> A larger problem for splitting columns across files is that there are a lot of assumptions about how data is laid out in both readers and writers. For example, aligning row groups and correctly handling split calculation is very complicated if you're trying to split rows across files. Other features are also impacted like deletes, which reference the file to which they apply and would need to account for deletes applying to multiple files and needing to update those references if columns are added.
I agree that there are many assumptions baked into the Content File layout, and we should preserve that abstraction.We can introduce a lower-level abstraction: the Physical File. This allows us to retain most of the existing assumptions at the Content File level, while delegating the responsibility of determining which Physical Files to access to the reader. One key advantage of introducing Physical Files is the flexibility to vary RowGroup sizes across columns. For instance, wide string columns could benefit from smaller RowGroups to reduce memory pressure, while numeric columns could use larger RowGroups to improve compression and scan efficiency. Rather than enforcing strict row group alignment across all columns, we can explore optimizing read split sizes and write-time RowGroup sizes independently - striking a balance that maximizes performance and storage costs for different data types and queries. On the topic of the view based wide tables: Using readers to join Physical File files closely resembles the shuffle-less join approach mentioned by Bryan, with the added benefit that any number of columns can be updated independently. In contrast, the multitable approach restricts independent updates to predefined column families (tables behind the view). Gang Wu <ust...@gmail.com> ezt írta (időpont: 2025. máj. 30., P, 8:09): > IMO, the main drawback for the view solution is the complexity of > maintaining consistency across tables if we want to use features like time > travel, incremental scan, branch & tag, encryption, etc. > > On Fri, May 30, 2025 at 12:55 PM Bryan Keller <brya...@gmail.com> wrote: > >> Fewer commit conflicts meaning the tables representing column families >> are updated independently, rather than having to serialize commits to a >> single table. Perhaps with a wide table solution the commit logic could be >> enhanced to support things like concurrent overwrites to independent column >> families, but it seems like it would be fairly involved. >> >> >> On May 29, 2025, at 7:16 PM, Steven Wu <stevenz...@gmail.com> wrote: >> >> Bryan, interesting approach to split horizontally across multiple tables. >> >> A few potential down sides >> * operational overhead. tables need to be managed consistently and >> probably in some coordinated way >> * complex read >> * maybe fragile to enforce correctness (during join). It is robust to >> enforce the stitching correctness at file group level in file reader and >> writer if built in the table format. >> >> > fewer commit conflicts >> >> Can you elaborate on this one? Are those tables populated by streaming or >> batch pipelines? >> >> On Thu, May 29, 2025 at 5:03 PM Bryan Keller <brya...@gmail.com> wrote: >> >>> Hi everyone, >>> >>> We have been investigating a wide table format internally for a similar >>> use case, i.e. we have wide ML tables with features generated by different >>> pipelines and teams but want a unified view of the data. We are comparing >>> that against separate tables joined together using a shuffle-less join >>> (e.g. storage partition join), along with a corresponding view. >>> >>> The join/view approach seems to give us much of we need, with some added >>> benefits like splitting up the metadata, fewer commit conflicts, and >>> ability to share, nest, and swap "column families". The downsides are table >>> management is split across multiple tables, it requires engine support of >>> shuffle-less joins for best performance, and even then, scans probably >>> won't be as optimal. >>> >>> I'm curious if anyone had further thoughts on the two? >>> >>> -Bryan >>> >>> >>> >>> On May 29, 2025, at 8:18 AM, Péter Váry <peter.vary.apa...@gmail.com> >>> wrote: >>> >>> I received feedback from Alkis regarding their Parquet optimization >>> work. Their internal testing shows promising results for reducing metadata >>> size and improving parsing performance. They plan to formalize a proposal >>> for these Parquet enhancements in the near future. >>> >>> Meanwhile, I'm putting together our horizontal sharding proposal as a >>> complementary approach. Even with the Parquet metadata improvements, >>> horizontal sharding would provide additional benefits for: >>> >>> - More efficient column-level updates >>> - Streamlined column additions >>> - Better handling of dominant columns that can cause RowGroup size >>> imbalances (placing these in separate files could significantly improve >>> performance) >>> >>> Thanks, Peter >>> >>> >>> >>> Péter Váry <peter.vary.apa...@gmail.com> ezt írta (időpont: 2025. máj. >>> 28., Sze, 15:39): >>> >>>> I would be happy to put together a proposal based on the inputs got >>>> here. >>>> >>>> Thanks everyone for your thoughts! >>>> I will try to incorporate all of this. >>>> >>>> Thanks, Peter >>>> >>>> Daniel Weeks <dwe...@apache.org> ezt írta (időpont: 2025. máj. 27., K, >>>> 20:07): >>>> >>>>> I feel like we have two different issues we're talking about here that >>>>> aren't necessarily tied (though solutions may address both): 1) wide >>>>> tables, 2) adding columns >>>>> >>>>> Wide tables are definitely a problem where parquet has limitations. >>>>> I'm optimistic about the ongoing work to help improve parquet >>>>> footers/stats >>>>> in this area that Fokko mentioned. There are always limitations in how >>>>> this scales as wide rows lead to small row groups and the cost to >>>>> reconstitute a row gets more expensive, but for cases that are read heavy >>>>> and projecting subsets of columns should significantly improve >>>>> performance. >>>>> >>>>> Adding columns to an existing dataset is something that comes up >>>>> periodically, but there's a lot of complexity involved in this. Parquet >>>>> does support referencing columns in separate files per the spec, but >>>>> there's no implementation that takes advantage of this to my knowledge. >>>>> This does allow for approaches where you separate/rewrite just the footers >>>>> or various other tricks, but these approaches get complicated quickly and >>>>> the number of readers that can consume those representations would >>>>> initially be very limited. >>>>> >>>>> A larger problem for splitting columns across files is that there are >>>>> a lot of assumptions about how data is laid out in both readers and >>>>> writers. For example, aligning row groups and correctly handling split >>>>> calculation is very complicated if you're trying to split rows across >>>>> files. Other features are also impacted like deletes, which reference the >>>>> file to which they apply and would need to account for deletes applying to >>>>> multiple files and needing to update those references if columns are >>>>> added. >>>>> >>>>> I believe there are a lot of interesting approaches to addressing >>>>> these use cases, but we'd really need a thorough proposal that explores >>>>> all >>>>> of these scenarios. The last thing we would want is to introduce >>>>> incompatibilities within the format that result in incompatible features. >>>>> >>>>> -Dan >>>>> >>>>> On Tue, May 27, 2025 at 10:02 AM Russell Spitzer < >>>>> russell.spit...@gmail.com> wrote: >>>>> >>>>>> Point definitely taken. We really should probably POC some of >>>>>> these ideas and see what we are actually dealing with. (He said without >>>>>> volunteering to do the work :P) >>>>>> >>>>>> On Tue, May 27, 2025 at 11:55 AM Selcuk Aya >>>>>> <selcuk....@snowflake.com.invalid> wrote: >>>>>> >>>>>>> Yes having to rewrite the whole file is not ideal but I believe most >>>>>>> of the cost of rewriting a file comes from decompression, encoding, >>>>>>> stats >>>>>>> calculations etc. If you are adding new values for some columns but are >>>>>>> keeping the rest of the columns the same in the file, then a bunch of >>>>>>> rewrite cost can be optimized away. I am not saying this is better than >>>>>>> writing to a separate file, I am not sure how much worse it is though. >>>>>>> >>>>>>> On Tue, May 27, 2025 at 9:40 AM Russell Spitzer < >>>>>>> russell.spit...@gmail.com> wrote: >>>>>>> >>>>>>>> I think that "after the fact" modification is one of the >>>>>>>> requirements here, IE: Updating a single column without rewriting the >>>>>>>> whole >>>>>>>> file. >>>>>>>> If we have to write new metadata for the file aren't we in the same >>>>>>>> boat as having to rewrite the whole file? >>>>>>>> >>>>>>>> On Tue, May 27, 2025 at 11:27 AM Selcuk Aya >>>>>>>> <selcuk....@snowflake.com.invalid> wrote: >>>>>>>> >>>>>>>>> If files represent column projections of a table rather than the >>>>>>>>> whole columns in the table, then any read that reads across these >>>>>>>>> files >>>>>>>>> needs to identify what constitutes a row. Lance DB for example has >>>>>>>>> vertical >>>>>>>>> partitioning across columns but also horizontal partitioning across >>>>>>>>> rows >>>>>>>>> such that in each horizontal partitioning(fragment), the same number >>>>>>>>> of >>>>>>>>> rows exist in each vertical partition, which I think is necessary to >>>>>>>>> make >>>>>>>>> whole/partial row construction cheap. If this is the case, there is no >>>>>>>>> reason not to achieve the same data layout inside a single columnar >>>>>>>>> file >>>>>>>>> with a lean header. I think the only valid argument for a separate >>>>>>>>> file is >>>>>>>>> adding a new set of columns to an existing table, but even then I am >>>>>>>>> not >>>>>>>>> sure a separate file is absolutely necessary for good performance. >>>>>>>>> >>>>>>>>> Selcuk >>>>>>>>> >>>>>>>>> On Tue, May 27, 2025 at 9:18 AM Devin Smith >>>>>>>>> <devinsm...@deephaven.io.invalid> wrote: >>>>>>>>> >>>>>>>>>> There's a `file_path` field in the parquet ColumnChunk structure, >>>>>>>>>> https://github.com/apache/parquet-format/blob/apache-parquet-format-2.11.0/src/main/thrift/parquet.thrift#L959-L962 >>>>>>>>>> >>>>>>>>>> I'm not sure what tooling actually supports this though. Could be >>>>>>>>>> interesting to see what the history of this is. >>>>>>>>>> https://lists.apache.org/thread/rcv1cxndp113shjybfcldh6nq1t3lcq3, >>>>>>>>>> https://lists.apache.org/thread/k5nv310yp315fttcz213l8o0vmnd7vyw >>>>>>>>>> >>>>>>>>>> On Tue, May 27, 2025 at 8:59 AM Russell Spitzer < >>>>>>>>>> russell.spit...@gmail.com> wrote: >>>>>>>>>> >>>>>>>>>>> I have to agree that while there can be some fixes in Parquet, >>>>>>>>>>> we fundamentally need a way to split a "row group" >>>>>>>>>>> or something like that between separate files. If that's >>>>>>>>>>> something we can do in the parquet project that would be great >>>>>>>>>>> but it feels like we need to start exploring more drastic >>>>>>>>>>> options than footer encoding. >>>>>>>>>>> >>>>>>>>>>> On Mon, May 26, 2025 at 8:42 PM Gang Wu <ust...@gmail.com> >>>>>>>>>>> wrote: >>>>>>>>>>> >>>>>>>>>>>> I agree with Steven that there are limitations that Parquet >>>>>>>>>>>> cannot do. >>>>>>>>>>>> >>>>>>>>>>>> In addition to adding new columns by rewriting all files, files >>>>>>>>>>>> of wide tables may suffer from bad performance like below: >>>>>>>>>>>> - Poor compression of row groups because there are too many >>>>>>>>>>>> columns and even a small number of rows can reach the row group >>>>>>>>>>>> threshold. >>>>>>>>>>>> - Dominating columns (e.g. blobs) may contribute to 99% size of >>>>>>>>>>>> a row group, leading to unbalanced column chunks and deteriorate >>>>>>>>>>>> the row >>>>>>>>>>>> group compression. >>>>>>>>>>>> - Similar to adding new columns, partial update also requires >>>>>>>>>>>> rewriting all columns of the affected rows. >>>>>>>>>>>> >>>>>>>>>>>> IIRC, some table formats already support splitting columns into >>>>>>>>>>>> different files: >>>>>>>>>>>> - Lance manifest splits a fragment [1] into one or more data >>>>>>>>>>>> files. >>>>>>>>>>>> - Apache Hudi has the concept of column family [2]. >>>>>>>>>>>> - Apache Paimon supports sequence groups [3] for partial update. >>>>>>>>>>>> >>>>>>>>>>>> Although Parquet can introduce the concept of logical file and >>>>>>>>>>>> physical file to manage the columns to file mapping, this looks >>>>>>>>>>>> like yet >>>>>>>>>>>> another manifest file design which duplicates the purpose of >>>>>>>>>>>> Iceberg. >>>>>>>>>>>> These might be something worth exploring in Iceberg. >>>>>>>>>>>> >>>>>>>>>>>> [1] https://lancedb.github.io/lance/format.html#fragments >>>>>>>>>>>> [2] >>>>>>>>>>>> https://github.com/apache/hudi/blob/master/rfc/rfc-80/rfc-80.md >>>>>>>>>>>> [3] >>>>>>>>>>>> https://paimon.apache.org/docs/0.9/primary-key-table/merge-engine/partial-update/#sequence-group >>>>>>>>>>>> >>>>>>>>>>>> Best, >>>>>>>>>>>> Gang >>>>>>>>>>>> >>>>>>>>>>>> >>>>>>>>>>>> >>>>>>>>>>>> On Tue, May 27, 2025 at 7:03 AM Steven Wu <stevenz...@gmail.com> >>>>>>>>>>>> wrote: >>>>>>>>>>>> >>>>>>>>>>>>> The Parquet metadata proposal (linked by Fokko) is mainly >>>>>>>>>>>>> addressing the read performance due to bloated metadata. >>>>>>>>>>>>> >>>>>>>>>>>>> What Peter described in the description seems useful for some >>>>>>>>>>>>> ML workload of feature engineering. A new set of features/columns >>>>>>>>>>>>> are added >>>>>>>>>>>>> to the table. Currently, Iceberg would require rewriting all >>>>>>>>>>>>> data files to >>>>>>>>>>>>> combine old and new columns (write amplification). Similarly, in >>>>>>>>>>>>> the past >>>>>>>>>>>>> the community also talked about the use cases of updating a >>>>>>>>>>>>> single column, >>>>>>>>>>>>> which would require rewriting all data files. >>>>>>>>>>>>> >>>>>>>>>>>>> On Mon, May 26, 2025 at 2:42 PM Péter Váry < >>>>>>>>>>>>> peter.vary.apa...@gmail.com> wrote: >>>>>>>>>>>>> >>>>>>>>>>>>>> Do you have the link at hand for the thread where this was >>>>>>>>>>>>>> discussed on the Parquet list? >>>>>>>>>>>>>> The docs seem quite old, and the PR stale, so I would like to >>>>>>>>>>>>>> understand the situation better. >>>>>>>>>>>>>> If it is possible to do this in Parquet, that would be great, >>>>>>>>>>>>>> but Avro, ORC would still suffer. >>>>>>>>>>>>>> >>>>>>>>>>>>>> Amogh Jahagirdar <2am...@gmail.com> ezt írta (időpont: 2025. >>>>>>>>>>>>>> máj. 26., H, 22:07): >>>>>>>>>>>>>> >>>>>>>>>>>>>>> Hey Peter, >>>>>>>>>>>>>>> >>>>>>>>>>>>>>> Thanks for bringing this issue up. I think I agree with >>>>>>>>>>>>>>> Fokko; the issue of wide tables leading to Parquet metadata >>>>>>>>>>>>>>> bloat and poor >>>>>>>>>>>>>>> Thrift deserialization performance is a long standing issue >>>>>>>>>>>>>>> that I believe >>>>>>>>>>>>>>> there's motivation in the community to address. So to me it >>>>>>>>>>>>>>> seems better to >>>>>>>>>>>>>>> address it in Parquet itself rather than Iceberg library >>>>>>>>>>>>>>> facilitate a >>>>>>>>>>>>>>> pattern which works around the limitations. >>>>>>>>>>>>>>> >>>>>>>>>>>>>>> Thanks, >>>>>>>>>>>>>>> Amogh Jahagirdar >>>>>>>>>>>>>>> >>>>>>>>>>>>>>> On Mon, May 26, 2025 at 1:42 PM Fokko Driesprong < >>>>>>>>>>>>>>> fo...@apache.org> wrote: >>>>>>>>>>>>>>> >>>>>>>>>>>>>>>> Hi Peter, >>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>> Thanks for bringing this up. Wouldn't it make more sense to >>>>>>>>>>>>>>>> fix this in Parquet itself? It has been a long-running issue >>>>>>>>>>>>>>>> on Parquet, >>>>>>>>>>>>>>>> and there is still active interest from the community. There >>>>>>>>>>>>>>>> is a PR to >>>>>>>>>>>>>>>> replace the footer with FlatBuffers, which dramatically >>>>>>>>>>>>>>>> improves performance >>>>>>>>>>>>>>>> <https://github.com/apache/arrow/pull/43793>. The >>>>>>>>>>>>>>>> underlying proposal can be found here >>>>>>>>>>>>>>>> <https://docs.google.com/document/d/1PQpY418LkIDHMFYCY8ne_G-CFpThK15LLpzWYbc7rFU/edit?tab=t.0#heading=h.atbrz9ch6nfa> >>>>>>>>>>>>>>>> . >>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>> Kind regards, >>>>>>>>>>>>>>>> Fokko >>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>> Op ma 26 mei 2025 om 20:35 schreef yun zou < >>>>>>>>>>>>>>>> yunzou.colost...@gmail.com>: >>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>> +1, I am really interested in this topic. Performance has >>>>>>>>>>>>>>>>> always been a problem when dealing with wide tables, not just >>>>>>>>>>>>>>>>> read/write, >>>>>>>>>>>>>>>>> but also during compilation. Most of the ML use cases >>>>>>>>>>>>>>>>> typically exhibit a >>>>>>>>>>>>>>>>> vectorized read/write pattern, I am also wondering if there >>>>>>>>>>>>>>>>> is any way at >>>>>>>>>>>>>>>>> the metadata level to help the whole compilation and >>>>>>>>>>>>>>>>> execution process. I >>>>>>>>>>>>>>>>> do not have any answer fo this yet, but I would be really >>>>>>>>>>>>>>>>> interested in >>>>>>>>>>>>>>>>> exploring this further. >>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>> Best Regards, >>>>>>>>>>>>>>>>> Yun >>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>> On Mon, May 26, 2025 at 9:14 AM Pucheng Yang >>>>>>>>>>>>>>>>> <py...@pinterest.com.invalid> wrote: >>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>> Hi Peter, I am interested in this proposal. What's more, >>>>>>>>>>>>>>>>>> I am curious if there is a similar story on the write side >>>>>>>>>>>>>>>>>> as well (how to >>>>>>>>>>>>>>>>>> generate these splitted files) and specifically, are you >>>>>>>>>>>>>>>>>> targeting feature >>>>>>>>>>>>>>>>>> backfill use cases in ML use? >>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>> On Mon, May 26, 2025 at 6:29 AM Péter Váry < >>>>>>>>>>>>>>>>>> peter.vary.apa...@gmail.com> wrote: >>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>>> Hi Team, >>>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>>> In machine learning use-cases, it's common to encounter >>>>>>>>>>>>>>>>>>> tables with a very high number of columns - sometimes even >>>>>>>>>>>>>>>>>>> in the range of >>>>>>>>>>>>>>>>>>> several thousand. I've seen cases with up to 15,000 >>>>>>>>>>>>>>>>>>> columns. Storing such >>>>>>>>>>>>>>>>>>> wide tables in a single Parquet file is often suboptimal, >>>>>>>>>>>>>>>>>>> as Parquet can >>>>>>>>>>>>>>>>>>> become a bottleneck, even when only a subset of columns is >>>>>>>>>>>>>>>>>>> queried. >>>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>>> A common approach to mitigate this is to split the data >>>>>>>>>>>>>>>>>>> across multiple Parquet files. With the upcoming File >>>>>>>>>>>>>>>>>>> Format API, we could >>>>>>>>>>>>>>>>>>> introduce a layer that combines these files into a single >>>>>>>>>>>>>>>>>>> iterator, >>>>>>>>>>>>>>>>>>> enabling efficient reading of wide and very wide tables. >>>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>>> To support this, we would need to revise the metadata >>>>>>>>>>>>>>>>>>> specification. Instead of the current `_file` column, we >>>>>>>>>>>>>>>>>>> could introduce a >>>>>>>>>>>>>>>>>>> _files column containing: >>>>>>>>>>>>>>>>>>> - `_file_column_ids`: the column IDs present in each file >>>>>>>>>>>>>>>>>>> - `_file_path`: the path to the corresponding file >>>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>>> Has there been any prior discussion around this idea? >>>>>>>>>>>>>>>>>>> Is anyone else interested in exploring this further? >>>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>>> Best regards, >>>>>>>>>>>>>>>>>>> Peter >>>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>> >>> >>
