lance-format

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.

| 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-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 = "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).

| 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 | next (unstable) | Map type, Blob v2, VariablePackedStruct, larger mini-blocks. Required for Map and Blob v2; encodings may still change |

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.