Lance v8 reference

Lance is an open columnar format for multimodal AI - "a columnar data format that is 100x faster than Parquet for random access." It is not one format but a stack of interoperating specs: a file format, a table format, index formats, catalog specs, and a namespace client spec. The Rust workspace at lance-format/lance implements all of them plus Python (pylance) and Java bindings.

This skill tracks v8.0.0-beta.14 (the lance-format/lance git tag). Pin against tags, not main - Lance ships beta tags every few days and next-format encodings can change.

The deep reference is references/lance-reference.md. Load it for any concrete schema, parameter, proto, or constraint. This file is the orientation: read it first, then jump into the reference section you need.

Lance vs LanceDB

These are two different things and conflating them produces wrong answers.

• Lance - the format and engine. The lance-format/lance repo; the lance /lance-* Rust crates; pylance. It gives you datasets, the file/table format, indexes, commits, scans. Consumed directly by DuckDB, Polars, Ray, Spark, PyTorch, DataFusion, or your own Rust/Python code. This skill is about Lance.
• LanceDB - a separate database product (lancedb/lancedb) built on top of Lance. It adds a query-builder API, an embedding registry, rerankers-as-API, multi-language SDK parity, and managed Cloud / Enterprise tiers. Not covered here.

If you are linking the lance crate in Cargo.toml, you are using Lance directly - use this skill. If a question is about LanceDB internals, the storage layer underneath it is still Lance, so this skill remains the authority for the format itself.

The crate workspace

25 crate directories under rust/. lance is the public entry point; the rest are layers beneath it. Full table with descriptions and citations in references/lance-reference.md section 2.

| Crate | Role | |-------|------| | lance | Public entry point - Dataset, scanner, indexes, commits | | lance-table | Table format - manifest, feature flags, commit handlers, row IDs | | lance-file | File format - file reader/writer | | lance-encoding | Structural encodings, compression (internal, not for external use) | | lance-index | Scalar / vector / FTS / system indexes | | lance-io | Object store, I/O schedulers | | lance-core | Shared Error/Result, cache, datatypes | | lance-datafusion | DataFusion glue (exec, expr, planner, UDFs) | | lance-linalg | SIMD L2 / dot / cosine / hamming kernels | | lance-select | Row-selection primitives - RowAddrMask, RowIdMask, IndexExprResult (extracted from lance-core/lance-index in v7.1.0-beta.2) | | lance-tokenizer | FTS tokenizer stack (simple, ngram, jieba, lindera, stemmers) | | lance-derive | #[derive(DeepSizeOf)] proc-macro for Arrow-aware memory accounting (new in v8, PR #6229; replaced the external deepsize crate) | | lance-geo | Geospatial UDFs (feature-gated geo) | | lance-namespace / -impls / -datafusion | Namespace trait, Directory/REST impls, DataFusion catalog bridge | | lance-arrow, lance-tools, fsst, lance-bitpacking, ... | Arrow extensions, CLI, compression sub-crates |

All share version = "8.0.0-beta.14" except lance-arrow-scalar, which is pinned at 58.0.0 to track Arrow. Workspace: edition 2024, rust-version = 1.91.0, resolver = "3"; notable deps arrow 58, datafusion 53, opendal 0.57, jieba-rs 0.10.

File format versions

The file format carries a single major.minor version. Selected per-dataset at creation via data_storage_version and fixed once the dataset exists (to change it, rewrite the dataset).

| Version | Status | Notes | |---------|--------|-------| | 0.1 (legacy) | read-only | Original format; no longer writable | | 2.0 | stable | Removed row groups; null support for lists/FSL/primitives | | 2.1 | current default (stable) | Adaptive structural encodings; better integer/string compression; nulls in struct fields; better nested random access. Default since Lance 5.0.0 | | 2.2 | unstable | Map type, Blob v2, VariablePackedStruct, larger mini-blocks. Required for Map and Blob v2; the real experimental frontier - encodings may still change | | 2.3 | unstable (next) | The current next alias target (V2_3 in the enum). Scaffolding only - no distinct encodings yet (6 refs vs 98 for 2.2 in lance-encoding); the docs version table still lists only 2.2 |

stable resolves to the default (2.1); next now resolves to 2.3 (not 2.2) in the running Lance release - pin an explicit number for deterministic behavior. In the version ladder 2.2 sits below next, so the code does not flag 2.2 as unstable even though the docs do; 2.2 is the version actually carrying the experimental features.

What's new in v8

The v7 -> v8 boundary unifies all index builds onto one segment-based lifecycle. The spine is feat!: migrate bitmap to index segment based (#6869): the old public Python Bitmap shard workflow (create_scalar_index(..., fragment_ids=) + merge_index_metadata(..., "BITMAP")) is gone, and bitmap now flows through the same create_index_uncommitted -> merge_existing_index_segments -> commit_existing_index_segments path as everything else. Five more breaking changes land with it: the standalone IndexSegmentBuilder API was removed from Rust/Python/Java (#6997, build_all() and target_segment_bytes gone); distributed BTree build moved to the segmented framework (#7013); file/index writers' finish() now returns FileWriteSummary { num_rows, size_bytes } instead of a bare row count (#7096); describe_indices()/list_indices() report full nested field paths and derive type from index details without opening the index (load_indices() removed, #6903); and index directories are no longer listed after writes - IndexFile metadata is propagated into the manifest instead (#7129).

Net-new in v8: the lance-derive crate (#6229, #[derive(DeepSizeOf)], replacing the external deepsize; workspace 24 -> 25 crates); the FM-Index scalar index (Ferragina- Manzini / Burrows-Wheeler substring, prefix, and regex search on raw bytes, built on the Segmented Index architecture); multi-bit IVF_RQ (RaBitQ num_bits 1..=9, no longer 1-bit-only, plus a raw-query distance estimator); independent per-worker vector index models for distributed builds; and the Volcengine TOS (tos://) and feature-gated GooseFS (goosefs://) object stores. merge_existing_index_segments now also covers zone-map segments. Unchanged at v8: still 15 transaction ops, next still resolves to 2.3 (default still 2.1), CommitConfig.num_retries still 20, the feature-flag bits, and the ConditionalPutCommitHandler routing. The v7 era - MemWAL, branches, the geo/RTree index, the lance-select crate, ICU FTS - all carry forward.

Later v8 betas (beta.9 -> beta.14) add a public vector-search approx_mode (fast/normal/accurate) for RaBitQ indexes (a breaking ann.proto change), a 4096 default IVF_RQ target_partition_size, a cleanup explain API, and full Tencent COS / GooseFS object-store config docs; they remove the table_version_storage_enabled / __manifest version path and brotli. All structural invariants above are unchanged. Details in references/lance-reference.md section 14.

Navigating the reference

references/lance-reference.md is the full v8 reference, regrounded against the v8.0.0-beta.14 source. Load the section for your task:

1. What Lance is - the lakehouse spec stack
2. Crate workspace - all 25 crates, what each does, the public entry point
3. File format - versions, container layout, structural encoding (mini-block / full-zip / constant / blob page types), compression schemes, blob encoding
4. Data types - Arrow type coverage, FixedSizeList for vectors, JSON (JSONB), blob, ML extension arrays (bfloat16, image types)
5. Table format - dataset directory layout, manifest contents, fragments, deletion files, base paths
6. Schema evolution - field IDs, zero-copy column add/drop/alter, why old rows read NULL
7. Versioning, tags, branches - manifest versions, time travel, tag pinning, branches
8. Row IDs - row address vs stable row ID, lineage, change-data-feed columns
9. Transactions and concurrency - the 15 transaction ops, OCC retry/rebase, commit handlers (conditional-put, DynamoDB), conflict resolution matrix
10. MemWAL - shards, MemTable/WAL/flush, the appender/tailer/flusher model, fencing
11. Indexes - vector (IVF/HNSW/PQ/SQ/RQ, multi-bit RQ), scalar (btree/bitmap/bloom/ labellist/ngram/zonemap/FM-Index), full-text (BM25, tokenizers), geo/RTree
12. Distributed write and indexing - two-phase commits, segment-based index builds
13. Object store - URI schemes, storage options, per-backend config
14. What changed in v8 - the full v7 -> v8 delta
15. Capability matrix - what Lance can and cannot do
16. Source map - where each spec and proto lives in the repo

Maintenance

Citations in references/lance-reference.md are path:line relative to the lance-format/lance repo; build a permalink as https://github.com/lance-format/lance/blob/v8.0.0-beta.14/<path>.

To refresh: git -C ~/pjv/lance-format/lance fetch --tags, check out the newest v8* tag, re-read the format spec under docs/src/format/ and the user guide under docs/src/guide/, re-verify the crate workspace, and bump metadata.upstream plus every v8.0.0-beta.14 reference. Line numbers in citations drift between tags - treat them as approximate.