omnigraph/docs/dev/lance.md

Lance Docs Index (for OmniGraph agents)

OmniGraph sits on top of Lance. Many problems — index lifecycle, branching, transactions, fragments, compaction, vector/FTS internals — are answered upstream in Lance's docs, not in this codebase.

This file is the curated entry point. When you hit a Lance-shaped problem, find the matching topic below and fetch the listed URL(s) before guessing. Don't grep our codebase for behavior that is documented authoritatively in Lance.

Base URL: https://lance.org. Fetch the FULL page content, not summaries — use curl -sL <url> | pandoc -f html -t markdown or paste the rendered page text manually. Tools that summarize pages (like Claude's WebFetch) routinely drop load-bearing details — defaults, pub(crate) blockers, sub-specs hidden behind navigation hubs. Never act on a summarized fetch alone. Keep this index curated to relevant material — the upstream sitemap has hundreds of URLs (notably the Namespace REST API model surface, Spark/Trino/Databricks integrations) that we don't use.

Substrate boundary check. Before fetching, recall docs/dev/invariants.md: if Lance already does the thing, we don't reimplement it. The most common reason to read these docs is to confirm a substrate behavior, not to learn what to clone.

Quick-start (read these once per project)

Read when	URL
Onboarding to Lance — concepts in 10 min	https://lance.org/quickstart/
Onboarding to vector search	https://lance.org/quickstart/vector-search/
Onboarding to full-text search	https://lance.org/quickstart/full-text-search/
Onboarding to versioning / time travel	https://lance.org/quickstart/versioning/
Lance's own AGENTS.md (its agent guide)	https://lance.org/format/AGENTS/

By problem domain

Storage format & file layout

Touching db/manifest, fragment lifecycle, dataset reconstruction, or anything that reads/writes raw Lance state.

Topic	URL
Lance file format overview	https://lance.org/format/
File-level format spec	https://lance.org/format/file/
File encoding	https://lance.org/format/file/encoding/
File-level versioning	https://lance.org/format/file/versioning/
Table layout (fragments, manifest)	https://lance.org/format/table/layout/
Table schema metadata	https://lance.org/format/table/schema/
Table-level versioning	https://lance.org/format/table/versioning/
Transactions (commit semantics, conflict types)	https://lance.org/format/table/transaction/
MemWAL (durability story)	https://lance.org/format/table/mem_wal/
Row-ID lineage (stable row IDs)	https://lance.org/format/table/row_id_lineage/
Branches & tags (Lance native)	https://lance.org/format/table/branch_tag/

Branching / tags / time travel

Touching graph-level branches, snapshots, run isolation, the commit graph.

Topic	URL
Branch & tag format	https://lance.org/format/table/branch_tag/
Tags & branches operational guide	https://lance.org/guide/tags_and_branches/
Versioning quick-start	https://lance.org/quickstart/versioning/
Table-level versioning spec	https://lance.org/format/table/versioning/

Indexes

Adding/changing index types, fixing coverage, debugging FTS or vector recall, designing the reconciler.

Topic	URL
Index spec overview	https://lance.org/format/index/
BTREE scalar index	https://lance.org/format/index/scalar/btree/
Bitmap scalar index	https://lance.org/format/index/scalar/bitmap/
Bloom-filter scalar index	https://lance.org/format/index/scalar/bloom_filter/
Label-list scalar index	https://lance.org/format/index/scalar/label_list/
Zone-map scalar index	https://lance.org/format/index/scalar/zonemap/
R-Tree scalar index (spatial)	https://lance.org/format/index/scalar/rtree/
Full-text search (FTS) index	https://lance.org/format/index/scalar/fts/
N-gram scalar index	https://lance.org/format/index/scalar/ngram/
Vector index	https://lance.org/format/index/vector/
Fragment-reuse system index	https://lance.org/format/index/system/frag_reuse/
MemWAL system index	https://lance.org/format/index/system/mem_wal/
HNSW Rust example	https://lance.org/examples/rust/hnsw/
Distributed indexing	https://lance.org/guide/distributed_indexing/
Tokenizer (FTS, n-gram)	https://lance.org/guide/tokenizer/

Reads & writes

Touching the bulk loader, mutation execution, merge_insert, WriteMode selection.

Topic	URL
Read-and-write guide	https://lance.org/guide/read_and_write/
Distributed write	https://lance.org/guide/distributed_write/
Rust example: write & read a dataset	https://lance.org/examples/rust/write_read_dataset/

Schema evolution

Touching apply_schema, the migration planner, additive evolution.

Topic	URL
Data-evolution guide	https://lance.org/guide/data_evolution/
Migration guide	https://lance.org/guide/migration/

Object store / S3

Touching storage.rs, S3-compatible backends (RustFS, MinIO), env vars.

Topic	URL
Object-store guide	https://lance.org/guide/object_store/

Data types

Touching schema-language scalar mappings, blob columns, JSON, list columns.

Topic	URL
Data types overview	https://lance.org/guide/data_types/
Arrays / list types	https://lance.org/guide/arrays/
Blobs (LargeBinary)	https://lance.org/guide/blob/
JSON	https://lance.org/guide/json/

Performance & tuning

Optimizing scans, fragment counts, cache behavior, memory pool sizing.

Topic	URL
Performance guide	https://lance.org/guide/performance/

Compaction & cleanup

Touching omnigraph optimize / cleanup, the underlying compact_files / cleanup_old_versions.

Topic	URL
Read-and-write guide (covers compact_files, cleanup_old_versions)	https://lance.org/guide/read_and_write/
Performance (compaction tradeoffs)	https://lance.org/guide/performance/
Fragment-reuse index	https://lance.org/format/index/system/frag_reuse/

DataFusion integration

The runtime substrate that may carry our query execution. See docs/dev/invariants.md: we don't rebuild relational machinery.

Topic	URL
DataFusion integration	https://lance.org/integrations/datafusion/

SDK reference

Looking up a specific Rust API (signature, return type, error variant).

Topic	URL
SDK docs landing	https://lance.org/sdk_docs/

What's not in this index (and why)

• Namespace REST API model surface (/format/namespace/client/operations/models/...) — hundreds of REST schema docs for the Lance Namespace catalog API. Omnigraph does not run a Lance Namespace server, so these are not reachable from our problem space.
• Spark / Trino / Databricks / Dataproc / Hive / Glue / Polaris / Iceberg / Unity / OneLake / Gravitino integrations — not part of OmniGraph's deployment surface.
• Python / TF / PyTorch / Hugging Face / Ray integrations — OmniGraph is Rust-only; Python notebooks aren't relevant.
• Community / governance / release / voting / PMC pages — meta, not technical.

If a future need pulls one of these into scope, add a row to the matching domain section above and link it from AGENTS.md's topic index.

Maintenance

When Lance ships a major release that changes any of the above (file format bump, new index type, transaction semantics change, new branching primitive), refresh this index in the same change as the omnigraph upgrade. Stale Lance pointers are worse than no pointers.

Last alignment audit: 2026-06-15 (Lance 7.0.0 upstream; omnigraph pinned at 7.0.0)

Migration from Lance 6.0.1 → 7.0.0 landed in this cycle. Arrow stayed 58, DataFusion stayed 53 (no change) — the only transitive bump is object_store 0.12.5 → 0.13.2. 141 upstream commits reviewed (6.0.1 → 7.0.0); no fixes lost (the 6.0.x release-branch backports are all forward-ported into 7.0.0). Behavior-affecting findings:

• object_store 0.13 moved convenience methods behind a new ObjectStoreExt trait (get/put/head/rename/delete; list/list_with_delimiter/put_opts stay on the core ObjectStore trait). Fix = add use object_store::ObjectStoreExt; to storage.rs and db/manifest/namespace.rs; no call-site changes. Mirrors Lance's own migration in PR #6672. The local-FS PutMode::Update gap is unchanged (still unimplemented upstream), so storage.rs::write_text_if_match's local content-token emulation stays.
• roaring must be pinned to 0.11.4 (cargo update -p roaring --precise 0.11.4). Lance 7.0.0's UpdatedFragmentOffsets newtype (PR #6650) derives Eq over HashMap<u64, RoaringBitmap>, which needs RoaringBitmap: Eq — added only in roaring 0.11.4 (roaring-rs PR #341). Lance's loose roaring = "0.11" constraint otherwise resolves the broken 0.11.3 and lance itself fails to compile (RoaringBitmap: Eq is not satisfied). roaring is transitive (no direct workspace dep); the pin lives only in Cargo.lock.
• _row_created_at_version for merge-insert INSERT rows now = the commit version (PR #6774; was a fallback of 1 / dataset-creation version). Flipped lance_version_columns.rs::lance_merge_insert_new_row_stamps_created_at_version to assert == v2. Production change-detection keys on _row_last_updated_at_version + ID-set membership, so classification logic is unaffected (the changes/mod.rs rationale comment was corrected).
• BTREE range-query bound inclusiveness fixed (PR #6796, issue #6792): x <= hi AND x > lo returned the wrong boundary row on 6.0.1. omnigraph today builds BTREE only on string @key columns (id/src/dst) and queries them by equality/IN, not range, so its current query patterns almost certainly never hit this bug — but the corrected boundary semantics are a contract we rely on the moment a BTREE-range path appears (BTREE-on-properties via the index-type tickets, or a range-on-key query). Pinned by lance_surface_guards.rs::btree_range_query_boundary_is_correct (reproduces #6792's 5-row + BTREE shape).
• WriteParams::auto_cleanup default flipped from on (every-20-commits) to None (PR #6755). On 6.0.1 the on-by-default hook could GC versions the __manifest pins for snapshots/time-travel. omnigraph owns cleanup explicitly (optimize.rs::cleanup_all_tables). Two parts to the fix, because auto_cleanup is create-time config only and has no effect on existing datasets (Lance write.rs docs): (1) auto_cleanup: None at all 11 WriteParams sites so new datasets store no cleanup config; (2) — the load-bearing half — skip_auto_cleanup: true on every commit path, because graphs created before the bump still carry the on-config in their datasets, and Lance's hook fires off the dataset's stored config at commit time (io/commit.rs: if !commit_config.skip_auto_cleanup). So the staged commit path (commit_staged → CommitBuilder::with_skip_auto_cleanup(true)), the __manifest publisher (MergeInsertBuilder::skip_auto_cleanup(true)), and the direct WriteParams paths all skip the hook. Without this, an upgraded graph would still auto-cleanup and delete __manifest-pinned versions. Pinned by lance_surface_guards.rs::skip_auto_cleanup_suppresses_version_gc (negative control + with-skip survival).
• Lance #6658 SHIPPED in 7.0.0 (DeleteBuilder::execute_uncommitted, exposed via PR #6781) → MR-A (migrate delete to the staged two-phase API) has since landed (dev-graph iss-950): delete_where is retired, deletes stage via TableStorage::stage_delete, and the guard was flipped to _compile_uncommitted_delete_field_shape (pins execute_uncommitted / UncommittedDelete). StagedWrite must carry UncommittedDelete.affected_rows through commit_staged so Lance's row-level rebase metadata is preserved. The parse-time D2 rule is retained as a deliberate boundary (constructive XOR destructive per query), not as scaffolding awaiting further work.
• The unenforced primary key is now immutable once set (lance::dataset::transaction, ~L2472–2480: if !primary_key_before.is_empty() && (writes_primary_key || primary_key_after != primary_key_before) → "the unenforced primary key is a reserved key and cannot be changed once set"). omnigraph marks __manifest.object_id as the unenforced PK (lance-schema:unenforced-primary-key) for merge-insert row-level CAS — baked into the manifest schema at init (db/manifest/state.rs). With the strand model there is no in-place migration, so the PK is only ever set at init: a graph that predates the annotation is refused on open (refuse_if_stamp_unsupported) and rebuilt via export/import, never re-keyed — which is also what Lance's immutability rule would require, since the wrong PK could not be changed once set. Pinned by lance_surface_guards.rs::unenforced_primary_key_is_immutable_once_set (red if Lance relaxes immutability).
• Native DirectoryNamespace no longer recognizes omnigraph's manifest-tracked tables (lance-namespace-impls dir.rs ~L1310): list/describe/create_table_version route through check_table_status, which reports an omnigraph table absent → TableNotFound. The decoupling is contingent on omnigraph's legacy boolean PK key, not an unconditional v7 property: v7's namespace eagerly adds the new lance-schema:unenforced-primary-key:position key to any __manifest lacking it; that write hits the immutable-PK rule above (the boolean key already set the PK), so ensure_manifest_table_up_to_date errors and the namespace silently falls back to directory listing. omnigraph keeps the boolean key deliberately — Lance honors it permanently (maps to PK position 0), and one uniform on-disk format beats a new-vs-old split (existing graphs can't be re-keyed to the position key under that same immutability rule). omnigraph production never uses Lance's native namespace (its publisher writes __manifest directly via merge_insert; its own namespace.rs impls are custom), so this is test-only — the test_directory_namespace_direct_publish_cannot_replace_native_omnigraph_write_path surface guard was realigned to the v7 behavior (it now asserts the native namespace is fully decoupled, which only strengthens the guard's thesis).
• Still NOT fixed in 7.0.0: vector-index two-phase (Lance #6666 open) — create_vector_index inline residual retained; blob-column compaction — compact_files_still_fails_on_blob_columns guard still red on a fix, optimize still skips blob tables behind LANCE_SUPPORTS_BLOB_COMPACTION.
• No Lance API surface omnigraph uses changed at compile time (the only compile break was object_store) — but two runtime behaviors did (the unenforced-PK immutability and the native-namespace TableNotFound, above), each caught by the full engine test suite rather than the build. CleanupPolicy, WriteParams (apart from the auto_cleanup default), CompactionOptions, the namespace models (resolved via lance-namespace-reqwest-client 0.7.7, unchanged across the bump), Operation, ManifestLocation, and MergeInsertBuilder shapes are all stable. Lesson: a clean build is not a clean alignment — run cargo test --workspace before declaring a Lance bump done.
• The v3→v4 migration-robustness surface guards were removed with the strand. An earlier cycle added dataset_open_missing_returns_not_found_variant and lance_error_incompatible_transaction_variant_exists to pin Lance error surfaces the migrate_v3_to_v4 backfill classified on. The strand retirement deleted that migration (storage is now strict-single-version — see invariants.md), so those guards and the legacy-read/stamp-bump code they pinned are gone. No current omnigraph code path classifies on those Lance variants.

Bump this date stanza on the next alignment pass.

Prior alignment audit: 2026-05-22 (Lance 6.0.1 upstream; omnigraph pinned at 6.0.1)

Migration from Lance 4.0.0 → 6.0.1 landed in this cycle (DataFusion 52 → 53, Arrow 57 → 58, lance-tokenizer 6.0.1 added, tantivy* removed). Direct 4 → 6 jump; v5.x was not used as an intermediate (rationale in ~/.claude/plans/shimmering-percolating-duckling.md). Behavior-affecting findings:

• DatasetIndexExt moved from lance-index to lance::index (Lance PR #6280, v5.0). Six import sites updated. lance-index::IndexType and lance-index::is_system_index stayed in lance-index. omnigraph-cli and omnigraph-server gained lance = { workspace = true } in their dev-dependencies.
• DescribeTableResponse gained is_only_declared: Option<bool> (lance-namespace 6.0+, v5.0 PR #6186). Set to Some(false) in both BranchManifestNamespace::describe_table and StagedTableNamespace::describe_table — every table we return is physically materialized via Dataset::open, never "declared-only."
• MergeInsertBuilder execute_reader return shape preserved (Arc<Dataset>, MergeStats); the publisher CAS chain at db/manifest/publisher.rs:370-391 works unchanged. Pinned by tests/lance_surface_guards.rs::_compile_merge_insert_builder_method_chain.
• LanceError::TooMuchWriteContention variant retained in v6.0.1 (no rename). The typed publisher translation at db/manifest/publisher.rs:417-430 continues to apply. Pinned by lance_surface_guards.rs::lance_error_too_much_write_contention_variant_exists.
• ManifestLocation field shape stable: .path: object_store::path::Path, .size: Option<u64>, .e_tag: Option<String>, .naming_scheme: ManifestNamingScheme. Pinned by lance_surface_guards.rs::manifest_location_field_shape.
• LanceFileVersion::default() flipped V2_0 → V2_1 (v5.0). No effect — every data_storage_version callsite explicitly pins Some(LanceFileVersion::V2_2) (load-bearing for blob v2: Blob v2 requires file version >= 2.2 enforced in lance/src/dataset/write.rs:748).
• Dataset::checkout_version(N).await?.restore().await?: restore() takes &mut self and returns Result<()> (mutates in place, does not consume + return a new dataset). The recovery rollback hammer at db/manifest/recovery.rs:505-522 continues to work. Pinned by lance_surface_guards.rs::_compile_checkout_version_then_restore_signature.
• DatasetBuilder::from_namespace(...).with_branch(...).with_version(...).load() surface preserved (the namespace builder chain at db/manifest/namespace.rs:162-174). Pinned by lance_surface_guards.rs::_compile_dataset_builder_from_namespace_signature.
• compact_files(&mut ds, CompactionOptions::default(), None) signature stable. CompactionOptions still does not expose data_storage_version; compact_files builds its own WriteParams { ..Default::default() }. Note: LanceFileVersion::default() is now V2_1 in v6, so optimize-rewritten fragments come out at V2_1 by default (was V2_0 in v4). Existing explicit V2_2 pins on creates/appends still apply.
• Dataset::optimize_indices(&mut self, &lance_index::optimize::OptimizeOptions) (via DatasetIndexExt) is a depended-on surface as of the index-coverage work: db/omnigraph/optimize.rs calls it after compact_files to fold appended/rewritten fragments into existing indexes (incremental merge, not retrain). It is a committing call (mutates in place, advances HEAD; no uncommitted variant in v6.0.1), so optimize treats it as an inline-commit residual under the SidecarKind::Optimize recovery sidecar. Signature pinned by lance_surface_guards.rs::_compile_optimize_indices_signature; the incremental-coverage behavior pinned by optimize_indices_extends_fragment_coverage (appended fragment uncovered before, covered after).
• Dataset::delete(predicate) returns DeleteResult { new_dataset: Arc<Dataset>, num_deleted_rows: u64 } — unchanged shape. Pinned by lance_surface_guards.rs::_compile_delete_result_field_shape. MR-A will repurpose this guard to the staged two-phase variant once DeleteBuilder::execute_uncommitted migration lands.
• File reader read methods now async (Lance PR #6710, v6.0). No effect — omnigraph reaches Lance exclusively through Dataset::scan and the staged-write API.
• Tokenizer vendored as lance-tokenizer (Lance PR #6512, v6.0). No effect — no direct tokenizer imports.
• Lance #6658 closed (2026-05-14) but DeleteBuilder::execute_uncommitted did not ship in v6.0.1 — binary search across the release stream shows it first appears in v7.0.0-beta.10 (the closing commits landed on main but didn't backport to the 6.x line). Tracked as MR-A: migrate delete_where to staged, retire the parse-time D2 mutation rule, extend recovery sidecar coverage. Gated on the Lance v7.x bump, not this PR. v7.0.0-rc.1 dropped 2026-05-21.
• Lance #6666 still open (build_index_metadata_from_segments public): vector-index two-phase blocked; inline create_vector_index residual retained.
• Lance #6877 still open (MergeInsertBuilder dup-rowid): PR #109's SourceDedupeBehavior::FirstSeen + check_batch_unique_by_keys precondition stay load-bearing.
• Dataset::force_delete_branch (branches().delete(name, force=true), dataset.rs:524) tolerates a missing branch-contents ref (vs plain delete_branch's RefNotFound), but on the local store still errors NotFound if the branch tree/ directory is fully absent (remove_dir_all's NotFound is not caught for Lance's native error variant, refs.rs:526-549). Both variants still refuse a branch with referencing descendants (RefConflict). TableStore::force_delete_branch wraps this to be fully idempotent (tolerates already-absent). The single-authority branch-delete redesign uses it for orphan reclamation (eager best-effort reclaim + cleanup reconciler). Pinned by lance_surface_guards.rs::force_delete_branch_semantics. Branch delete is "flip the ref atomically, then remove_dir_all(tree/{branch})"; branch-exclusive data lives under tree/{branch}/ so a drop reclaims it immediately without touching main.
• Lance blob-v2 compact_files bug (no public issue found as of 2026-06): compact_files disables binary-copy for blob datasets and forces BlobHandling::AllBinary on the read side; the v2.1+ structural decoder then mis-counts column infos for the blob-v2 struct and fails with Invalid user input: there were more fields in the schema than provided column indices / infos (lance-encoding/src/decoder.rs::ColumnInfoIter::expect_next). This fails even a pristine uniform-V2_2 multi-fragment blob table; vector/list/scalar/ragged columns and mixed file versions all compact fine. Reads/queries use descriptor handling (BlobHandling::default()) and are unaffected. optimize skips blob-bearing tables behind LANCE_SUPPORTS_BLOB_COMPACTION = false (db/omnigraph/optimize.rs), reporting SkipReason::BlobColumnsUnsupportedByLance. Pinned by lance_surface_guards.rs::compact_files_still_fails_on_blob_columns, which turns red when the bug is fixed → flip the gate, remove the skip branch + the maintenance.rs::optimize_skips_blob_table_and_reports_skip skip assertions.

Surface guards added: crates/omnigraph/tests/lance_surface_guards.rs (10 named guards; 5 runtime + 5 compile-only; plus the index-coverage work's _compile_optimize_indices_signature and optimize_indices_extends_fragment_coverage). Future Lance bumps re-run this file first as the smoke check. Two additional guards from the original plan deferred to follow-up (manifest_cas_returns_row_level_contention_variant needs full publisher-race harness; table_version_metadata_byte_compatible_with_v4 needs pub(crate) reach extension).