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Minor: Improve comments on GenericByteViewArray::bytes_iter(), prefix_iter() and suffix_iter() #6306

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Aug 28, 2024
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Minor: Improve comments on GenericByteViewArray::bytes_iter(), prefix_iter() and suffix_iter() #6306

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19 changes: 14 additions & 5 deletions arrow-array/src/array/byte_view_array.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -302,7 +302,7 @@ impl<T: ByteViewType + ?Sized> GenericByteViewArray<T> {
ArrayIter::new(self)
}

/// Returns an iterator over the bytes of this array.
/// Returns an iterator over the bytes of this array, including null values
pub fn bytes_iter(&self) -> impl Iterator<Item = &[u8]> {
self.views.iter().map(move |v| {
let len = *v as u32;
Expand All @@ -317,8 +317,11 @@ impl<T: ByteViewType + ?Sized> GenericByteViewArray<T> {
})
}

/// Returns an iterator over the prefix bytes of this array with respect to the prefix length.
/// If the prefix length is larger than the string length, it will return the empty slice.
/// Returns an iterator over the first `prefix_len` bytes of each array
/// element, including null values.
///
/// If `prefix_len` is larger than the element's length, the iterator will
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This feels rather surprising to me?

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Is that a statement or a question?

If you have a suggestion of how to make it less surprising perhaps @xinlifoobar would be willing to give it a try

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I guess I would have thought returning Option<&[u8]> would be perhaps more intuitive and likely perform similarly? That being said I am surprised that skipping the null mask yields performance returns, as this hasn't been my experience with regular StringArray, so my intuition about performance may be off

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@xinlifoobar xinlifoobar Aug 27, 2024 •
edited
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Thanks for the suggestions! @tustvold.

I created a new PR #6312 to put this discussion. I did this in the early version but didn't see performance gains like this time - it did well for inline values. We might do a combination based on the bench result, e.g., null masks for prefix_iter and slice for suffix_iter.

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The results on #6312 seem to show that using null masks does indeed reduce performance

So is the conclusion that these APIs are good to go?

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I left some comments, I think this API would be better if it returned None for a prefix too long, even if it continues to not inspect null masks

/// return an empty slice (`&[]`).
pub fn prefix_bytes_iter(&self, prefix_len: usize) -> impl Iterator<Item = &[u8]> {
self.views().into_iter().map(move |v| {
let len = (*v as u32) as usize;
Expand All @@ -341,8 +344,14 @@ impl<T: ByteViewType + ?Sized> GenericByteViewArray<T> {
})
}

/// Returns an iterator over the suffix bytes of this array with respect to the suffix length.
/// If the suffix length is larger than the string length, it will return the empty slice.
/// Returns an iterator over the last `suffix_len` bytes of each array
/// element, including null values.
///
/// Note that for [`StringViewArray`] the last bytes may start in the middle
/// of a UTF-8 codepoint, and thus may not be a valid `&str`.
///
/// If `suffix_len` is larger than the element's length, the iterator will
/// return an empty slice (`&[]`).
pub fn suffix_bytes_iter(&self, suffix_len: usize) -> impl Iterator<Item = &[u8]> {
self.views().into_iter().map(move |v| {
let len = (*v as u32) as usize;
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