feat: implement semantic sub-query decomposition and parallel retrieval with RRF merge

backend/app/rag/retriever.py

@@ -72,6 +72,41 @@ def _accumulate(results: List[Dict[str, Any]]) -> None:
     return merged
 
 
+def rrf_merge_multiple(
+    ranked_lists: List[List[Dict[str, Any]]],
+    k: int = 60,
+) -> List[Dict[str, Any]]:
+    """Merge multiple ranked lists using Reciprocal Rank Fusion."""
+    rrf_scores: Dict[str, float] = {}
+    chunk_store: Dict[str, Dict[str, Any]] = {}
+
+    def _key(chunk: Dict[str, Any]) -> str:
+        for field in ("id", "chunk_id"):
+            if chunk.get(field):
+                return str(chunk[field])
+        text = str(chunk.get("text", ""))
+        return "|".join([
+            str(chunk.get("document_id", "")),
+            str(chunk.get("page", "")),
+            text[:200],
+        ])
+
+    for lst in ranked_lists:
+        for rank, chunk in enumerate(lst, start=1):
+            key = _key(chunk)
+            rrf_scores[key] = rrf_scores.get(key, 0.0) + 1.0 / (k + rank)
+            if key not in chunk_store or chunk.get("score", 0) > chunk_store[key].get("score", 0):
+                chunk_store[key] = chunk
+
+    merged = []
+    for key, rrf_score in sorted(rrf_scores.items(), key=lambda t: t[1], reverse=True):
+        chunk = chunk_store[key].copy()
+        chunk["rrf_score"] = round(rrf_score, 6)
+        merged.append(chunk)
+
+    return merged
+
+
 # ── Query helpers ─────────────────────────────────────────────────────────────
 
 def transform_query(query: str) -> List[str]:
@@ -98,15 +133,18 @@ def _generate_query_variants(query: str) -> List[str]:
 
     client = InferenceClient(token=settings.HF_TOKEN)
     prompt = (
-        "Rewrite the user question into concise semantic search queries for document retrieval. "
-        "Split independent topics into separate queries. Return a JSON array of strings only. "
+        "Decompose the user's complex multi-part question into simple, distinct semantic sub-queries. "
+        "Each sub-query should focus on a single question, topic, or comparison. "
+        "Return a JSON array of strings only. "
+        "Example question: 'Compare treatment A and treatment B for diabetes'\n"
+        "Example output: [\"treatment A for diabetes\", \"treatment B for diabetes\", \"diabetes treatments comparison\"]\n"
         f"User question: {query}"
     )
     response = client.chat_completion(
         messages=[
             {
                 "role": "system",
-                "content": "You create optimized search queries for a RAG retriever.",
+                "content": "You decompose complex search queries into list of search sub-queries for a RAG retriever.",
             },
             {"role": "user", "content": prompt},
         ],
@@ -221,10 +259,11 @@ def retrieve(
     """
     effective_top_k = top_k if top_k is not None else settings.TOP_K_RETRIEVAL
 
-    # ── Stage 1: Hybrid retrieval with query transformation ───────────────────
-    all_candidates: List[Dict[str, Any]] = []
+    # ── Stage 1: Parallel retrieval of sub-queries and RRF merging ───────────
+    sub_queries = transform_query(query)
+    sub_query_results: List[List[Dict[str, Any]]] = []
 
-    for search_query in transform_query(query):
+    def retrieve_single_query(search_query: str) -> List[Dict[str, Any]]:
         query_vector = embed_query(search_query)
 
         # Vector results (always)
@@ -257,14 +296,28 @@ def retrieve(
             for chunk in merged:
                 chunk["score"] = chunk.pop("rrf_score")
 
-            all_candidates.extend(merged)
+            return merged
         else:
-            all_candidates.extend(vector_results)
+            return vector_results
+
+    import concurrent.futures
+    with concurrent.futures.ThreadPoolExecutor(max_workers=len(sub_queries) or 1) as executor:
+        future_to_query = {executor.submit(retrieve_single_query, sq): sq for sq in sub_queries}
+        for future in concurrent.futures.as_completed(future_to_query):
+            try:
+                results = future.result()
+                sub_query_results.append(results)
+            except Exception as e:
+                sq = future_to_query[future]
+                logger.error("Failed retrieval for sub-query '%s': %s", sq, e)
 
-    if not all_candidates:
+    if not sub_query_results:
         return []
 
-    candidates = _merge_candidates(all_candidates)
+    # Merge all sub-query candidate lists using generalized RRF
+    candidates = rrf_merge_multiple(sub_query_results, k=settings.RRF_K)
+    for chunk in candidates:
+        chunk["score"] = chunk.pop("rrf_score")
 
     # ── Stage 2: Cross-encoder reranking ─────────────────────────────────────
     reranker = get_reranker()