Lance v7 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 v7.1.0-beta.2 (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

24 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.

All share version = "7.1.0-beta.2" except lance-arrow-scalar, which is pinned at 58.0.0 to track Arrow. Workspace: edition 2024, rust-version = 1.91.0, resolver = "3".

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).

stable and next are aliases resolved by the running Lance release - pin an explicit number for deterministic behavior.

What's new in v7

The v6 -> v7 boundary is one breaking change: feat!: make dataset object store access base-aware (#6647) - object-store access is now scoped to a dataset base rather than a flat global path, which underpins multi-base storage (hot/cold tiering, shallow clones).

The dominant theme across the v7 betas is MemWAL - an experimental LSM / write-ahead-log architecture for high-throughput streaming writes (WAL appender/tailer primitives, shard writers, a Lance-native in-memory HNSW index, the shared-memory:// object-store scheme). Also landing in the v7 era: branches (Git-like, alongside tags), segmented and distributed index builds (FTS, bitmap, btree), newer scalar indexes (zonemap, bloom filter, ngram), the geo / RTree index and lance-geo crate, manifest version hints for fast latest-version lookup, and a formal split of the catalog / namespace / table / index specifications. The v7.1.0-beta.1 tag opens the v7.1 line and adds a materialized-view namespace API. v7.1.0-beta.2 then extracts mask code into a new lance-select crate (#6879) and lands two MemWAL correctness fixes - flushed memtables now build their secondary indexes so vector rows are visible to fast_search (#6901), and a per-source PK-hash block-list post-filter suppresses stale LSM vector reads when the fresh row falls out of its source's top-k (#6899). Details in references/lance-reference.md section 14.

Navigating the reference

references/lance-reference.md is the full v7 reference, regrounded against the v7.1.0-beta.2 source. Load the section for your task:

1. What Lance is - the lakehouse spec stack
2. Crate workspace - all 23 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), scalar (btree/bitmap/bloom/labellist/ngram/ zonemap), full-text (BM25, tokenizers), geo/RTree
12. Distributed write and indexing - two-phase commits, segmented index builds
13. Object store - URI schemes, storage options, per-backend config
14. What changed in v7 - the full v7 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/v7.1.0-beta.2/<path>.

To refresh: git -C ~/pjv/lance-format/lance fetch --tags, check out the newest v7* 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 v7.1.0-beta.2 reference. Line numbers in citations drift between tags - treat them as approximate.