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BHAWESHBHASKAR
a9146d536a
## What kind of change does this PR introduce?
Feature. Exposes the NGram scalar index in the Python SDK.
## What is the current behavior?
The NGram scalar index already exists in the Rust core (`Index::NGram`) and is accepted over the remote protocol (the `"NGRAM"` type), but the Python SDK has no way to create it. `lancedb.index` offers `BTree`, `Bitmap`, `LabelList`, `Fm`, and the vector indexes, but not `NGram`, so it cannot be built through `create_index(config=...)`.
## What is the new behavior?
Adds an `NGram` config to `lancedb.index` and wires it through the pyo3 binding:
```python
from lancedb.index import NGram
await table.create_index("text", config=NGram())
```
The NGram index accelerates substring filters such as `contains(col, 'needle')`, along with some `LIKE` and regular-expression predicates, on UTF-8 string columns. It is parameter free and uses Lance's default trigram parameters.
Changes:
- `lancedb.index`: new `NGram` dataclass, added to `__all__`.
- pyo3 binding (`python/src/index.rs`): a match arm mapping `NGram` to `LanceDbIndex::NGram`, plus the import and the error message.
- Type hints: `NGram` added to the `create_index` config unions and the isinstance check in `table.py`, the `IndexConfigType` union, and the `_lancedb.pyi` stubs; `"NGRAM"` added to the `IndexType` and `IndexStatistics.index_type` literals.
- Test: `test_create_ngram_index` covers creation, `list_indices`, `index_stats`, and that a substring filter uses the index.
- Docs: an NGram section in the scalar index guide.
This mirrors the approach used for the RTree index in https://github.com/lancedb/lancedb/pull/4555 (that PR keeps the RTree scope; this one adds the remaining parameter free string index).