fix: do not min/max-normalize already normalized similarity scores to not distort them

pkg/datastore/lib/scores/scores.go

@@ -40,6 +40,9 @@ func NormalizeDocScores(docs []vs.Document) []vs.Document {
 
 // NormalizeScore normalizes a single score
 func NormalizeScore(score float32, minScore float32, maxScore float32) float32 {
+	if maxScore == 0 {
+		return 0
+	}
 	if maxScore-minScore == 0 {
 		return 1 // Avoid division by zero - also, this happens for a single document, so we want a score of 1 here
 	}

pkg/datastore/retrievers/bm25.go

@@ -29,6 +29,10 @@ func (r *BM25Retriever) Name() string {
 	return BM25RetrieverName
 }
 
+func (r *BM25Retriever) NormalizedScores() bool {
+	return false
+}
+
 func (r *BM25Retriever) DecodeConfig(cfg map[string]any) error {
 	return DefaultConfigDecoder(r, cfg)
 }

pkg/datastore/retrievers/merging.go

@@ -32,6 +32,10 @@ func (r *MergingRetriever) Name() string {
 	return MergingRetrieverName
 }
 
+func (r *MergingRetriever) NormalizedScores() bool {
+	return true
+}
+
 func (r *MergingRetriever) DecodeConfig(cfg map[string]any) error {
 	if err := mapstructure.Decode(cfg, &r); err != nil {
 		return fmt.Errorf("failed to decode merging retriever configuration: %w", err)
@@ -75,7 +79,11 @@ func (r *MergingRetriever) Retrieve(ctx context.Context, store store.Store, quer
 
 		slog.Debug("Retrieved documents from retriever", "retriever", retriever.Name, "numDocs", len(retrievedDocs))
 
-		min, max := scores.FindMinMaxScores(retrievedDocs)
+		normalized := r.retrievers[ri].NormalizedScores()
+		var minScore, maxScore float32
+		if !normalized {
+			minScore, maxScore = scores.FindMinMaxScores(retrievedDocs)
+		}
 
 	docLoop:
 		for _, retrievedDoc := range retrievedDocs {
@@ -85,7 +93,11 @@ func (r *MergingRetriever) Retrieve(ctx context.Context, store store.Store, quer
 					// Note that this was found by another retriever and note it's similarityScore
 					resultDocs[i].Metadata["retriever"] = fmt.Sprintf("%s,%s", resultDocs[i].Metadata["retriever"], retriever.Name)
 					resultDocs[i].Metadata["retrieverScore::"+retriever.Name] = retrievedDoc.SimilarityScore
-					normalizedScore := scores.NormalizeScore(retrievedDoc.SimilarityScore, min, max)
+					normalizedScore := retrievedDoc.SimilarityScore
+					if !normalized {
+						normalizedScore = scores.NormalizeScore(retrievedDoc.SimilarityScore, minScore, maxScore)
+						slog.Debug("Normalized score", "retriever", retriever.Name, "score", retrievedDoc.SimilarityScore, "minScore", minScore, "maxScore", maxScore, "normalizedScore", normalizedScore)
+					}
 					resultDocs[i].Metadata["retrieverScoreNormalized::"+retriever.Name] = normalizedScore
 					resultDocs[i].SimilarityScore += normalizedScore * z.Dereference(retriever.Weight)
 					continue docLoop
@@ -94,7 +106,7 @@ func (r *MergingRetriever) Retrieve(ctx context.Context, store store.Store, quer
 			// not in resultDocs yet, add it
 			retrievedDoc.Metadata["retriever"] = retriever.Name
 			retrievedDoc.Metadata["retrieverScore::"+retriever.Name] = retrievedDoc.SimilarityScore
-			normalizedScore := scores.NormalizeScore(retrievedDoc.SimilarityScore, min, max)
+			normalizedScore := scores.NormalizeScore(retrievedDoc.SimilarityScore, minScore, maxScore)
 			retrievedDoc.Metadata["retrieverScoreNormalized::"+retriever.Name] = normalizedScore
 			retrievedDoc.SimilarityScore = normalizedScore * z.Dereference(retriever.Weight)
 			resultDocs = append(resultDocs, retrievedDoc)

pkg/datastore/retrievers/retrievers.go

@@ -21,6 +21,7 @@ type Retriever interface {
 	Retrieve(ctx context.Context, store store.Store, query string, datasetIDs []string, where map[string]string, whereDocument []chromem.WhereDocument) ([]vs.Document, error)
 	Name() string
 	DecodeConfig(cfg map[string]any) error
+	NormalizedScores() bool // whether the retriever returns normalized scores
 }
 
 func GetRetriever(name string) (Retriever, error) {
@@ -68,6 +69,10 @@ func (r *BasicRetriever) Name() string {
 	return BasicRetrieverName
 }
 
+func (r *BasicRetriever) NormalizedScores() bool {
+	return true
+}
+
 func (r *BasicRetriever) DecodeConfig(cfg map[string]any) error {
 	return DefaultConfigDecoder(r, cfg)
 }

pkg/datastore/retrievers/routing.go

@@ -25,6 +25,10 @@ func (r *RoutingRetriever) Name() string {
 	return RoutingRetrieverName
 }
 
+func (r *RoutingRetriever) NormalizedScores() bool {
+	return true
+}
+
 func (r *RoutingRetriever) DecodeConfig(cfg map[string]any) error {
 	return DefaultConfigDecoder(r, cfg)
 }

pkg/datastore/retrievers/subquery.go

@@ -28,6 +28,10 @@ func (s *SubqueryRetriever) Name() string {
 	return SubqueryRetrieverName
 }
 
+func (s *SubqueryRetriever) NormalizedScores() bool {
+	return true
+}
+
 func (s *SubqueryRetriever) DecodeConfig(cfg map[string]any) error {
 	return DefaultConfigDecoder(s, cfg)
 }