fix: add better check for empty results in hybrid search (#2252)

fixes: https://github.com/lancedb/lancedb/issues/2249

python/python/lancedb/query.py

@@ -1220,6 +1220,10 @@ class LanceHybridQueryBuilder(LanceQueryBuilder):
             vector_results = LanceHybridQueryBuilder._rank(vector_results, "_distance")
             fts_results = LanceHybridQueryBuilder._rank(fts_results, "_score")
 
+        original_distances = None
+        original_scores = None
+        original_distance_row_ids = None
+        original_score_row_ids = None
         # normalize the scores to be between 0 and 1, 0 being most relevant
         # We check whether the results (vector and FTS) are empty, because when
         # they are, they often are missing the _rowid column, which causes an error
@@ -1249,7 +1253,7 @@ class LanceHybridQueryBuilder(LanceQueryBuilder):
 
         check_reranker_result(results)
 
-        if "_distance" in results.column_names:
+        if "_distance" in results.column_names and original_distances is not None:
             # restore the original distances
             indices = pc.index_in(
                 results["_rowid"], original_distance_row_ids, skip_nulls=True
@@ -1258,7 +1262,7 @@ class LanceHybridQueryBuilder(LanceQueryBuilder):
             distance_i = results.column_names.index("_distance")
             results = results.set_column(distance_i, "_distance", original_distances)
 
-        if "_score" in results.column_names:
+        if "_score" in results.column_names and original_scores is not None:
             # restore the original scores
             indices = pc.index_in(
                 results["_rowid"], original_score_row_ids, skip_nulls=True

python/python/tests/test_rerankers.py

@@ -368,6 +368,26 @@ def test_rrf_reranker_distance():
             assert score == fts_scores[rowid], "Score mismatch"
     assert found_match, "No results matched between hybrid and fts search"
 
+    # Test for empty fts results
+    fts_results = (
+        table.search("abcxyz" * 100, query_type="fts").with_row_id(True).to_list()
+    )
+    hybrid_results = (
+        table.search(query_type="hybrid")
+        .vector([0.0] * 32)
+        .text("abcxyz" * 100)
+        .with_row_id(True)
+        .rerank(reranker)
+        .to_list()
+    )
+    assert len(fts_results) == 0
+    # confirm if _rowid, _score, _distance & _relevance_score are present in hybrid
+    assert len(hybrid_results) > 0
+    assert "_rowid" in hybrid_results[0]
+    assert "_score" in hybrid_results[0]
+    assert "_distance" in hybrid_results[0]
+    assert "_relevance_score" in hybrid_results[0]
+
 
 @pytest.mark.skipif(
     os.environ.get("COHERE_API_KEY") is None, reason="COHERE_API_KEY not set"