I'm going to bump this because it would be good to get feedback. In particular it would be nice to get feedback on the suggested format change[1]. We are currently moving forward on coming up with an IPC format proposal which we will share when ready.
The two interesting points that jump out to me are: * Should we encode "run lengths" or "run ends"? For example, should 5,5,5,6,6,7,7,7,7 be encoded with "run lengths" 3, 2, 4 or "run ends" 3, 5, 9. In the proposal the latter is preferred as that leads to O(log(N)) random access (instead of O(N)) and it's not clear there are any disadvantages. * Should the run ends be encoded as a buffer or a child array? The values are definitely a child array. However, encoding the run ends as a buffer (similar to list array for example) makes it difficult to calculate offsets. Translating an array offset to a buffer offset takes O(log(N)) time. If the run ends are encoded as a child array (so the RLE array has no buffers and two child arrays) then this problem goes away. [1] https://github.com/apache/arrow/pull/13333/files On Thu, Aug 25, 2022 at 10:35 AM Tobias Zagorni <tob...@zagorni.eu.invalid> wrote: > > Hello Everyone, > > Recently, I have implemented support for run-length encoding in Arrow > C++. So far my implementation is split into different subtasks of > ARROW-16771 (https://issues.apache.org/jira/browse/ARROW-16771). > > I have (draft) PRs available for: > - general handling of RLE in arrow C++, Type, Arrow, Builder > subclasses, etc. > (subtasks 1-9) > - encode, decode kernels (fixed size only): > (https://issues.apache.org/jira/browse/ARROW-16772) > - filter kernel (fixed size only): > (https://issues.apache.org/jira/browse/ARROW-16774) > - simple benchmark for the RLE kernels > (https://issues.apache.org/jira/browse/ARROW-17026) > - adding RLE to Arrow Columnar format document > (https://issues.apache.org/jira/browse/ARROW-16773) > > What is not yet implemented: > - converting RLE to formats like Parquet, JSON, IPC. > - caching of physical offsets when working with sliced arrays - finding > these offsets is an O(log(n)) binary search which could be avoided in > a lot of cases > > I'm interested in any feedback on the code and I'm wondering what would > be the best way to get this merged. > > A lot of the PRs depend on earlier ones. I ordered the subtasks in a > way they could be merged. The first would be: > 1. Handling of array-only types using VisitTypeInline: > https://issues.apache.org/jira/browse/ARROW-17258 > 2. Adding RLE type / array class (only builds on #1): > https://issues.apache.org/jira/browse/ARROW-17261 > - also, since it has no dependencies: adding RLE to Arrow Columnar > format document > https://issues.apache.org/jira/browse/ARROW-16773 > > Best, > Tobias
