[I] Reuse `MaskSelection`'s cached selectors when converting a mask-backed `RowSelection` to selectors [arrow-rs]

["haohuaijin (via GitHub)" <gi...@apache.org>]

haohuaijin opened a new issue, #10422:
URL: https://github.com/apache/arrow-rs/issues/10422

   **Is your feature request related to a problem or challenge? Please describe what you are trying to do.**
   
   Follow up to #10141, which made `RowSelection` optionally backed by a `BooleanBuffer` bitmap.
   
   For mask-backed selections, `RowSelection::iter()` lazily materializes an RLE
   selector cache inside `MaskSelection` (a `OnceLock<Vec<RowSelector>>`). However,
   `RowSelection::into_selectors_vec()` (used by `From<RowSelection> for Vec<RowSelector>`
   and `From<RowSelection> for VecDeque<RowSelector>`) ignores that cache and calls
   `mask_to_selectors()` a second time. `selectors_mut()` has the same issue when
   promoting a mask-backed selection to selector backing in place.
   
   This means a caller that first iterates a selection and then consumes it converts
   the entire bitmap to RLE twice. DataFusion's
   `ParquetAccessPlan::into_overall_row_selection` does exactly this sequence: it
   calls `iter()` for validation and then converts the selection into selectors.
   
   Originally reported by @hhhizzz in
   https://github.com/apache/arrow-rs/pull/10141#discussion_r3608544008
   
   **Describe the solution you'd like**
   
   When the lazy cache has already been populated, consume it instead of re-running
   the conversion, e.g. take the `OnceLock`'s value in `into_selectors_vec()` /
   `selectors_mut()` (`OnceLock::into_inner` / `OnceLock::take`) and fall back to
   `mask_to_selectors()` only when the cache is empty.
   
   **Describe alternatives you've considered**
   
   Leave as is — the conversion is O(bitmap) either way, but doing it twice is pure
   waste for large selections.
   
   **Additional context**
   
   Per review feedback on #10141, this should land with benchmark coverage, e.g.
   extending `parquet/benches/row_selection_cursor.rs` with an
   "iterate-then-consume" scenario over a mask-backed selection
   (`cargo bench -p parquet --bench row_selection_cursor`).


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Re: [I] Reuse `MaskSelection`'s cached selectors when converting a mask-backed `RowSelection` to selectors [arrow-rs]

["alamb (via GitHub)" <gi...@apache.org>]

alamb closed issue #10422: Reuse `MaskSelection`'s cached selectors when converting a mask-backed `RowSelection` to selectors
URL: https://github.com/apache/arrow-rs/issues/10422


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