Lei XuandClaude Opus 5.5
1772f4ef83
## Problem
`permutation_builder(table).execute()` and `StreamingDataset(table,
...)` (which builds a permutation at construction) panic with
`RustPanic` on any table whose Arrow schema has schema-level metadata:
```python
schema = pa.schema([pa.field("x", pa.int64())], metadata={b"huggingface": b"{}"})
t = lancedb.connect(tmp).create_table("t", pa.table({"x": list(range(100))}, schema=schema))
permutation_builder(t).split_random(fixed=2, seed=1).execute() # RustPanic
```
Every Hugging Face Lance dataset triggers this, e.g.
`hf://datasets/lance-format/lerobot-xvla-soft-fold/data` table `frames`
carries `source-column-name-map`.
## Root cause
`PermutationBuilder::add_config_metadata` replaced the stream schema's
metadata with the permutation config and then called
`batch.with_schema(schema).unwrap()`. The base-table scan's batches
carry the base schema metadata, and `RecordBatch::with_schema` requires
the target to be a superset of the current schema, metadata included. It
returned `SchemaError("target schema is not superset of current schema
...")`, and the `unwrap` turned that into a panic on a tokio worker.
## Fix
- The permutation table deliberately does **not** inherit base-table
schema metadata. That metadata describes columns the permutation table
(`row_id`, `split_id`) does not have, and its keys can collide with
config keys such as `split_names`. A merge would let a base table's
`split_names` leak into, or override, the permutation's split names.
Batches are rebuilt with `RecordBatch::try_new(schema, columns)`.
- Errors are propagated (`map_ok` → `and_then`) instead of `unwrap`.
- The same panic-on-worker pattern in the splitter's streaming closures
(`split.rs`: `try_new`/`filter(...).unwrap()`) now propagates errors
too. Nothing else in `dataloader/` used `with_schema(...).unwrap()`.
## Tests
- Rust: `test_permutation_over_table_with_schema_metadata` in
`builder.rs` builds a random split + shuffled permutation over a table
with `huggingface` and a colliding `split_names` key. It panics with the
original `SchemaError` on the old code and passes now.
- Python: `test_permutation_over_table_with_schema_metadata` in
`test_permutation.py` covers the repro and reads both splits through
`Permutations`/`Permutation.from_tables`.
`test_streaming_dataset_over_table_with_schema_metadata` in
`test_elastic_dataloader.py` covers `StreamingDataset` construction and
iteration.
- `cargo fmt`, `cargo clippy --features remote --tests --examples`, the
dataloader Rust tests (42), `test_permutation.py` (72), and
`test_elastic_dataloader.py` all pass.
`test_torch.py::test_remote_permutation_dataloader_multiprocessing`
fails locally with `take query result did not include ordering column
'__lancedb_take_row_offset'`. It uses `Permutation.identity`, which does
not go through `PermutationBuilder`, so this change does not cause it.
🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>
LanceDB Python SDK
A Python library for LanceDB.
Installation
pip install lancedb
If no wheel is published for your platform, pip builds lancedb from the source distribution. That build needs a Rust toolchain and protoc (the Protocol Buffers compiler) on your PATH, and can take a while.
Pre-Haswell x86_64 hosts: lancedb-compat
The default lancedb wheel targets x86-64-haswell (AVX2 + FMA + F16C) for full performance on modern hardware. Pre-Haswell hosts — Intel Sandy Bridge / Ivy Bridge / Westmere; AMD Bulldozer / Piledriver / Steamroller — don't have AVX2 and crash with Illegal instruction at import lancedb.
For those hosts, install the lancedb-compat package instead:
pip install lancedb-compat
Same Python API (import lancedb works as usual). The compat wheel is compiled at the x86-64-v2 baseline (Nehalem-class) and uses runtime SIMD dispatch in the embedded lance crate to pick the right kernel tier (scalar / AVX / AVX+FMA / AVX2+FMA / AVX-512) at load time, so it still goes fast on modern hardware while running cleanly on the pre-Haswell silicon. Use lance.simd_info() from Python to verify which tier was selected.
lancedb and lancedb-compat install to the same lancedb/ namespace and conflict at install time. Pick one. To switch, pip uninstall lancedb first, then pip install lancedb-compat (or vice-versa).
If you need a custom baseline (or lancedb-compat isn't yet published for your platform), build from source with the override:
RUSTFLAGS="-C target-cpu=x86-64-v2" maturin build --release
pip install ./target/wheels/lancedb-*.whl
Preview Releases
Stable releases are created about every 2 weeks. For the latest features and bug fixes, you can install the preview release. These releases receive the same level of testing as stable releases, but are not guaranteed to be available for more than 6 months after they are released. Once your application is stable, we recommend switching to stable releases.
pip install --pre --extra-index-url https://pypi.fury.io/lancedb/ lancedb
Threading in CPU-limited containers
LanceDB uses separate pools for compute work and storage I/O. On a container with two visible CPUs, current releases intentionally use one compute worker by default; no manual configuration is needed. If every query logs an I/O core reservation warning on a two-CPU container, upgrade from LanceDB 0.21.1 or earlier.
The two commonly tuned environment variables control different resources:
LANCE_CPU_THREADSoverrides the number of compute workers. One worker is the appropriate setting for a two-CPU container when an explicit override is needed.LANCE_IO_THREADScontrols concurrent storage operations, not reserved CPU cores. Its default can be greater than the number of CPUs because I/O workers spend much of their time waiting for storage.
Keep the defaults unless measurements show that the workload benefits from an override. See the Lance threading model for the current defaults and tuning guidance.
Usage
Basic Example
import lancedb
db = lancedb.connect('<PATH_TO_LANCEDB_DATASET>')
table = db.open_table('my_table')
results = table.search([0.1, 0.3]).limit(20).to_list()
print(results)
Development
See CONTRIBUTING.md for information on how to contribute to LanceDB.