feat(python): add CombinedFieldsQuery for BM25F full text search

Add a typed Python `CombinedFieldsQuery` for BM25F search across multiple columns.

The query mirrors Lance's combined-fields semantics with ordered per-column boosts and AND/OR term matching. Convert it in both directions at the PyO3 boundary, including nested Boolean queries, and document the Python typed convenience entry.

Closes #4652.

python/python/lancedb/query.py

@@ -445,6 +445,7 @@ class FullTextQueryType(str, Enum):
     MATCH_PHRASE = "match_phrase"
     BOOST = "boost"
     MULTI_MATCH = "multi_match"
+    COMBINED_FIELDS = "combined_fields"
     BOOLEAN = "boolean"
 
 
@@ -669,6 +670,42 @@ class MultiMatchQuery(FullTextQuery):
         return FullTextQueryType.MULTI_MATCH
 
 
+@pydantic.dataclasses.dataclass
+class CombinedFieldsQuery(FullTextQuery):
+    """
+    Combined-fields (BM25F) query for full-text search.
+
+    Unlike `MultiMatchQuery`, which scores each column on its own and keeps
+    the best one, this query treats the columns as a single virtual field, so
+    term statistics are blended across them. A query term can then match in
+    any of the columns, e.g. a first name in one column and a last name in
+    another.
+
+    Parameters
+    ----------
+    query : str
+        The query string to match against.
+    columns : list[str]
+        The columns combined into the virtual field. Every column must have the
+        same tokenizer configuration, and at least one must have an FTS index.
+    boosts : list[float], optional
+        The weight of each column, aligned with `columns`. Each weight must be
+        at least 1. If not provided, every column has a weight of 1.
+    operator : FullTextOperator, default OR
+        The operator to use for combining the query terms.
+        If `AND`, every term must appear in at least one of the columns.
+        If `OR`, at least one term must match.
+    """
+
+    query: str
+    columns: list[str]
+    boosts: Optional[list[float]] = pydantic.Field(None, kw_only=True)
+    operator: FullTextOperator = pydantic.Field(FullTextOperator.OR, kw_only=True)
+
+    def query_type(self) -> FullTextQueryType:
+        return FullTextQueryType.COMBINED_FIELDS
+
+
 @pydantic.dataclasses.dataclass
 class BooleanQuery(FullTextQuery):
     """

python/python/tests/test_fts.py

@@ -26,7 +26,9 @@ from lancedb.db import DBConnection
 from lancedb.index import FTS
 from lancedb.query import (
     BoostQuery,
+    CombinedFieldsQuery,
     DocumentGranularity,
+    FullTextOperator,
     MatchQuery,
     MultiMatchQuery,
     PhraseQuery,
@@ -1367,6 +1369,103 @@ def test_match_query_fuzziness_search(tmp_db, options, expected):
     assert sorted(row["text"] for row in results) == sorted(expected)
 
 
+@pytest.mark.parametrize(
+    "operator,combined_ids,multi_match_ids",
+    [
+        # Combined fields treat the columns as one field, so AND also matches
+        # row 0, where "john" is in the title and "smith" is in the body.
+        (FullTextOperator.AND, {0, 1}, {1}),
+        (FullTextOperator.OR, {0, 1, 2}, {0, 1, 2}),
+    ],
+)
+def test_combined_fields_query(mem_db, operator, combined_ids, multi_match_ids):
+    table = mem_db.create_table(
+        "people",
+        pa.table(
+            {
+                "id": [0, 1, 2],
+                "title": ["john", "john smith", "john"],
+                "body": ["smith", "foo", "alice"],
+            }
+        ),
+    )
+    table.create_index("title", config=FTS())
+    table.create_index("body", config=FTS())
+
+    def ids(query):
+        return {row["id"] for row in table.search(query).select(["id"]).to_list()}
+
+    combined = CombinedFieldsQuery("john smith", ["title", "body"], operator=operator)
+    multi_match = MultiMatchQuery("john smith", ["title", "body"], operator=operator)
+    assert ids(combined) == combined_ids
+    assert ids(multi_match) == multi_match_ids
+
+
+def test_combined_fields_query_boosts(mem_db):
+    table = mem_db.create_table(
+        "docs", pa.table({"title": ["hello"], "body": ["world"]})
+    )
+    table.create_index("title", config=FTS())
+    table.create_index("body", config=FTS())
+
+    results = table.search(
+        CombinedFieldsQuery("hello", ["title", "body"], boosts=[1.5, 1.0])
+    ).to_list()
+    assert len(results) == 1
+
+    # Each weight must be at least 1, and there must be one per column.
+    with pytest.raises(ValueError, match="boost"):
+        table.search(
+            CombinedFieldsQuery("hello", ["title", "body"], boosts=[0.5, 1.0])
+        ).to_list()
+    with pytest.raises(ValueError, match="boosts"):
+        table.search(
+            CombinedFieldsQuery("hello", ["title", "body"], boosts=[1.0])
+        ).to_list()
+
+
+@pytest.mark.asyncio
+async def test_combined_fields_query_round_trip(mem_db_async):
+    table = await mem_db_async.create_table(
+        "docs", pa.table({"title": ["hello"], "body": ["world"]})
+    )
+    query = CombinedFieldsQuery(
+        "hello world",
+        ["title", "body"],
+        boosts=[2.0, 1.0],
+        operator=FullTextOperator.AND,
+    )
+
+    query_obj = table.query().nearest_to_text(query).to_query_object()
+
+    assert query_obj.full_text_query.query == query
+
+
+@pytest.mark.asyncio
+async def test_combined_fields_query_nested_round_trip(mem_db_async):
+    table = await mem_db_async.create_table(
+        "docs", pa.table({"title": ["hello"], "body": ["world"]})
+    )
+    query = BooleanQuery(
+        [
+            (
+                Occur.MUST,
+                CombinedFieldsQuery(
+                    "hello world",
+                    ["title", "body"],
+                    boosts=[2.0, 1.0],
+                    operator=FullTextOperator.AND,
+                ),
+            ),
+            (Occur.MUST_NOT, MatchQuery("draft", "body")),
+        ]
+    )
+
+    query_obj = table.query().nearest_to_text(query).to_query_object()
+
+    assert query_obj.full_text_query.query == query
+
+
 def test_fts_query_to_json():
     """Test that FTS query to_json() produces valid JSON strings with exact format."""
 
@@ -1504,6 +1603,17 @@ def test_fts_query_to_json():
     )
     assert json_str == expected
 
+    # Test CombinedFieldsQuery
+    combined_fields = CombinedFieldsQuery(
+        "python", ["tags", "title"], boosts=[2.0, 1.0], operator=FullTextOperator.AND
+    )
+    json_str = combined_fields.to_json()
+    expected = (
+        '{"combined_fields":{"query":"python","columns":["tags","title"],'
+        '"boost":[2.0,1.0],"operator":"And"}}'
+    )
+    assert json_str == expected
+
     # Test complex nested BooleanQuery
     inner1 = BooleanQuery(
         [

python/src/query.rs

@@ -17,8 +17,8 @@ use arrow::pyarrow::FromPyArrow;
 use arrow::pyarrow::IntoPyArrow;
 use arrow::pyarrow::ToPyArrow;
 use lancedb::index::scalar::{
-    BooleanQuery, BoostQuery, DocumentGranularity, FtsQuery, FullTextSearchQuery, MatchQuery,
-    MultiMatchQuery, Occur, Operator, PhraseQuery,
+    BooleanQuery, BoostQuery, CombinedFieldsQuery, DocumentGranularity, FtsQuery,
+    FullTextSearchQuery, MatchQuery, MultiMatchQuery, Occur, Operator, PhraseQuery,
 };
 use lancedb::query::AnalyzePlanDistributedMetrics;
 use lancedb::query::QueryBase;
@@ -148,6 +148,26 @@ impl<'a, 'py> FromPyObject<'a, 'py> for PyLanceDB<FtsQuery> {
 
                 Ok(Self(q.with_operator(op).into()))
             }
+            "CombinedFieldsQuery" => {
+                let query = ob.getattr("query")?.extract()?;
+                let columns = ob.getattr("columns")?.extract()?;
+                let boosts: Option<Vec<f32>> = ob.getattr("boosts")?.extract()?;
+                let operator: String = ob.getattr("operator")?.extract()?;
+
+                let q = CombinedFieldsQuery::try_new(query, columns)
+                    .map_err(|e| PyValueError::new_err(format!("Invalid query: {}", e)))?;
+                let q = if let Some(boosts) = boosts {
+                    q.try_with_boosts(boosts)
+                        .map_err(|e| PyValueError::new_err(format!("Invalid boosts: {}", e)))?
+                } else {
+                    q
+                };
+
+                let op = Operator::try_from(operator.as_str())
+                    .map_err(|e| PyValueError::new_err(format!("Invalid operator: {}", e)))?;
+
+                Ok(Self(q.with_operator(op).into()))
+            }
             "BooleanQuery" => {
                 let queries: Vec<(String, Self)> = ob.getattr("queries")?.extract()?;
                 let mut sub_queries = Vec::with_capacity(queries.len());
@@ -178,9 +198,6 @@ impl<'py> IntoPyObject<'py> for PyLanceDB<FtsQuery> {
             .expect("Failed to import namespace");
 
         match self.0 {
-            FtsQuery::CombinedFields(_) => Err(PyValueError::new_err(
-                "Combined-fields FTS queries are not yet supported in Python",
-            )),
             FtsQuery::Match(query) => {
                 let kwargs = PyDict::new(py);
                 kwargs.set_item("boost", query.boost)?;
@@ -236,6 +253,18 @@ impl<'py> IntoPyObject<'py> for PyLanceDB<FtsQuery> {
                     .getattr(intern!(py, "MultiMatchQuery"))?
                     .call((first.terms.clone(), columns), Some(&kwargs))
             }
+            FtsQuery::CombinedFields(query) => {
+                let (columns, boosts): (Vec<_>, Vec<_>) = query
+                    .weighted_columns()
+                    .map(|(column, boost)| (column.to_string(), boost))
+                    .unzip();
+                let kwargs = PyDict::new(py);
+                kwargs.set_item("boosts", boosts)?;
+                kwargs.set_item::<_, &str>("operator", query.operator().into())?;
+                namespace
+                    .getattr(intern!(py, "CombinedFieldsQuery"))?
+                    .call((query.terms(), columns), Some(&kwargs))
+            }
             FtsQuery::Boolean(query) => {
                 let mut queries: Vec<(&str, Bound<'py, PyAny>)> = Vec::with_capacity(
                     query.should.len() + query.must.len() + query.must_not.len(),