feat: Implement hybrid search in SQLite-vec (#2312)

# What does this PR do?
Add support for hybrid search mode in SQLite-vec provider, which
combines
keyword and vector search for better results. The implementation:

- Adds hybrid search mode as a new option alongside vector and keyword
search
- Implements query_hybrid method in SQLiteVecIndex that:
  - First performs keyword search to get candidate matches
  - Then applies vector similarity search on those candidates
- Updates documentation to reflect the new search mode

This change improves search quality by leveraging both semantic
similarity
and keyword matching, while maintaining backward compatibility with
existing
vector and keyword search modes.

## Test Plan
```
pytest tests/unit/providers/vector_io/test_sqlite_vec.py -v -s --tb=short
/Users/vnarsing/miniconda3/envs/stack-client/lib/python3.10/site-packages/pytest_asyncio/plugin.py:217: PytestDeprecationWarning: The configuration option "asyncio_default_fixture_loop_scope" is unset.
The event loop scope for asynchronous fixtures will default to the fixture caching scope. Future versions of pytest-asyncio will default the loop scope for asynchronous fixtures to function scope. Set the default fixture loop scope explicitly in order to avoid unexpected behavior in the future. Valid fixture loop scopes are: "function", "class", "module", "package", "session"

  warnings.warn(PytestDeprecationWarning(_DEFAULT_FIXTURE_LOOP_SCOPE_UNSET))
=============================================================================================== test session starts ===============================================================================================
platform darwin -- Python 3.10.16, pytest-8.3.5, pluggy-1.5.0 -- /Users/vnarsing/miniconda3/envs/stack-client/bin/python
cachedir: .pytest_cache
metadata: {'Python': '3.10.16', 'Platform': 'macOS-14.7.6-arm64-arm-64bit', 'Packages': {'pytest': '8.3.5', 'pluggy': '1.5.0'}, 'Plugins': {'html': '4.1.1', 'json-report': '1.5.0', 'timeout': '2.4.0', 'metadata': '3.1.1', 'anyio': '4.8.0', 'asyncio': '0.26.0', 'nbval': '0.11.0', 'cov': '6.1.1'}}
rootdir: /Users/vnarsing/go/src/github/meta-llama/llama-stack
configfile: pyproject.toml
plugins: html-4.1.1, json-report-1.5.0, timeout-2.4.0, metadata-3.1.1, anyio-4.8.0, asyncio-0.26.0, nbval-0.11.0, cov-6.1.1
asyncio: mode=strict, asyncio_default_fixture_loop_scope=None, asyncio_default_test_loop_scope=function
collected 10 items                                                                                                                                                                                                

tests/unit/providers/vector_io/test_sqlite_vec.py::test_add_chunks PASSED
tests/unit/providers/vector_io/test_sqlite_vec.py::test_query_chunks_vector PASSED
tests/unit/providers/vector_io/test_sqlite_vec.py::test_query_chunks_full_text_search PASSED
tests/unit/providers/vector_io/test_sqlite_vec.py::test_query_chunks_hybrid PASSED
tests/unit/providers/vector_io/test_sqlite_vec.py::test_query_chunks_full_text_search_k_greater_than_results PASSED
tests/unit/providers/vector_io/test_sqlite_vec.py::test_chunk_id_conflict PASSED
tests/unit/providers/vector_io/test_sqlite_vec.py::test_generate_chunk_id PASSED
tests/unit/providers/vector_io/test_sqlite_vec.py::test_query_chunks_hybrid_no_keyword_matches PASSED
tests/unit/providers/vector_io/test_sqlite_vec.py::test_query_chunks_hybrid_score_threshold PASSED
tests/unit/providers/vector_io/test_sqlite_vec.py::test_query_chunks_hybrid_different_embedding PASSED
```

---------

Signed-off-by: Varsha Prasad Narsing <varshaprasad96@gmail.com>

docs/source/providers/vector_io/sqlite-vec.md

@@ -66,25 +66,126 @@ To use sqlite-vec in your Llama Stack project, follow these steps:
 2. Configure your Llama Stack project to use SQLite-Vec.
 3. Start storing and querying vectors.
 
-## Supported Search Modes
+The SQLite-vec provider supports three search modes:
 
-The sqlite-vec provider supports both vector-based and keyword-based (full-text) search modes.
-
-When using the RAGTool interface, you can specify the desired search behavior via the `mode` parameter in
-`RAGQueryConfig`. For example:
+1. **Vector Search** (`mode="vector"`): Performs pure vector similarity search using the embeddings.
+2. **Keyword Search** (`mode="keyword"`): Performs full-text search using SQLite's FTS5.
+3. **Hybrid Search** (`mode="hybrid"`): Combines both vector and keyword search for better results. First performs keyword search to get candidate matches, then applies vector similarity search on those candidates.
 
+Example with hybrid search:
 ```python
-from llama_stack.apis.tool_runtime.rag import RAGQueryConfig
+response = await vector_io.query_chunks(
+    vector_db_id="my_db",
+    query="your query here",
+    params={"mode": "hybrid", "max_chunks": 3, "score_threshold": 0.7},
+)
 
-query_config = RAGQueryConfig(max_chunks=6, mode="vector")
+# Using RRF ranker
+response = await vector_io.query_chunks(
+    vector_db_id="my_db",
+    query="your query here",
+    params={
+        "mode": "hybrid",
+        "max_chunks": 3,
+        "score_threshold": 0.7,
+        "ranker": {"type": "rrf", "impact_factor": 60.0},
+    },
+)
 
-results = client.tool_runtime.rag_tool.query(
-    vector_db_ids=[vector_db_id],
-    content="what is torchtune",
-    query_config=query_config,
+# Using weighted ranker
+response = await vector_io.query_chunks(
+    vector_db_id="my_db",
+    query="your query here",
+    params={
+        "mode": "hybrid",
+        "max_chunks": 3,
+        "score_threshold": 0.7,
+        "ranker": {"type": "weighted", "alpha": 0.7},  # 70% vector, 30% keyword
+    },
 )
 ```
 
+Example with explicit vector search:
+```python
+response = await vector_io.query_chunks(
+    vector_db_id="my_db",
+    query="your query here",
+    params={"mode": "vector", "max_chunks": 3, "score_threshold": 0.7},
+)
+```
+
+Example with keyword search:
+```python
+response = await vector_io.query_chunks(
+    vector_db_id="my_db",
+    query="your query here",
+    params={"mode": "keyword", "max_chunks": 3, "score_threshold": 0.7},
+)
+```
+
+## Supported Search Modes
+
+The SQLite vector store supports three search modes:
+
+1. **Vector Search** (`mode="vector"`): Uses vector similarity to find relevant chunks
+2. **Keyword Search** (`mode="keyword"`): Uses keyword matching to find relevant chunks
+3. **Hybrid Search** (`mode="hybrid"`): Combines both vector and keyword scores using a ranker
+
+### Hybrid Search
+
+Hybrid search combines the strengths of both vector and keyword search by:
+- Computing vector similarity scores
+- Computing keyword match scores
+- Using a ranker to combine these scores
+
+Two ranker types are supported:
+
+1. **RRF (Reciprocal Rank Fusion)**:
+   - Combines ranks from both vector and keyword results
+   - Uses an impact factor (default: 60.0) to control the weight of higher-ranked results
+   - Good for balancing between vector and keyword results
+   - The default impact factor of 60.0 comes from the original RRF paper by Cormack et al. (2009) [^1], which found this value to provide optimal performance across various retrieval tasks
+
+2. **Weighted**:
+   - Linearly combines normalized vector and keyword scores
+   - Uses an alpha parameter (0-1) to control the blend:
+     - alpha=0: Only use keyword scores
+     - alpha=1: Only use vector scores
+     - alpha=0.5: Equal weight to both (default)
+
+Example using RAGQueryConfig with different search modes:
+
+```python
+from llama_stack.apis.tools import RAGQueryConfig, RRFRanker, WeightedRanker
+
+# Vector search
+config = RAGQueryConfig(mode="vector", max_chunks=5)
+
+# Keyword search
+config = RAGQueryConfig(mode="keyword", max_chunks=5)
+
+# Hybrid search with custom RRF ranker
+config = RAGQueryConfig(
+    mode="hybrid",
+    max_chunks=5,
+    ranker=RRFRanker(impact_factor=50.0),  # Custom impact factor
+)
+
+# Hybrid search with weighted ranker
+config = RAGQueryConfig(
+    mode="hybrid",
+    max_chunks=5,
+    ranker=WeightedRanker(alpha=0.7),  # 70% vector, 30% keyword
+)
+
+# Hybrid search with default RRF ranker
+config = RAGQueryConfig(
+    mode="hybrid", max_chunks=5
+)  # Will use RRF with impact_factor=60.0
+```
+
+Note: The ranker configuration is only used in hybrid mode. For vector or keyword modes, the ranker parameter is ignored.
+
 ## Installation
 
 You can install SQLite-Vec using pip:
@@ -96,3 +197,5 @@ pip install sqlite-vec
 ## Documentation
 
 See [sqlite-vec's GitHub repo](https://github.com/asg017/sqlite-vec/tree/main) for more details about sqlite-vec in general.
+
+[^1]: Cormack, G. V., Clarke, C. L., & Buettcher, S. (2009). [Reciprocal rank fusion outperforms condorcet and individual rank learning methods](https://dl.acm.org/doi/10.1145/1571941.1572114). In Proceedings of the 32nd international ACM SIGIR conference on Research and development in information retrieval (pp. 758-759).