[GitHub] [arrow-rs] msalib opened a new issue, #3228: better document when we need `LargeUtf8` instead of `Utf8`

[GitBox <gi...@apache.org>]

msalib opened a new issue, #3228:
URL: https://github.com/apache/arrow-rs/issues/3228

   **Is your feature request related to a problem or challenge? Please describe what you are trying to do.**
   
   When I filed https://github.com/apache/arrow-rs/issues/3123, I was surprised to discover that concatenating lots of `Utf8` elements is supposed to panic when the total size is over 2 GB, even though the individual sizes are much smaller. That constraint was really unexpected! It makes sense if you understand the storage model, but I didn't and so was very surprised.
   
   **Describe the solution you'd like**
   
   I'm not sure how to surface this knowledge better. When I first skimmed the data type docs, I walked away thinking that `LargeUtf8` is for cases where an individual element is large (I wasn't even clear that large meant > 2 GB) and that I should use `Utf8` for everything else. But I should've understood the constraint as "use `LargeUtf8` everywhere except places where you can guarantee that you'll never have an array with more than 2 GB of text total".
   
   Maybe we just need a big statement in the [Physical Memory Layout](https://arrow.apache.org/docs/format/Columnar.html#physical-memory-layout) guide and the [`DataType`](https://docs.rs/arrow/latest/arrow/datatypes/enum.DataType.html#variant.Utf8) doc string explaining that you cannot ever build an array where the total text size is over 2 GB if you use `Utf8`
   
   **Describe alternatives you've considered**
   
   This feels like a landmine and I wish Arrow could transparently convert between these types as needed. Ideally there should just be a `Utf8` type that internally specifies what type it uses to manage offsets.
   
   Alternatively, I wish the `concat` kernel could return a more explicit failure message by explicitly checking for this sort of overflow, something like "I've been asked to concat 2 `Utf8` arrays into an array that will be over 2 GB and I cannot do that: these arrays need to be `LargeUtf8` instead". I mean, when you're doing the concatenation, you can check lengths explicitly ahead of time.


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[GitHub] [arrow-rs] viirya closed issue #3228: better document when we need `LargeUtf8` instead of `Utf8`

[GitBox <gi...@apache.org>]

viirya closed issue #3228: better document when we need `LargeUtf8` instead of `Utf8`
URL: https://github.com/apache/arrow-rs/issues/3228


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