feat(scan): deferred materialization for heavy columns#175
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Xuanwo merged 2 commits intolance-format:mainfrom Mar 18, 2026
Merged
feat(scan): deferred materialization for heavy columns#175Xuanwo merged 2 commits intolance-format:mainfrom
Xuanwo merged 2 commits intolance-format:mainfrom
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…shdown fails When DuckDB's filter cannot be pushed down to Lance SDK (unsupported functions, CAST+LIKE, join predicates, etc.), Lance reads all columns eagerly since it has no filter to optimize around. This wastes I/O on heavy columns (BLOBs, large vectors) for rows that DuckDB will filter out post-scan. This adds C++-side deferred materialization for this specific gap: 1. During scan init, detect heavy columns via `bytes_on_disk` stats (>1KB/row threshold) with type-based fallback (BLOB, LIST of numeric/BLOB, ARRAY>=256, MAP, recursive STRUCT). 2. Remove heavy columns from the Lance scan projection, add `_rowid`. 3. After DuckDB applies its post-scan filter, call `lance_create_dataset_take_stream_unfiltered` with surviving row IDs to fetch only the heavy columns for surviving rows. 4. Merge deferred columns into the output. The optimization only activates when: - Filter pushdown to Lance failed (`!filter_pushed_down`) - DuckDB has pending table filters - Heavy columns exist in the projection but not in the filter Controlled by: `SET lance_deferred_materialization = true|false` (default: true). Benchmark (20K rows, 5 fragments, 100KB blob/row, ~2GB): - Filter 3% selectivity: 555ms → 53ms (10.5x faster) - Filter 10% selectivity: 558ms → 162ms (3.4x faster) Note: When filter IS pushed to Lance, Lance SDK's internal MaterializationStyle::Heuristic already handles late materialization (LanceRead → TakeExec), so this optimization correctly does not activate in that case. Also adds: - Rust FFI: `lance_dataset_list_named_field_stats` for stats-based heavy column detection (resolves field IDs to column names) - Test: `scan_deferred_materialization.test` with plan verification, correctness, setting toggle, and multi-fragment coverage Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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Summary
MaterializationStyle::Heuristicalready handles late materialization — this optimization correctly does not activatelance_dataset_list_named_field_statsfor stats-based heavy column detectionSET lance_deferred_materialization = true|false(default: true)How it works
bytes_on_diskstats (>1KB/row threshold) with type-based fallback (BLOB, LIST, ARRAY≥256, MAP)_rowidfrom Lance (without heavy cols)take_rows(surviving_rowids, heavy_cols)fetches heavy data only for surviving rowsBenchmark
20K rows, 5 fragments, 100KB blob/row, ~2GB on disk, non-pushable filter (
id::VARCHAR LIKE '%42%'):Test plan
test/sql/scan_deferred_materialization.test(37 assertions)SET lance_deferred_materialization = falsedisables itCloses #170
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