Jack Ye
1564ec10c8
## Context
`StreamingDataset`'s shipped split strategies (`split_random`,
`split_sequential`, `split_hash`, `split_calculated`) all share one
trait: every row in the base table is assigned to exactly one split,
exactly once. They only differ in *how* that one-to-one assignment is
chosen -- none of them can express "give me a resampled stream of N
rows, allowing repeats."
That's a distinct, common need in ML training: **the bootstrap** -- the
statistical technique (Efron, 1979) of estimating a dataset's properties
by resampling it *with replacement*. It's the same sense scikit-learn's
bagging classifiers mean by `bootstrap=True`. `SplitStrategy::Bootstrap`
adds exactly this to the data loader: sample rows uniformly with
replacement from the (optionally filtered) base table, independently per
split.
## Main use cases
- **Bagging / ensemble training** (e.g. random forests): each split
trains on an independent bootstrap resample of the training set.
- **Oversampling a rare class**: filter to the minority class, then
sample it up to whatever count balances it against the majority, instead
of exhausting its small set once per epoch.
- **Fixed-size training steps independent of dataset size**: want
exactly N batches per epoch regardless of whether the table has 50 rows
or 50 million? Set `num_samples` directly instead of it being implicitly
`len(dataset)`.
- **Weighted sampling without a weight column**: filter to "eligible"
rows (e.g. a windowed-sampling workload filtering to valid window-start
positions), then bootstrap the filtered set -- an item with more
eligible rows is naturally drawn proportionally more often, with no
explicit per-row weight needed.
## Example (an actual run, not a hypothetical one)
A 5-row base table (`row_id` 0..4), sampled via `Bootstrap {
num_samples: 8, sizes: Fixed(2), seed: Some(42) }`:
```
row_id split_id
0 0
2 0
4 0
4 0 <- row 4 repeated within split 0
4 1 <- row 4 also drawn in split 1 (independent draw)
4 1
1 1
2 1
```
8 output rows from a 5-row table -- not 5. Row 4 drawn 4 times total,
row 3 never drawn. That's with-replacement sampling actually behaving
like with-replacement sampling, not an even round-robin dressed up with
a new name.
## What changed
Adds `SplitStrategy::Bootstrap` (uniform sampling with replacement,
optionally filtered, independently seeded per split -- a given split's
content doesn't depend on how many other splits there are, the same
elastic-determinism property documented for the loader's other
strategies). No new `PermutationBuilder` Rust method was needed:
`with_split_strategy` already accepts any `SplitStrategy` value
generically. Python binding:
`PermutationBuilder.bootstrap_sample(num_samples, *, ratios=None,
counts=None, fixed=None, seed=None, split_names=None)`, mirroring
`split_random`'s existing shape.
Also adds a generic `split_strategy` parameter to `StreamingDataset`'s
constructor (a callable taking/returning a `PermutationBuilder`), since
it previously could only reach `split_random`/`split_sequential` even
though `split_hash`/`split_calculated` already shipped. This unblocks
`Bootstrap` and any future strategy without further constructor changes.
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.