Thanks Weston. That all sounds reasonable to me. > with the caveat that the primary layout must be emitted if the user does not > specifically request the alternative layout
This implies that each canonical alternative layout would codify a primary layout as its "fallback." This seems reasonable but it opens up some cans of worms, such as how two components communicating through an Arrow interface would negotiate which layout is supported. I suppose such details should be discussed in a separate thread, but I raise this here just to point out that it implies an expansion in the scope of what Arrow interfaces can do. On Tue, Jun 6, 2023 at 6:17 PM Weston Pace <weston.p...@gmail.com> wrote: > > From Micah: > > > This sounds reasonable to me but my main concern is, I'm not sure there is > > a great mechanism to enforce canonical layouts don't somehow become > default > > (or the only implementation). > > I'm not sure I understand. Is the concern that an alternative layout is > eventually > used more and more by implementations until it is used more often than the > primary > layouts? In that case I think that is ok and we can promote the alternative > to a primary layout. > > Or is the concern that some applications will only support the alternative > layouts and > not the primary layout? In that case I would argue the application is not > "arrow compatible". > I don't know that we prevent or enforce this today either. An author can > always falsely > claim they support Arrow even if they are using their own bespoke format. > > From Ian: > > > It seems to me that most projects that are implementing Arrow today > > are not aiming to provide complete coverage of Arrow; rather they are > > adopting Arrow because of its role as a standard and they are > > implementing only as much of the Arrow standard as they require to > > achieve some goal. I believe that such projects are important Arrow > > stakeholders, and I believe that this proposed notion of canonical > > alternative layouts will serve them well and will create efficiencies > > by standardizing implementations around a shared set of alternatives. > > > > However I think that the documentation for canonical alternative > > layouts should strongly encourage implementers to default to using the > > primary layouts defined in the core spec and only use alternative > > layouts in cases where the primary layouts do not meet their needs. > > I'd maybe take a slightly harsher stance. I don't think an application > needs to > support all types. For example, an Arrow-native string processing library > might > not worry about the integer types. That would be fine. I think it would > still > be fair to call it an "arrow compatible string processing library". > > However, an application must support primary layouts in addition to > alternative > layouts. For example, a string processing library that expects all strings > to be > delivered as a single buffer sequence of null-terminated strings would not > be "an > arrow compatible string processing library" unless it also fully supported > the > standard (lengths + data) variable-sized list layout for strings defined at > [1]. > > In other words: > > * Only receives and emits alternative layouts - not arrow compatible > * Only receives and emits primary layouts - arrow compatible > * Receives and emits both primary and alternative layouts - arrow > compatible† > > † - with the caveat that the primary layout must be emitted if the user > does not > specifically request the alternative layout. > > [1] > https://arrow.apache.org/docs/format/Columnar.html#variable-size-list-layout > > On Tue, Jun 6, 2023 at 2:45 PM Ian Cook <ianmc...@apache.org> wrote: > > > To clarify why we cannot simply propose adding ListView as a new > > “canonical extension type”: The extension type mechanism in Arrow > > depends on the underlying data being organized in an existing Arrow > > layout—that way an implementation that does not support the extension > > type can still handle the underlying data. But ListView is a wholly > > new layout. > > > > I strongly agree with Weston’s idea that it is a good time for Arrow > > to introduce the notion of “canonical alternative layouts.” > > > > Taken together, I think that canonical extension types and canonical > > alternative layouts could serve as an “incubator” for proposed new > > representations. For example, if a proposed canonical alternative > > layout ends up being broadly adopted, then that will serve as a signal > > that we should consider adding it as a primary layout in the core > > spec. > > > > It seems to me that most projects that are implementing Arrow today > > are not aiming to provide complete coverage of Arrow; rather they are > > adopting Arrow because of its role as a standard and they are > > implementing only as much of the Arrow standard as they require to > > achieve some goal. I believe that such projects are important Arrow > > stakeholders, and I believe that this proposed notion of canonical > > alternative layouts will serve them well and will create efficiencies > > by standardizing implementations around a shared set of alternatives. > > > > However I think that the documentation for canonical alternative > > layouts should strongly encourage implementers to default to using the > > primary layouts defined in the core spec and only use alternative > > layouts in cases where the primary layouts do not meet their needs. > > > > > > On Sat, May 27, 2023 at 7:44 PM Micah Kornfield <emkornfi...@gmail.com> > > wrote: > > > > > > This sounds reasonable to me but my main concern is, I'm not sure there > > is > > > a great mechanism to enforce canonical layouts don't somehow become > > default > > > (or the only implementation). > > > > > > Even for these new layouts, I think it might be worth rethinking binding > > a > > > layout into the schema versus having a different concept of encoding (and > > > changing some of the corresponding data structures). > > > > > > > > > On Mon, May 22, 2023 at 10:37 AM Weston Pace <weston.p...@gmail.com> > > wrote: > > > > > > > Trying to settle on one option is a fruitless endeavor. Each type has > > pros > > > > and cons. I would also predict that the largest existing usage of > > Arrow is > > > > shuttling data from one system to another. The newly proposed format > > > > doesn't appear to have any significant advantage for that use case (if > > > > anything, the existing format is arguably better as it is more > > compact). > > > > > > > > I am very biased towards historical precedent and avoiding breaking > > > > changes. > > > > > > > > We have "canonical extension types", perhaps it is time for "canonical > > > > alternative layouts". We could define it as such: > > > > > > > > * There are one or more primary layouts > > > > * Existing layouts are automatically considered primary layouts, > > even if > > > > they wouldn't > > > > have been primary layouts initially (e.g. large list) > > > > * A new layout, if it is semantically equivalent to another, is > > considered > > > > an alternative layout > > > > * An alternative layout still has the same requirements for adoption > > (two > > > > implementations and a vote) > > > > * An implementation should not feel pressured to rush and implement > > the > > > > new layout. > > > > It would be good if they contribute in the discussion and > > consider the > > > > layout and vote > > > > if they feel it would be an acceptable design. > > > > * We can define and vote and approve as many canonical alternative > > layouts > > > > as we want: > > > > * A canonical alternative layout should, at a minimum, have some > > > > reasonable justification, such as improved performance for > > algorithm X > > > > * Arrow implementations MUST support the primary layouts > > > > * An Arrow implementation MAY support a canonical alternative, > > however: > > > > * An Arrow implementation MUST first support the primary layout > > > > * An Arrow implementation MUST support conversion to/from the > > primary > > > > and canonical layout > > > > * An Arrow implementation's APIs MUST only provide data in the > > > > alternative > > > > layout if it is explicitly asked for (e.g. schema inference should > > > > prefer the primary layout). > > > > * We can still vote for new primary layouts (e.g. promoting a > > canonical > > > > alternative) but, in these > > > > votes we don't only consider the value (e.g. performance) of the > > layout > > > > but also the interoperability. > > > > In other words, a layout can only become a primary layout if there > > is > > > > significant evidence that most > > > > implementations plan to adopt it. > > > > > > > > This lets us evolve support for new layouts more naturally. We can > > > > generally assume that users will not, initially, be aware of these > > > > alternative layouts. However, everything will just work. They may > > start > > > > to see a performance penalty stemming from a lack of support for these > > > > layouts. If this performance penalty becomes significant then they > > will > > > > discover it and become aware of the problem. They can then ask > > whatever > > > > library they are using to add support for the alternative layout. As > > > > enough users find a need for it then libraries will add support. > > > > Eventually, enough libraries will support it that we can adopt it as a > > > > primary layout. > > > > > > > > Also, it allows libraries to adopt alternative layouts more > > aggressively if > > > > they would like while still hopefully ensuring that we eventually all > > > > converge on the same implementation of the alternative layout. > > > > > > > > On Mon, May 22, 2023 at 9:35 AM Will Jones <will.jones...@gmail.com> > > > > wrote: > > > > > > > > > Hello Arrow devs, > > > > > > > > > > I don't understand why we would start deprecating features in the > > Arrow > > > > > > format. Even starting this talk might already be a bad idea > > PR-wise. > > > > > > > > > > > > > > > > I agree we don't want to make breaking changes to the Arrow format. > > But > > > > > several maintainers have already stated they have no interest in > > > > > maintaining both list types with full compute functionality [1][2], > > so I > > > > > think it's very likely one list type or the other will be > > > > > implicitly preferred in the ecosystem if this data type was added. > > If > > > > > that's the case, I'd prefer that we agreed as a community which one > > > > should > > > > > be preferred. Maybe that's not the best path; it's just one way for > > us to > > > > > balance stability, maintenance burden, and relevance. > > > > > > > > > > Can someone help distill down the primary rationale and usecase for > > > > > > adding ArrayView to the Arrow Spec? > > > > > > > > > > > > > > > > Looking back at that old thread, I think one of the main motivations > > is > > > > to > > > > > try to prevent query engine implementers from feeling there is a > > tradeoff > > > > > between having state-of-the-art performance and being Arrow-native. > > For > > > > > some of the new array types, we had both Velox and DuckDB use them, > > so it > > > > > was reasonable to expect they were innovations that might > > proliferate. > > > > I'm > > > > > not sure if the ArrayView is part of that. From Wes earlier [3]: > > > > > > > > > > The idea is that in a world of data and query federation (for > > example, > > > > > > consider [1] where Arrow is being used as a data federation layer > > with > > > > > many > > > > > > query engines), we want to increase the amount of data in-flight > > and > > > > > > in-memory that is in Arrow format. So if query engines are having > > to > > > > > depart > > > > > > substantially from the Arrow format to get performance, then this > > > > > creates a > > > > > > potential lose-lose situation: * Depart from Arrow: get better > > > > > performance > > > > > > but pay serialization costs to read and write Arrow (the > > performance > > > > and > > > > > > resource utilization benefits outweigh the serialization costs). > > This > > > > > puts > > > > > > additional pressure on query engines to build specialized > > components > > > > for > > > > > > solving problems rather than making use of off-the-shelf components > > > > that > > > > > > use Arrow. This has knock-on effects on ecosystem fragmentation. * > > Or > > > > use > > > > > > Arrow, and accept suboptimal query processing performance > > > > > > > > > > > > > > > > > > > > > Will mentions one usecase is Velox consuming python UDF output, which > > > > seems > > > > > > to be mostly about how fast Velox can consume this format, not how > > fast > > > > > it > > > > > > can be written. Are there other usecases? > > > > > > > > > > > > > > > > To be clear, I don't know if that's the use case they want. That's > > just > > > > me > > > > > speculating. > > > > > > > > > > I still have some questions myself: > > > > > > > > > > 1. Is this array type currently only used in Velox? (not DuckDB like > > some > > > > > of the other new types?) What evidence do we have that it will become > > > > used > > > > > outside of Velox? > > > > > 2. We already have three list types: list, large list (64-bit > > offsets), > > > > and > > > > > fixed size list. Do we think we will only want a view version of the > > > > 32-bit > > > > > offset variable length list? Or are we potentially talking about view > > > > > variants for all three? > > > > > > > > > > Best, > > > > > > > > > > Will Jones > > > > > > > > > > [1] https://lists.apache.org/thread/smn13j1rnt23mb3fwx75sw3f877k3nwx > > > > > [2] https://lists.apache.org/thread/cc4w3vs3foj1fmpq9x888k51so60ftr3 > > > > > [3] https://lists.apache.org/thread/mk2yn62y6l8qtngcs1vg2qtwlxzbrt8t > > > > > > > > > > On Mon, May 22, 2023 at 3:48 AM Andrew Lamb <al...@influxdata.com> > > > > wrote: > > > > > > > > > > > Can someone help distill down the primary rationale and usecase for > > > > > > adding ArrayView to the Arrow Spec? > > > > > > > > > > > > From the above discussions, the stated rationale seems to be fast > > > > > > (zero-copy) interchange with Velox. > > > > > > > > > > > > This thread has qualitatively enumerated the benefits of > > (offset+len) > > > > > > encoding over the existing Arrow ListArray (offets) approach, but I > > > > > haven't > > > > > > seen any performance measurements that might help us to gauge the > > > > > tradeoff > > > > > > in additional complexity vs runtime overhead. > > > > > > > > > > > > Will mentions one usecase is Velox consuming python UDF output, > > which > > > > > seems > > > > > > to be mostly about how fast Velox can consume this format, not how > > fast > > > > > it > > > > > > can be written. Are there other usecases? > > > > > > > > > > > > Do we have numbers showing how much overhead converting to /from > > > > Velox's > > > > > > internal representation and the existing ListArray adds? Has > > anyone in > > > > > > Velox land considered adding faster support for Arrow style > > ListArray > > > > > > encoding? > > > > > > > > > > > > > > > > > > Andrew > > > > > > > > > > > > On Mon, May 22, 2023 at 4:38 AM Antoine Pitrou <anto...@python.org > > > > > > > > wrote: > > > > > > > > > > > > > > > > > > > > Hi, > > > > > > > > > > > > > > I don't understand why we would start deprecating features in the > > > > Arrow > > > > > > > format. Even starting this talk might already be a bad idea > > PR-wise. > > > > > > > > > > > > > > As for implementing conversions at the I/O boundary, it's a > > > > reasonably > > > > > > > policy, but it still requires work by implementors and it's not > > > > granted > > > > > > > that all consumers of the Arrow format will grow such conversions > > > > > > > if/when we add non-trivial types such as ListView or StringView. > > > > > > > > > > > > > > Regards > > > > > > > > > > > > > > Antoine. > > > > > > > > > > > > > > > > > > > > > Le 22/05/2023 à 00:39, Will Jones a écrit : > > > > > > > > One more thing: Looking back on the previous discussion[1] > > (which > > > > > > Weston > > > > > > > > pointed out in his earlier message), Jorge suggested that the > > old > > > > > list > > > > > > > > types might be deprecated in favor of view variants [2]. Others > > > > were > > > > > > > > worried that it might undermine the perception that the Arrow > > > > format > > > > > is > > > > > > > > stable. I think it might be worth thinking about "soft > > deprecating" > > > > > the > > > > > > > old > > > > > > > > list type: suggesting new implementations prefer the list > > view, but > > > > > > > > reassuring that implementations should support the old format, > > even > > > > > if > > > > > > > they > > > > > > > > just convert to the new format. To be clear, this wouldn't > > mean we > > > > > plan > > > > > > > to > > > > > > > > create breaking changes in the format; but if we ever did for > > other > > > > > > > > reasons, the old list type might go. > > > > > > > > > > > > > > > > Arrow compute libraries could choose either format for compute > > > > > support, > > > > > > > and > > > > > > > > plan to do conversion at the boundaries. Libraries that use > > the new > > > > > > type > > > > > > > > will have cheap conversion when reading the old type. Meanwhile > > > > those > > > > > > > that > > > > > > > > are still on the old type will have some incentive to move > > towards > > > > > the > > > > > > > new > > > > > > > > one, since that conversion will not be as efficient. > > > > > > > > > > > > > > > > [1] > > > > https://lists.apache.org/thread/49qzofswg1r5z7zh39pjvd1m2ggz2kdq > > > > > > > > [2] > > > > https://lists.apache.org/thread/smn13j1rnt23mb3fwx75sw3f877k3nwx > > > > > > > > > > > > > > > > On Sun, May 21, 2023 at 3:07 PM Will Jones < > > > > will.jones...@gmail.com> > > > > > > > wrote: > > > > > > > > > > > > > > > >> Hello, > > > > > > > >> > > > > > > > >> I think Sasha brings up a good point, that the advantages of > > this > > > > > > format > > > > > > > >> seem to be primarily about query processing. Other encodings > > like > > > > > REE > > > > > > > and > > > > > > > >> dictionary have space-saving advantages that justify them > > simply > > > > in > > > > > > > terms > > > > > > > >> of space efficiency (although they have query processing > > > > advantages > > > > > as > > > > > > > >> well). As discussed, most use cases are already well served by > > > > > > existing > > > > > > > >> list types and dictionary encoding. > > > > > > > >> > > > > > > > >> I agree that there are cases where transferring this type > > without > > > > > > > >> conversion would be ideal. One use case I can think of is if > > Velox > > > > > > > wants to > > > > > > > >> be able to take Arrow-based UDFs (possibly written with > > PyArrow, > > > > for > > > > > > > >> example) that operate on this column type and therefore wants > > > > > > zero-copy > > > > > > > >> exchange over the C Data Interface. > > > > > > > >> > > > > > > > >> One big question I have: we already have three list types: > > list, > > > > > large > > > > > > > >> list (64-bit offsets), and fixed size list. Do we think we > > will > > > > only > > > > > > > want a > > > > > > > >> view version of the 32-bit offset variable length list? Or > > are we > > > > > > > >> potentially talking about view variants for all three? > > > > > > > >> > > > > > > > >> Best, > > > > > > > >> > > > > > > > >> Will Jones > > > > > > > >> > > > > > > > >> > > > > > > > >> On Sun, May 21, 2023 at 2:19 PM Felipe Oliveira Carvalho < > > > > > > > >> felipe...@gmail.com> wrote: > > > > > > > >> > > > > > > > >>> The benefit of having a memory format that’s friendly to > > > > > > > non-deterministic > > > > > > > >>> order writes is unlocked by the transport and processing of > > the > > > > > data > > > > > > > being > > > > > > > >>> agnostic to the physical order as much as possible. > > > > > > > >>> > > > > > > > >>> Requiring a conversion could cancel out that benefit. But it > > can > > > > > be a > > > > > > > >>> provisory step for compatibility between systems that don’t > > > > > > understand > > > > > > > the > > > > > > > >>> format yet. This is similar to the situation with compression > > > > > schemes > > > > > > > like > > > > > > > >>> run-end encoding — the goal is processing the compressed data > > > > > > directly > > > > > > > >>> without an expansion step whenever possible. > > > > > > > >>> > > > > > > > >>> This is why having it as part of the open Arrow format is so > > > > > > important: > > > > > > > >>> everyone can agree on a format that’s friendly to parallel > > and/or > > > > > > > >>> vectorized compute kernels without introducing multiple > > > > > incompatible > > > > > > > >>> formats to the ecosystem and without imposing a conversion > > step > > > > > > between > > > > > > > >>> the > > > > > > > >>> different systems. > > > > > > > >>> > > > > > > > >>> — > > > > > > > >>> Felipe > > > > > > > >>> > > > > > > > >>> On Sat, 20 May 2023 at 20:04 Aldrin > > <octalene....@pm.me.invalid> > > > > > > > wrote: > > > > > > > >>> > > > > > > > >>>> I don't feel like this representation is necessarily a > > detail of > > > > > the > > > > > > > >>> query > > > > > > > >>>> engine, but I am also not sure why this representation would > > > > have > > > > > to > > > > > > > be > > > > > > > >>>> converted to a non-view format when serializing. Could you > > > > clarify > > > > > > > >>> that? My > > > > > > > >>>> impression is that this representation could be used for > > > > > persistence > > > > > > > or > > > > > > > >>>> data transfer, though it can be more complex to guarantee > > the > > > > > > portion > > > > > > > of > > > > > > > >>>> the buffer that an index points to is also present in > > memory. > > > > > > > >>>> > > > > > > > >>>> Sent from Proton Mail for iOS > > > > > > > >>>> > > > > > > > >>>> > > > > > > > >>>> On Sat, May 20, 2023 at 15:00, Sasha Krassovsky < > > > > > > > >>> krassovskysa...@gmail.com > > > > > > > >>>> > > <On+Sat,+May+20,+2023+at+15:00,+Sasha+Krassovsky+%3C%3Ca+href=>> > > > > > > > wrote: > > > > > > > >>>> > > > > > > > >>>> Hi everyone, > > > > > > > >>>> I understand that there are numerous benefits to this > > > > > representation > > > > > > > >>>> during query processing, but would it be fair to say that > > this > > > > is > > > > > an > > > > > > > >>>> implementation detail of the query engine? Query engines > > don’t > > > > > > > >>> necessarily > > > > > > > >>>> need to conform to the Arrow format internally, only at > > > > > > ingest/egress > > > > > > > >>>> points, and performing a conversion from the non-view to > > view > > > > > format > > > > > > > >>> seems > > > > > > > >>>> like it would be very cheap (though I understand not > > necessarily > > > > > the > > > > > > > >>> other > > > > > > > >>>> way around, but you’d need to do that anyway if you’re > > > > > serializing). > > > > > > > >>>> > > > > > > > >>>> Sasha Krassovsky > > > > > > > >>>> > > > > > > > >>>>> 20 мая 2023 г., в 13:00, Will Jones < > > will.jones...@gmail.com> > > > > > > > >>>> написал(а): > > > > > > > >>>>> > > > > > > > >>>>> Thanks for sharing these details, Pedro. The conditional > > > > > branches > > > > > > > >>>> argument > > > > > > > >>>>> makes a lot of sense to me. > > > > > > > >>>>> > > > > > > > >>>>> The tensors point brings up some interesting issues. For > > now, > > > > > we've > > > > > > > >>>> defined > > > > > > > >>>>> our only tensor extension type to be built on a fixed size > > > > list. > > > > > > If a > > > > > > > >>> use > > > > > > > >>>>> case of this might be manipulating tensors with zero copy, > > > > > perhaps > > > > > > > >>> that > > > > > > > >>>>> suggests that we want a fixed size list variant? In > > addition, > > > > > would > > > > > > > we > > > > > > > >>>> have > > > > > > > >>>>> to define another extension type to be a ListView variant? > > Or > > > > > would > > > > > > > we > > > > > > > >>>> want > > > > > > > >>>>> to think about making extension types somehow valid across > > > > > various > > > > > > > >>>>> encodings of the same "logical type"? > > > > > > > >>>>> > > > > > > > >>>>>> On Fri, May 19, 2023 at 1:59 PM Pedro Eugenio Rocha > > Pedreira > > > > > > > >>>>>> <pedro...@meta.com.invalid> wrote: > > > > > > > >>>>>> > > > > > > > >>>>>> Hi all, > > > > > > > >>>>>> > > > > > > > >>>>>> This is Pedro from the Velox team at Meta. This is my > > first > > > > time > > > > > > > >>> here, > > > > > > > >>>> so > > > > > > > >>>>>> nice to e-meet you! > > > > > > > >>>>>> > > > > > > > >>>>>> Adding to what Felipe said, the main reason we created > > > > > “ListView” > > > > > > > >>>> (though > > > > > > > >>>>>> we just call them ArrayVector/MapVector in Velox) is that, > > > > along > > > > > > > with > > > > > > > >>>>>> StringViews for strings, they allow us to write any > > columnar > > > > > > buffer > > > > > > > >>>>>> out-or-order, regardless of their types or encodings. > > This is > > > > > > > >>> naturally > > > > > > > >>>>>> doable for all primitive types (fixed-size), but not for > > types > > > > > > that > > > > > > > >>>> don’t > > > > > > > >>>>>> have fixed size and are required to be contiguous. The > > > > > StringView > > > > > > > and > > > > > > > >>>>>> ListView formats allow us to keep this invariant in Velox. > > > > > > > >>>>>> > > > > > > > >>>>>> Being able to write vectors out-of-order is useful when > > > > > executing > > > > > > > >>>>>> conditionals like IF/SWITCH statements, which are > > pervasive > > > > > among > > > > > > > our > > > > > > > >>>>>> workloads. To fully vectorize it, one first evaluates the > > > > > > > expression, > > > > > > > >>>> then > > > > > > > >>>>>> generate a bitmap containing which rows take the THEN and > > > > which > > > > > > take > > > > > > > >>> the > > > > > > > >>>>>> ELSE branch. Then you populate all rows that match the > > first > > > > > > branch > > > > > > > >>> by > > > > > > > >>>>>> evaluating the THEN expression in a vectorized > > (branch-less > > > > and > > > > > > > cache > > > > > > > >>>>>> friendly) way, and subsequently the ELSE branch. If you > > can’t > > > > > > write > > > > > > > >>> them > > > > > > > >>>>>> out-of-order, you would either have a big branch per row > > > > > > dispatching > > > > > > > >>> to > > > > > > > >>>> the > > > > > > > >>>>>> right expression (slow), or populate two distinct vectors > > then > > > > > > > >>> merging > > > > > > > >>>> them > > > > > > > >>>>>> at the end (potentially even slower). How much faster our > > > > > approach > > > > > > > is > > > > > > > >>>>>> highly depends on the buffer sizes and expressions, but we > > > > found > > > > > > it > > > > > > > >>> to > > > > > > > >>>> be > > > > > > > >>>>>> faster enough on average to justify us extending the > > > > underlying > > > > > > > >>> layout. > > > > > > > >>>>>> > > > > > > > >>>>>> With that said, this is all within a single thread of > > > > execution. > > > > > > > >>>>>> Parallelization is done by feeding each thread its own > > > > > > vector/data. > > > > > > > >>> As > > > > > > > >>>>>> pointed out in a previous message, this also gives you the > > > > > > > >>> flexibility > > > > > > > >>>> to > > > > > > > >>>>>> implement cardinality increasing/reducing operations, but > > we > > > > > don’t > > > > > > > >>> use > > > > > > > >>>> it > > > > > > > >>>>>> for that purpose. Operations like filtering, joining, > > > > unnesting > > > > > > and > > > > > > > >>>> similar > > > > > > > >>>>>> are done by wrapping the internal vector in a dictionary, > > as > > > > > these > > > > > > > >>> need > > > > > > > >>>> to > > > > > > > >>>>>> work not only on “ListViews” but on any data types with > > any > > > > > > > encoding. > > > > > > > >>>> There > > > > > > > >>>>>> are more details on Section 4.2.1 in [1] > > > > > > > >>>>>> > > > > > > > >>>>>> Beyond this, it also gives function/kernel developers more > > > > > > > >>> flexibility > > > > > > > >>>> to > > > > > > > >>>>>> implement operations that manipulate Arrays/Maps. For > > example, > > > > > > > >>>> operations > > > > > > > >>>>>> that slice these containers can be implemented in a > > zero-copy > > > > > > manner > > > > > > > >>> by > > > > > > > >>>>>> just rearranging the lengths/offsets indices, without ever > > > > > > touching > > > > > > > >>> the > > > > > > > >>>>>> larger internal buffers. This is a similar motivation as > > for > > > > > > > >>> StringView > > > > > > > >>>>>> (think of substr(), trim(), and similar). One nice last > > > > property > > > > > > is > > > > > > > >>> that > > > > > > > >>>>>> this layout allows for overlapping ranges. This is > > something > > > > > > > >>> discussed > > > > > > > >>>> with > > > > > > > >>>>>> our ML people to allow deduping feature values in a tensor > > > > > (which > > > > > > is > > > > > > > >>>> fairly > > > > > > > >>>>>> common), but not something we have leveraged just yet. > > > > > > > >>>>>> > > > > > > > >>>>>> [1] - https://vldb.org/pvldb/vol15/p3372-pedreira.pdf > > > > > > > >>>>>> > > > > > > > >>>>>> Best, > > > > > > > >>>>>> -- > > > > > > > >>>>>> Pedro Pedreira > > > > > > > >>>>>> ________________________________ > > > > > > > >>>>>> From: Felipe Oliveira Carvalho <felipe...@gmail.com> > > > > > > > >>>>>> Sent: Friday, May 19, 2023 10:01 AM > > > > > > > >>>>>> To: dev@arrow.apache.org <dev@arrow.apache.org> > > > > > > > >>>>>> Cc: Pedro Eugenio Rocha Pedreira <pedro...@meta.com> > > > > > > > >>>>>> Subject: Re: [DISCUSS][Format] Starting the draft > > > > implementation > > > > > > of > > > > > > > >>> the > > > > > > > >>>>>> ArrayView array format > > > > > > > >>>>>> > > > > > > > >>>>>> +pedroerp On Thu, 11 May 2023 at 17: 51 Raphael > > Taylor-Davies > > > > > <r. > > > > > > > >>>>>> taylordavies@ googlemail. com. invalid> wrote: Hi All, > > > if > > > > we > > > > > > > added > > > > > > > >>>>>> this, do we think many Arrow and query > engine > > > > implementations > > > > > > (for > > > > > > > >>>>>> example, DataFusion) will be > > > > > > > >>>>>> ZjQcmQRYFpfptBannerStart > > > > > > > >>>>>> This Message Is From an External Sender > > > > > > > >>>>>> > > > > > > > >>>>>> ZjQcmQRYFpfptBannerEnd > > > > > > > >>>>>> +pedroerp > > > > > > > >>>>>> > > > > > > > >>>>>> On Thu, 11 May 2023 at 17:51 Raphael Taylor-Davies > > > > > > > >>>>>> <r.taylordav...@googlemail.com.invalid> wrote: > > > > > > > >>>>>> Hi All, > > > > > > > >>>>>> > > > > > > > >>>>>>> if we added this, do we think many Arrow and query > > > > > > > >>>>>>> engine implementations (for example, DataFusion) will be > > > > eager > > > > > to > > > > > > > >>> add > > > > > > > >>>>>> full > > > > > > > >>>>>>> support for the type, including compute kernels? Or are > > they > > > > > > likely > > > > > > > >>> to > > > > > > > >>>>>> just > > > > > > > >>>>>>> convert this type to ListArray at import boundaries? > > > > > > > >>>>>> I can't speak for query engines in general, but at least > > for > > > > > > > arrow-rs > > > > > > > >>>>>> and by extension DataFusion, and based on my current > > > > > understanding > > > > > > > of > > > > > > > >>>>>> the use-cases I would be rather hesitant to add support > > to the > > > > > > > >>> kernels > > > > > > > >>>>>> for this array type, definitely instead favouring > > conversion > > > > at > > > > > > the > > > > > > > >>>>>> edges. We already have issues with the amount of code > > > > generation > > > > > > > >>>>>> resulting in binary bloat and long compile times, and I > > worry > > > > > this > > > > > > > >>> would > > > > > > > >>>>>> worsen this situation whilst not really providing > > compelling > > > > > > > >>> advantages > > > > > > > >>>>>> for the vast majority of workloads that don't interact > > with > > > > > Velox. > > > > > > > >>>>>> Whilst I can definitely see that the ListView > > representation > > > > is > > > > > > > >>> probably > > > > > > > >>>>>> a better way to represent variable length lists than what > > > > arrow > > > > > > > >>> settled > > > > > > > >>>>>> upon, I'm not yet convinced it is sufficiently better to > > > > > > incentivise > > > > > > > >>>>>> broad ecosystem adoption. > > > > > > > >>>>>> > > > > > > > >>>>>> Kind Regards, > > > > > > > >>>>>> > > > > > > > >>>>>> Raphael Taylor-Davies > > > > > > > >>>>>> > > > > > > > >>>>>>> On 11/05/2023 21:20, Will Jones wrote: > > > > > > > >>>>>>> Hi Felipe, > > > > > > > >>>>>>> > > > > > > > >>>>>>> Thanks for the additional details. > > > > > > > >>>>>>> > > > > > > > >>>>>>> > > > > > > > >>>>>>>> Velox kernels benefit from being able to append data to > > the > > > > > > array > > > > > > > >>> from > > > > > > > >>>>>>>> different threads without care for strict ordering. > > Only the > > > > > > > >>> offsets > > > > > > > >>>>>> array > > > > > > > >>>>>>>> has to be written according to logical order but that is > > > > > > > >>> potentially a > > > > > > > >>>>>> much > > > > > > > >>>>>>>> smaller buffer than the values buffer. > > > > > > > >>>>>>>> > > > > > > > >>>>>>> It still seems to me like applications are still pretty > > > > niche, > > > > > > as I > > > > > > > >>>>>> suspect > > > > > > > >>>>>>> in most cases the benefits are outweighed by the costs. > > The > > > > > > benefit > > > > > > > >>>> here > > > > > > > >>>>>>> seems pretty limited: if you are trying to split work > > between > > > > > > > >>> threads, > > > > > > > >>>>>>> usually you will have other levels such as array chunks > > to > > > > > > > >>> parallelize. > > > > > > > >>>>>> And > > > > > > > >>>>>>> if you have an incoming stream of row data, you'll want > > to > > > > > append > > > > > > > in > > > > > > > >>>>>>> predictable order to match the order of the other > > arrays. Am > > > > I > > > > > > > >>> missing > > > > > > > >>>>>>> something? > > > > > > > >>>>>>> > > > > > > > >>>>>>> And, IIUC, the cost of using ListView with out-of-order > > > > values > > > > > > over > > > > > > > >>>>>>> ListArray is you lose memory locality; the values of > > element > > > > 2 > > > > > > are > > > > > > > >>> no > > > > > > > >>>>>>> longer adjacent to the values of element 1. What do you > > think > > > > > > about > > > > > > > >>>> that > > > > > > > >>>>>>> tradeoff? > > > > > > > >>>>>>> > > > > > > > >>>>>>> I don't mean to be difficult about this. I'm excited for > > both > > > > > the > > > > > > > >>> REE > > > > > > > >>>> and > > > > > > > >>>>>>> StringView arrays, but this one I'm not so sure about > > yet. I > > > > > > > suppose > > > > > > > >>>>>> what I > > > > > > > >>>>>>> am trying to ask is, if we added this, do we think many > > Arrow > > > > > and > > > > > > > >>> query > > > > > > > >>>>>>> engine implementations (for example, DataFusion) will be > > > > eager > > > > > to > > > > > > > >>> add > > > > > > > >>>>>> full > > > > > > > >>>>>>> support for the type, including compute kernels? Or are > > they > > > > > > likely > > > > > > > >>> to > > > > > > > >>>>>> just > > > > > > > >>>>>>> convert this type to ListArray at import boundaries? > > > > > > > >>>>>>> > > > > > > > >>>>>>> Because if it turns out to be the latter, then we might > > as > > > > well > > > > > > ask > > > > > > > >>>> Velox > > > > > > > >>>>>>> to export this type as ListArray and save the rest of the > > > > > > ecosystem > > > > > > > >>>> some > > > > > > > >>>>>>> work. > > > > > > > >>>>>>> > > > > > > > >>>>>>> Best, > > > > > > > >>>>>>> > > > > > > > >>>>>>> Will Jones > > > > > > > >>>>>>> > > > > > > > >>>>>>> On Thu, May 11, 2023 at 12:32 PM Felipe Oliveira > > Carvalho < > > > > > > > >>>>>>> felipe...@gmail.com<mailto:felipe...@gmail.com>> wrote: > > > > > > > >>>>>>> > > > > > > > >>>>>>>> Initial reason for ListView arrays in Arrow is zero-copy > > > > > > > >>> compatibility > > > > > > > >>>>>> with > > > > > > > >>>>>>>> Velox which uses this format. > > > > > > > >>>>>>>> > > > > > > > >>>>>>>> Velox kernels benefit from being able to append data to > > the > > > > > > array > > > > > > > >>> from > > > > > > > >>>>>>>> different threads without care for strict ordering. > > Only the > > > > > > > >>> offsets > > > > > > > >>>>>> array > > > > > > > >>>>>>>> has to be written according to logical order but that is > > > > > > > >>> potentially a > > > > > > > >>>>>> much > > > > > > > >>>>>>>> smaller buffer than the values buffer. > > > > > > > >>>>>>>> > > > > > > > >>>>>>>> Acero kernels could take advantage of that in the > > future. > > > > > > > >>>>>>>> > > > > > > > >>>>>>>> In implementing ListViewArray/Type I was able to reuse > > some > > > > > C++ > > > > > > > >>>>>> templates > > > > > > > >>>>>>>> used for ListArray which can reduce some of the burden > > on > > > > > kernel > > > > > > > >>>>>>>> implementations that aim to work with all the types. > > > > > > > >>>>>>>> > > > > > > > >>>>>>>> I’m can fix Acero kernels for working with ListView. > > This is > > > > > > > >>> similar > > > > > > > >>>> to > > > > > > > >>>>>> the > > > > > > > >>>>>>>> work I’ve doing in kernels dealing with run-end encoded > > > > > arrays. > > > > > > > >>>>>>>> > > > > > > > >>>>>>>> — > > > > > > > >>>>>>>> Felipe > > > > > > > >>>>>>>> > > > > > > > >>>>>>>> > > > > > > > >>>>>>>> On Wed, 26 Apr 2023 at 01:03 Will Jones < > > > > > > will.jones...@gmail.com > > > > > > > >>>>>> <mailto:will.jones...@gmail.com>> wrote: > > > > > > > >>>>>>>> > > > > > > > >>>>>>>>> I suppose one common use case is materializing list > > columns > > > > > > after > > > > > > > >>>> some > > > > > > > >>>>>>>>> expanding operation like a join or unnest. That's a > > case > > > > > where > > > > > > I > > > > > > > >>>> could > > > > > > > >>>>>>>>> imagine a lot of repetition of values. Haven't yet > > thought > > > > of > > > > > > > >>> common > > > > > > > >>>>>>>> cases > > > > > > > >>>>>>>>> where there is overlap but not full duplication, but am > > > > eager > > > > > > to > > > > > > > >>> hear > > > > > > > >>>>>>>> any. > > > > > > > >>>>>>>>> The dictionary encoding point Raphael makes is > > interesting, > > > > > > > >>>> especially > > > > > > > >>>>>>>>> given the existence of LargeList and FixedSizeList. For > > > > many > > > > > > > >>>>>> operations, > > > > > > > >>>>>>>> it > > > > > > > >>>>>>>>> might make more sense to just compose those existing > > types. > > > > > > > >>>>>>>>> > > > > > > > >>>>>>>>> IIUC the operations that would be unique to the > > ArrayView > > > > are > > > > > > > ones > > > > > > > >>>>>>>> altering > > > > > > > >>>>>>>>> the shape. One could truncate each array to a certain > > > > length > > > > > > > >>> cheaply > > > > > > > >>>>>>>> simply > > > > > > > >>>>>>>>> by replacing the sizes buffer. Or perhaps there are > > > > > interesting > > > > > > > >>>>>>>> operations > > > > > > > >>>>>>>>> on tensors that would benefit. > > > > > > > >>>>>>>>> > > > > > > > >>>>>>>>> On Tue, Apr 25, 2023 at 7:47 PM Raphael Taylor-Davies > > > > > > > >>>>>>>>> <r.taylordav...@googlemail.com.invalid> wrote: > > > > > > > >>>>>>>>> > > > > > > > >>>>>>>>>> Unless I am missing something, I think the selection > > > > > use-case > > > > > > > >>> could > > > > > > > >>>> be > > > > > > > >>>>>>>>>> equally well served by a dictionary-encoded > > > > > > > BinarArray/ListArray, > > > > > > > >>>> and > > > > > > > >>>>>>>>> would > > > > > > > >>>>>>>>>> have the benefit of not requiring any modifications > > to the > > > > > > > >>> existing > > > > > > > >>>>>>>>> format > > > > > > > >>>>>>>>>> or kernels. > > > > > > > >>>>>>>>>> > > > > > > > >>>>>>>>>> The major additional flexibility of the proposed > > encoding > > > > > > would > > > > > > > >>> be > > > > > > > >>>>>>>>>> permitting disjoint or overlapping ranges, are these > > > > common > > > > > > > >>> enough > > > > > > > >>>> in > > > > > > > >>>>>>>>>> practice to represent a meaningful bottleneck? > > > > > > > >>>>>>>>>> > > > > > > > >>>>>>>>>> > > > > > > > >>>>>>>>>> On 26 April 2023 01:40:14 BST, David Li < > > > > > lidav...@apache.org > > > > > > > >>>> <mailto: > > > > > > > >>>>>> lidav...@apache.org>> wrote: > > > > > > > >>>>>>>>>>> Is there a need for a 64-bit offsets version the > > same way > > > > > we > > > > > > > >>> have > > > > > > > >>>>>> List > > > > > > > >>>>>>>>>> and LargeList? > > > > > > > >>>>>>>>>>> And just to be clear, the difference with List is > > that > > > > the > > > > > > > lists > > > > > > > >>>>>> don't > > > > > > > >>>>>>>>>> have to be stored in their logical order (or in other > > > > words, > > > > > > > >>> offsets > > > > > > > >>>>>> do > > > > > > > >>>>>>>>> not > > > > > > > >>>>>>>>>> have to be nondecreasing and so we also need sizes)? > > > > > > > >>>>>>>>>>> On Wed, Apr 26, 2023, at 09:37, Weston Pace wrote: > > > > > > > >>>>>>>>>>>> For context, there was some discussion on this back > > in > > > > > [1]. > > > > > > At > > > > > > > >>>> that > > > > > > > >>>>>>>>>> time > > > > > > > >>>>>>>>>>>> this was called "sequence view" but I do not like > > that > > > > > name. > > > > > > > >>>>>>>> However, > > > > > > > >>>>>>>>>>>> array-view array is a little confusing. Given this > > is > > > > > > similar > > > > > > > >>> to > > > > > > > >>>>>>>> list > > > > > > > >>>>>>>>>> can > > > > > > > >>>>>>>>>>>> we go with list-view array? > > > > > > > >>>>>>>>>>>> > > > > > > > >>>>>>>>>>>>> Thanks for the introduction. I'd be interested to > > hear > > > > > > about > > > > > > > >>> the > > > > > > > >>>>>>>>>>>>> applications Velox has found for these vectors, > > and in > > > > > what > > > > > > > >>>>>>>>> situations > > > > > > > >>>>>>>>>>>> they > > > > > > > >>>>>>>>>>>>> are useful. This could be contrasted with the > > current > > > > > > > >>> ListArray > > > > > > > >>>>>>>>>>>>> implementations. > > > > > > > >>>>>>>>>>>> I believe one significant benefit is that take (and > > by > > > > > > proxy, > > > > > > > >>>>>>>> filter) > > > > > > > >>>>>>>>>> and > > > > > > > >>>>>>>>>>>> sort are O(# of items) with the proposed format and > > O(# > > > > of > > > > > > > >>> bytes) > > > > > > > >>>>>>>> with > > > > > > > >>>>>>>>>> the > > > > > > > >>>>>>>>>>>> current format. Jorge did some profiling to this > > effect > > > > in > > > > > > > [1]. > > > > > > > >>>>>>>>>>>> > > > > > > > >>>>>>>>>>>> [1] > > > > > > > >>>>>>>> > > > > > > https://lists.apache.org/thread/49qzofswg1r5z7zh39pjvd1m2ggz2kdq< > > > > > > > >>>>>> > > > > > https://lists.apache.org/thread/49qzofswg1r5z7zh39pjvd1m2ggz2kdq> > > > > > > > >>>>>>>>>>>> On Tue, Apr 25, 2023 at 3:13 PM Will Jones < > > > > > > > >>>> will.jones...@gmail.com > > > > > > > >>>>>> <mailto:will.jones...@gmail.com> > > > > > > > >>>>>>>>>> wrote: > > > > > > > >>>>>>>>>>>>> Hi Felipe, > > > > > > > >>>>>>>>>>>>> > > > > > > > >>>>>>>>>>>>> Thanks for the introduction. I'd be interested to > > hear > > > > > > about > > > > > > > >>> the > > > > > > > >>>>>>>>>>>>> applications Velox has found for these vectors, > > and in > > > > > what > > > > > > > >>>>>>>>> situations > > > > > > > >>>>>>>>>> they > > > > > > > >>>>>>>>>>>>> are useful. This could be contrasted with the > > current > > > > > > > >>> ListArray > > > > > > > >>>>>>>>>>>>> implementations. > > > > > > > >>>>>>>>>>>>> > > > > > > > >>>>>>>>>>>>> IIUC it would be fairly cheap to transform a > > ListArray > > > > to > > > > > > an > > > > > > > >>>>>>>>>> ArrayView, but > > > > > > > >>>>>>>>>>>>> expensive to go the other way. > > > > > > > >>>>>>>>>>>>> > > > > > > > >>>>>>>>>>>>> Best, > > > > > > > >>>>>>>>>>>>> > > > > > > > >>>>>>>>>>>>> Will Jones > > > > > > > >>>>>>>>>>>>> > > > > > > > >>>>>>>>>>>>> On Tue, Apr 25, 2023 at 3:00 PM Felipe Oliveira > > > > Carvalho > > > > > < > > > > > > > >>>>>>>>>>>>> felipe...@gmail.com<mailto:felipe...@gmail.com>> > > > > wrote: > > > > > > > >>>>>>>>>>>>> > > > > > > > >>>>>>>>>>>>>> Hi folks, > > > > > > > >>>>>>>>>>>>>> > > > > > > > >>>>>>>>>>>>>> I would like to start a public discussion on the > > > > > inclusion > > > > > > > >>> of a > > > > > > > >>>>>>>> new > > > > > > > >>>>>>>>>> array > > > > > > > >>>>>>>>>>>>>> format to Arrow — array-view array. The name is > > also > > > > up > > > > > > for > > > > > > > >>>>>>>> debate. > > > > > > > >>>>>>>>>>>>>> This format is inspired by Velox's ArrayVector > > format > > > > > [1]. > > > > > > > >>>>>>>>> Logically, > > > > > > > >>>>>>>>>>>>> this > > > > > > > >>>>>>>>>>>>>> array represents an array of arrays. Each element > > is > > > > an > > > > > > > >>>>>>>> array-view > > > > > > > >>>>>>>>>>>>> (offset > > > > > > > >>>>>>>>>>>>>> and size pair) that points to a range within a > > nested > > > > > > > >>> "values" > > > > > > > >>>>>>>>> array > > > > > > > >>>>>>>>>>>>>> (called "elements" in Velox docs). The nested > > array > > > > can > > > > > be > > > > > > > of > > > > > > > >>>> any > > > > > > > >>>>>>>>>> type, > > > > > > > >>>>>>>>>>>>>> which makes this format very flexible and > > powerful. > > > > > > > >>>>>>>>>>>>>> > > > > > > > >>>>>>>>>>>>>> [image: ../_images/array-vector.png] > > > > > > > >>>>>>>>>>>>>> < > > > > > > > >>>>>>>>> > > > > > > > >>> > > > > https://facebookincubator.github.io/velox/_images/array-vector.png > > > > > < > > > > > > > >>>>>> > > > > > > https://facebookincubator.github.io/velox/_images/array-vector.png > > > > > > > >> > > > > > > > >>>>>>>>>>>>>> I'm currently working on a C++ implementation and > > plan > > > > > to > > > > > > > >>> work > > > > > > > >>>>>>>> on a > > > > > > > >>>>>>>>>> Go > > > > > > > >>>>>>>>>>>>>> implementation to fulfill the two-implementations > > > > > > > requirement > > > > > > > >>>> for > > > > > > > >>>>>>>>>> format > > > > > > > >>>>>>>>>>>>>> changes. > > > > > > > >>>>>>>>>>>>>> > > > > > > > >>>>>>>>>>>>>> The draft design: > > > > > > > >>>>>>>>>>>>>> > > > > > > > >>>>>>>>>>>>>> - 3 buffers: [validity_bitmap, int32 offsets > > buffer, > > > > > int32 > > > > > > > >>> sizes > > > > > > > >>>>>>>>>> buffer] > > > > > > > >>>>>>>>>>>>>> - 1 child array: "values" as an array of the type > > > > > > parameter > > > > > > > >>>>>>>>>>>>>> > > > > > > > >>>>>>>>>>>>>> validity_bitmap is used to differentiate between > > empty > > > > > > array > > > > > > > >>>>>>>> views > > > > > > > >>>>>>>>>>>>>> (sizes[i] == 0) and NULL array views > > > > (validity_bitmap[i] > > > > > > == > > > > > > > >>> 0). > > > > > > > >>>>>>>>>>>>>> > > > > > > > >>>>>>>>>>>>>> When the validity_bitmap[i] is 0, both sizes and > > > > offsets > > > > > > are > > > > > > > >>>>>>>>>> undefined > > > > > > > >>>>>>>>>>>>> (as > > > > > > > >>>>>>>>>>>>>> usual), and when sizes[i] == 0, offsets[i] is > > > > > undefined. 0 > > > > > > > is > > > > > > > >>>>>>>>>> recommended > > > > > > > >>>>>>>>>>>>>> if setting a value is not an issue to the system > > > > > producing > > > > > > > >>> the > > > > > > > >>>>>>>>>> arrays. > > > > > > > >>>>>>>>>>>>>> offsets buffer is not required to be ordered and > > views > > > > > > don't > > > > > > > >>>> have > > > > > > > >>>>>>>>> to > > > > > > > >>>>>>>>>> be > > > > > > > >>>>>>>>>>>>>> disjoint. > > > > > > > >>>>>>>>>>>>>> > > > > > > > >>>>>>>>>>>>>> [1] > > > > > > > >>>>>>>>>>>>>> > > > > > > > >>>>>>>> > > > > > > > >>>>>> > > > > > > > >>>> > > > > > > > >>> > > > > > > > > > > > > > > > > > > > > > > > > https://facebookincubator.github.io/velox/develop/vectors.html#arrayvector > > > > > > > >>>>>> < > > > > > > > >>>>>> > > > > > > > >>>> > > > > > > > >>> > > > > > > > > > > > > > > > > > > > > > > > > https://facebookincubator.github.io/velox/develop/vectors.html#arrayvector > > > > > > > >>>>>>> > > > > > > > >>>>>>>>>>>>>> Thanks, > > > > > > > >>>>>>>>>>>>>> Felipe O. Carvalho > > > > > > > >>>>>>>>>>>>>> > > > > > > > >>>>>> > > > > > > > >>>> > > > > > > > >>>> > > > > > > > >>> > > > > > > > >> > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > >
