more testing for prefix scanning

cozo-core/src/query/ra.rs

@@ -1507,6 +1507,9 @@ impl StoredRA {
 }
 
 fn join_is_prefix(right_join_indices: &[usize]) -> bool {
+    // We do not consider partial index match to be "prefix", e.g. [a, u => c]
+    // with a, c bound and u unbound is not "prefix", as it is not clear that
+    // using prefix scanning in this case will really save us computation.
     let mut indices = right_join_indices.to_vec();
     indices.sort();
     let l = indices.len();

cozo-core/src/runtime/tests.rs

@@ -920,6 +920,19 @@ fn filtering() {
     ")
         .unwrap();
     assert_eq!(0, res.rows.len());
+
+    let res = db.run_default(r"
+        {
+            ?[x, u, y] <- [[1, 0, 2]]
+            :create _rel {x, u => y}
+            :returning
+        }
+        {
+            ?[x, y] := x = 1, *_rel{x, y: 3}, y = 2
+        }
+    ")
+        .unwrap();
+    assert_eq!(0, res.rows.len());
 }
 
 #[test]