String type maps to a >> >>>> > List<Utf8> should be sufficient and allows us to re-use the >> >>>> > serialization code for ListArrays for these types. >> >>>> > >> >>>> >> >>>> It is simpler for the IPC serde code-path. I'll let Jacques comment >> >>>> but one downside of having strings as a nested type is that there are >> >>>> certain code paths (for example: Parquet-related) which deal with the >> >>>> flat table case. To make a Parquet analogy, there is the special >> >>>> BYTE_ARRAY primitive type, even though you could technically represent >> >>>> variable-length binary data using a repeated field and using >> >>>> repetition/definition levels (but the encoding/decoding overhead for >> >>>> this in Parquet is much more significant than Arrow). There may be >> >>>> other reasons. >> >>>> >> >>> >> >>> I'm a bit confused about what everyone means. I didn't actually realize >> >>> that this [1] had been merged yet but I'm generally on board with how >> it is >> >>> constructed. >> >>> >> >>> With regards to the c++ implementation of the items at [1], abstracting >> >>> shared physical representations out seems fine to me but I don't think >> we >> >>> should necessitate effective 3NF for [1]. >> >>> >> >>> One of the key points that I'm focused on in the Java space is that I'd >> >>> like to move to an always nullable pattern. This is vastly simplifying >> from >> >>> a code generation, casting and complexity perspective and is a nominal >> cost >> >>> when using column execution. If binary and varchar are primitive types >> as >> >>> there there is no weird special casing of avoiding the nullability >> bitmap >> >>> in the case of variable width items (for the offsets). But that is an >> >>> implementation detail of the Java library. >> >>> >> >>> So in general, I like the scheme at [1] for the concepts that we all >> are >> >>> talking about (as opposed to eliminating lines 67 & 68) >> >>> >> >>> [1] https://github.com/apache/arrow/blob/master/format/Message.fbs >> >>> >> >> >> >> Well, the issue is that mapping of metadata onto memory layout for IPC >> >> purposes, at least. You can use the List code path for arbitrary List >> >> types as well as strings and binary. It sounds like either way on the >> >> Java side you're going to collapse UTF8 / BINARY into a primitive so >> >> that you don't have to manage a separate never-used bitmap for the >> >> string/binary data. It seems useful enough to me to have a primitive >> >> variable-length binary/UTF8 type but I do not feel strongly about it. >> >> >> >>> >> >>> >> >>>> > For case 2, it would be nice to utilize the type system of the host >> >>>> > programming language to express the semantics of a function call >> (e.g. >> >>>> > ParseString(StringArray strings) vs ParseString(ListArray strings), >> >>>> > but I think this can be implemented without requiring a new >> primitive >> >>>> > type in the spec. >> >>>> > >> >>>> > The more interesting thing to me is if we should have a new >> primitive >> >>>> > type for fixed length lists (e.g. the logical type CHAR). The >> >>>> > offsets array isn't necessary in this case for random access. >> >>>> > >> >>>> > Also, the way the VARCHAR types (based on a comment in the C++ >> >>>> > ( >> https://github.com/apache/arrow/blob/master/cpp/src/arrow/type.h#L63) >> >>>> > are currently described as a null terminated UTF8 is problematic. I >> >>>> > believe null bytes are valid UTF8 characters. >> >>>> >> >>>> >> >>>> > >> >>>> >> >>>> Good point, sorry about that. We probably would need to length-prefix >> >>>> the values, then. >> >>>> >> >>> >> >>> >> >>> Is this an input/output interface? Arrow structures should all be 4 >> byte >> >>> offset based and be neither length prefixed nor null terminated. >> >> >> >> This was a question around the VARCHAR(k) type (which in many >> >> databases is distinct from a TEXT type in which any value can be >> >> arbitrary length). So if you have a VARCHAR(50), you guarantee that no >> >> value exceeds 50 characters. In Arrow I suppose this is just metadata >> >> because you have the offsets encoding length (pardon the jet lag). >> >> Micah -- I think we can nix the `VarcharType` in the C++ code, >> >> leftovers from my earliest draft implementation. >> >> >> >> - Wes >>
