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 >
