[GitHub] [arrow] waynexia commented on pull request #8856: Arrow-10355: [Rust] Add support for list_sort

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

waynexia commented on pull request #8856:
URL: https://github.com/apache/arrow/pull/8856#issuecomment-739533505


   There are some points I'm not very clear about:
   1. Ordering. I didn't find a definition of `List` in the documentation or CPP implementation. So I use something like lexicographical order (refer to `sort_list()` in sort.rs) to compare two lists. Please let me know if I missed anything about the ordering's definition.
   2. Nulls. It seems that nulls element in list array is different than [documentation](http://arrow.apache.org/docs/format/Columnar.html#variable-size-list-layout). As `From<ArrayData>` requires buffers number should be 1 (0 in `FixedSizeListArray`) which is not the same as the spec. It says that variable sized list has two buffers for "validate bitmap" and "offsets". In the current implementation validate bitmap is placed in `ArrayData`'s `null_bitmap`. I think nullable array in `List` cannot be represented in this way?


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