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alamb merged 2 commits into from
Aug 21, 2025
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arrow-row: Document dictionary handling #8168

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arrow-row: Document dictionary handling
alamb Aug 18, 2025
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Merge branch 'main' into alamb/row_coverter_docs
alamb Aug 20, 2025
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34 changes: 33 additions & 1 deletion arrow-row/src/lib.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -97,7 +97,7 @@
//! assert_eq!(&c2_values, &["a", "f", "c", "e"]);
//! ```
//!
//! # Lexsort
//! # Lexicographic Sorts (lexsort)
//!
//! The row format can also be used to implement a fast multi-column / lexicographic sort
//!
Expand All @@ -117,13 +117,41 @@
//! }
//! ```
//!
//! # Flattening Dictionaries
//!
//! For performance reasons, dictionary arrays are flattened ("hydrated") to their
//! underlying values during row conversion. See [the issue] for more details.
//!
//! This means that the arrays that come out of [`RowConverter::convert_rows`]
//! may not have the same data types as the input arrays. For example, encoding
//! a `Dictionary<Int8, Utf8>` and then will come out as a `Utf8` array.
//!
//! ```
//! # use arrow_array::{Array, ArrayRef, DictionaryArray};
//! # use arrow_array::types::Int8Type;
//! # use arrow_row::{RowConverter, SortField};
//! # use arrow_schema::DataType;
//! # use std::sync::Arc;
//! // Input is a Dictionary array
//! let dict: DictionaryArray::<Int8Type> = ["a", "b", "c", "a", "b"].into_iter().collect();
//! let sort_fields = vec![SortField::new(dict.data_type().clone())];
//! let arrays = vec![Arc::new(dict) as ArrayRef];
//! let converter = RowConverter::new(sort_fields).unwrap();
//! // Convert to rows
//! let rows = converter.convert_columns(&arrays).unwrap();
//! let converted = converter.convert_rows(&rows).unwrap();
//! // result was a Utf8 array, not a Dictionary array
//! assert_eq!(converted[0].data_type(), &DataType::Utf8);
//! ```
//!
//! [non-comparison sorts]: https://en.wikipedia.org/wiki/Sorting_algorithm#Non-comparison_sorts
//! [radix sort]: https://en.wikipedia.org/wiki/Radix_sort
//! [normalized for sorting]: http://wwwlgis.informatik.uni-kl.de/archiv/wwwdvs.informatik.uni-kl.de/courses/DBSREAL/SS2005/Vorlesungsunterlagen/Implementing_Sorting.pdf
//! [`memcmp`]: https://www.man7.org/linux/man-pages/man3/memcmp.3.html
//! [`lexsort`]: https://docs.rs/arrow-ord/latest/arrow_ord/sort/fn.lexsort.html
//! [compared]: PartialOrd
//! [compare]: PartialOrd
//! [the issue]: https://github.com/apache/arrow-rs/issues/4811

#![doc(
html_logo_url = "https://arrow.apache.org/img/arrow-logo_chevrons_black-txt_white-bg.svg",
Expand Down Expand Up @@ -661,6 +689,8 @@ impl RowConverter {
///
/// See [`Row`] for information on when [`Row`] can be compared
///
/// See [`Self::convert_rows`] for converting [`Rows`] back into [`ArrayRef`]
///
/// # Panics
///
/// Panics if the schema of `columns` does not match that provided to [`RowConverter::new`]
Expand Down Expand Up @@ -768,6 +798,8 @@ impl RowConverter {

/// Convert [`Rows`] columns into [`ArrayRef`]
///
/// See [`Self::convert_columns`] for converting [`ArrayRef`] into [`Rows`]
///
/// # Panics
///
/// Panics if the rows were not produced by this [`RowConverter`]
Expand Down
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