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 >>>>>>>>
