fix: return error if searching against BM25 output field with incorrect metric type (#36910)

issue: https://github.com/milvus-io/milvus/issues/36835

currently searching BM25 output field using IP will end up in an error
in segcore which is hard to understand. now returning error in query
node delegator and provide more useful error message

Signed-off-by: Buqian Zheng <zhengbuqian@gmail.com>

internal/proxy/task_index.go

@@ -154,8 +154,8 @@ func (cit *createIndexTask) parseFunctionParamsToIndex(indexParamsMap map[string
 		}
 
 		if metricType, ok := indexParamsMap["metric_type"]; !ok {
-			indexParamsMap["metric_type"] = "BM25"
-		} else if metricType != "BM25" {
+			indexParamsMap["metric_type"] = metric.BM25
+		} else if metricType != metric.BM25 {
 			return fmt.Errorf("index metric type of BM25 function output field must be BM25, got %s", metricType)
 		}
 

internal/querynodev2/delegator/delegator.go

@@ -263,8 +263,13 @@ func (sd *shardDelegator) search(ctx context.Context, req *querypb.SearchRequest
 		}()
 	}
 
-	// build idf for bm25 search
-	if req.GetReq().GetMetricType() == metric.BM25 || (req.GetReq().GetMetricType() == metric.EMPTY && sd.isBM25Field[req.GetReq().GetFieldId()]) {
+	searchAgainstBM25Field := sd.isBM25Field[req.GetReq().GetFieldId()]
+
+	if searchAgainstBM25Field {
+		if req.GetReq().GetMetricType() != metric.BM25 && req.GetReq().GetMetricType() != metric.EMPTY {
+			return nil, merr.WrapErrParameterInvalid("BM25", req.GetReq().GetMetricType(), "must use BM25 metric type when searching against BM25 Function output field")
+		}
+		// build idf for bm25 search
 		avgdl, err := sd.buildBM25IDF(req.GetReq())
 		if err != nil {
 			return nil, err

internal/querynodev2/delegator/delegator_test.go

@@ -1267,3 +1267,23 @@ func TestDelegatorTSafeListenerClosed(t *testing.T) {
 	assert.Equal(t, sd.Serviceable(), false)
 	assert.Equal(t, sd.Stopped(), true)
 }
+
+func TestDelegatorSearchBM25InvalidMetricType(t *testing.T) {
+	paramtable.Init()
+	searchReq := &querypb.SearchRequest{
+		Req: &internalpb.SearchRequest{
+			Base: commonpbutil.NewMsgBase(),
+		},
+	}
+
+	searchReq.Req.FieldId = 101
+	searchReq.Req.MetricType = metric.IP
+
+	sd := &shardDelegator{
+		isBM25Field: map[int64]bool{101: true},
+	}
+
+	_, err := sd.search(context.Background(), searchReq, []SnapshotItem{}, []SegmentEntry{})
+	require.Error(t, err)
+	assert.Contains(t, err.Error(), "must use BM25 metric type when searching against BM25 Function output field")
+}