I should also add that we could (with some effort) use the MetadataVersion V4/V5 indicator to offer backward compatibility for old serialized union data
In any case, if there is consensus about this, we would need to have a vote and get busy with implementing and testing the changes. I could assist with the C++ changes On Wed, Jun 24, 2020 at 1:14 PM Wes McKinney <wesmck...@gmail.com> wrote: > > On Wed, Jun 24, 2020 at 1:07 PM Francois Saint-Jacques > <fsaintjacq...@gmail.com> wrote: > > > > OTOH, > > > > how do we handle NullType -> UnionType<T...> cast conversion? Do we > > require some convention like the first children ArrayData null bitmap > > to be set and all tags set to 0? > > Sure, that sounds like a reasonable implementation should this > operation actually be required. > > > François > > > > On Wed, Jun 24, 2020 at 1:09 PM Antoine Pitrou <anto...@python.org> wrote: > > > > > > > > > Le 24/06/2020 à 18:34, Wes McKinney a écrit : > > > > On Wed, Jun 24, 2020 at 11:08 AM Antoine Pitrou <anto...@python.org> > > > > wrote: > > > >> > > > >> > > > >> Le 24/06/2020 à 16:57, Wes McKinney a écrit : > > > >>> hi folks, > > > >>> > > > >>> As discussed on the recent GitHub PR [1], as a means of reconciling > > > >>> the long-standing cross-implementation incompatibilities with Union > > > >>> types, it's been proposed to remove the top-level validity bitmap from > > > >>> the Union data layout and let validity be determined exclusively by > > > >>> the child arrays of the union. So the only additional data needed to > > > >>> form a union are the type ids (and for the dense union, the offsets). > > > >>> > > > >>> I do not think this change meaningfully alters the semantics of Union > > > >>> types and I think it also simplifies their construction, so I would be > > > >>> in favor of making it for 1.0.0. > > > >> > > > >> So it sounds like this may break compatibility with existing only uses > > > >> of Arrow C++ (and the relevant bindings: PyArrow, Arrow C/GLib, Red > > > >> Arrow); not only on the API side, but on the data side. > > > > > > > > Right. However, I don't think these changes will be very disruptive, > > > > and we always knew that this disruption was possible because of the > > > > hitherto unreconciled issues with Unions. The applications that I'm > > > > aware of that use Union serialization (e.g. Ray) use it only for > > > > ephemeral serialization. > > > > > > Ok, that's a convincing argument. > > > > > > > In general, I think that we should be bumping the metadata version [1] > > > > for 1.0.0 to create a forcing function for upgrade to the > > > > format-stable line of libraries. The C++/Python libraries could have a > > > > "compatibility mode" (like the "write_legacy_ipc_format" options) that > > > > writes MetadataVersion::V4 (v0.8.0 -> v0.17.1) with certain features > > > > (like unions -- which are not needed for Spark for example) disabled. > > > > > > Hmm, I hope we can keep the negotiation minimal. We should take from > > > the Jon Postel principle - be liberal in what you accept, strict in what > > > you emit. > > > > > > So the IPC reader can have a simple detection that goes this way: > > > > > > * if we receive 1 buffer for sparse union or 2 buffers for dense union > > > => it's the new-style format, there's nothing to do > > > > > > * if we receive 2 (non-null) buffers for sparse union or 3 (non-null) > > > buffers for dense union > > > => it's the old format, we should AND the parent bitmap into each of the > > > child bitmaps > > > > > > We can also add a flag to IpcOptions to enable/disable compatibility > > > tricks. > > > > > > Regards > > > > > > Antoine.
