perf: optimize stable row_id index build from O(rows) to O(fragments)

[jiaoew1991]

Summary

When enable_stable_row_id is enabled, the first take_rows() call triggers a full RowIdIndex build. On large datasets this cold start was extremely slow (18.9s for 968M rows, 220s for 4.26B rows).

Root Causes

1. O(total_rows) segment expansion in decompose_sequence

The original code expanded every U64Segment element-by-element, even for Range segments with no deletions. For a Range(0..273711) with no deletions, this meant 273K iterations, deletion vector checks, temporary allocations, and re-compression — only to produce the same Range back. Across 18,243 fragments averaging 233K rows, this totaled 4.26 billion iterations with ~32 GB of temporary allocations.

2. O(N²) fragment lookup in load_row_id_index

The original code called fragments.iter().find() (O(N) linear search) for each of N fragments, resulting in O(N²) comparisons. try_join_all spawned all N futures at once, overwhelming the async runtime. get_deletion_vector() was called unconditionally even for fragments without deletion files.

Solution

Fix 1: O(1) fast path for Range segments without deletions. When a fragment has no deletions and its row_id sequence is a Range, construct the index chunk directly without iterating.

Fix 2: HashMap lookup + conditional deletion vector loading. Use a HashMap<u32, &FileFragment> for O(1) lookup, buffer_unordered for controlled concurrency, and skip get_deletion_vector() when there's no deletion file.

Results

Dataset	Before	After	Speedup
968M rows, 3,540 fragments	18.9s	150ms	126x
4.26B rows, 18,243 fragments	220s	89ms	2,471x

Test plan

• All existing rowids tests pass (45 tests)
• Clippy clean
• New test test_large_range_segments_no_deletions validates fast path correctness at boundaries and performance (100 fragments × 250K rows completes < 1s)
• Verify on real datasets with deletions to ensure slow path correctness

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 97.26027% with 4 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
rust/lance-table/src/rowids/index.rs	97.69%	2 Missing and 1 partial ⚠️
rust/lance/src/dataset/rowids.rs	93.75%	1 Missing ⚠️

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