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## Why `optimize(cleanup_older_than=timedelta(0))` deletes the data files of every version except the latest. Any other handle still on an older version then fails with a missing-file error, and nothing tells the caller this can happen. #2470 asks for a docs note and a runtime warning. Fixes #2470 ## Scope - `AsyncTable.optimize` emits a `UserWarning` when `cleanup_older_than` rounds to 0 ms or less. `LanceTable.optimize` runs through it, so sync callers get the warning too. - The `cleanup_older_than` docstring on `Table.optimize`, `LanceTable.optimize`, and `AsyncTable.optimize` gets a `.. warning::` block, in the same style as the existing `delete_unverified` one. - Removed the function-local `import warnings` in `AsyncTable.optimize`. `warnings` is already imported at module level, and the local import would shadow it for the new check (`UnboundLocalError`). - `test_optimize` and `test_optimize_delete_unverified` intentionally use a 0 cleanup, so they now assert the warning with `pytest.warns`. Nodejs `cleanupOlderThan` is out of scope, since the issue is filed against the Python SDK. ## Tradeoffs Like the existing `retrain` deprecation warning, the warning is raised inside the async coroutine. For sync callers it is attributed to `asyncio/events.py` rather than the caller's line. Warning from both the sync and async entry points would duplicate the check for a small gain. ## Verification - Repro: a second handle checked out at an older version, then `optimize(cleanup_older_than=timedelta(0))` on the first. The second handle's read fails with `LanceError(IO) ... Not found: .../data/<file>.lance`. Before this change no warning is emitted. After it, the `UserWarning` fires. - New tests `test_optimize_warns_on_zero_cleanup` (async, and checks that `timedelta(days=1)` and the default don't warn) and `test_optimize_warns_on_zero_cleanup_sync` fail on main (`DID NOT WARN`) and pass with the fix. The failing tests are in their own commit before the fix. - `pytest python/tests/test_table.py -m "not slow and not s3_test"`: all pass except `test_merge_insert`. That test's `timeout=timedelta(0)` assertion fails the same way on unmodified main on my machine. - `ruff format --check` and `ruff check` are clean on the changed files. Tests ran against the source tree with the `_lancedb` extension from the matching `0.40.0b11` wheel. The change touches no Rust. This PR was written by an AI coding agent (Claude Code) and reviewed before submission. 🤖 Generated with [Claude Code](https://claude.com/claude-code) --------- Co-authored-by: Jay Zhou <190699450+jayzhou2309@users.noreply.github.com> Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>
LanceDB Python SDK
A Python library for LanceDB.
Installation
pip install lancedb
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.