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Minor: refine row selection example more #5850

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Jun 11, 2024
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Minor: refine row selection example more #5850

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29 changes: 24 additions & 5 deletions parquet/src/arrow/arrow_reader/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -155,24 +155,43 @@ impl<T> ArrowReaderBuilder<T> {
///
/// # Example
///
/// Given a parquet file with 3 row groups, and a row group filter of
/// `[0, 2]`, in order to only scan rows 50-100 in row group 2:
/// Given a parquet file with 4 row groups, and a row group filter of `[0,
/// 2, 3]`, in order to scan rows 50-100 in row group 2 and rows 200-300 in
/// row group 3:
///
/// ```text
/// Row Group 0, 1000 rows (selected)
/// Row Group 1, 1000 rows (skipped)
/// Row Group 2, 1000 rows (selected, but want to only scan rows 50-100)
/// Row Group 3, 1000 rows (selected, but want to only scan rows 200-300)
/// ```
///
/// You would pass the following [`RowSelection`]:
/// You could pass the following [`RowSelection`]:
///
/// ```text
/// Select 1000 (scan all rows in row group 0)
/// Select 50-100 (scan rows 50-100 in row group 2)
/// Skip 50 (skip the first 50 rows in row group 2)
/// Select 50 (scan rows 50-100 in row group 2)
/// Skip 900 (skip the remaining rows in row group 2)
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Not showing this "skip remaining rows in row group" I think was part of what was confusing

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I don't remember if this is actually required, the readers actually trim trailing skips, but good to document regardless

/// Skip 200 (skip the first 200 rows in row group 3)
/// Select 100 (scan rows 200-300 in row group 3)
/// Skip 700 (skip the remaining rows in row group 3)
/// ```
///
/// Note there is no entry for the (entirely) skipped row group 1.
///
/// Note you can represent the same selection with fewer entries. Instead of
///
/// ```text
/// Skip 900 (skip the remaining rows in row group 2)
/// Skip 200 (skip the first 200 rows in row group 3)
/// ```
///
/// you could use
///
/// ```text
/// Skip 1100 (skip the remaining 900 rows in row group 2 and the first 200 rows in row group 3)
/// ```
///
/// [`Index`]: crate::file::page_index::index::Index
pub fn with_row_selection(self, selection: RowSelection) -> Self {
Self {
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