diff --git a/hugegraph-core/src/main/java/org/apache/hugegraph/backend/query/ConditionQueryFlatten.java b/hugegraph-core/src/main/java/org/apache/hugegraph/backend/query/ConditionQueryFlatten.java
index 693b944be9..55af99442a 100644
--- a/hugegraph-core/src/main/java/org/apache/hugegraph/backend/query/ConditionQueryFlatten.java
+++ b/hugegraph-core/src/main/java/org/apache/hugegraph/backend/query/ConditionQueryFlatten.java
@@ -51,23 +51,7 @@ public static List<ConditionQuery> flatten(ConditionQuery query) {
     public static List<ConditionQuery> flatten(ConditionQuery query,
                                                boolean supportIn) {
         if (query.isFlattened() && !query.mayHasDupKeys(SPECIAL_KEYS)) {
-            Relations relations = new Relations();
-            List<Condition> noRelations = new ArrayList<>();
-            for (Condition condition : query.conditions()) {
-                if (condition.isRelation()) {
-                    relations.add((Relation) condition);
-                } else {
-                    noRelations.add(condition);
-                }
-            }
-            relations = optimizeRelations(relations);
-            if (relations != null) {
-                ConditionQuery cq = newQueryFromRelations(query, relations);
-                cq.query(noRelations);
-                return ImmutableList.of(cq);
-            }
-
-            return ImmutableList.of(query);
+            return flattenRelations(query);
         }
 
         List<ConditionQuery> queries = new ArrayList<>();
@@ -267,6 +251,25 @@ private static ConditionQuery newQueryFromRelations(ConditionQuery query,
         return cq;
     }
 
+    private static ImmutableList<ConditionQuery> flattenRelations(ConditionQuery query) {
+        Relations relations = new Relations();
+        List<Condition> noRelations = new ArrayList<>();
+        for (Condition condition : query.conditions()) {
+            if (condition.isRelation()) {
+                relations.add((Relation) condition);
+            } else {
+                noRelations.add(condition);
+            }
+        }
+        relations = optimizeRelations(relations);
+        if (relations != null) {
+            ConditionQuery cq = newQueryFromRelations(query, relations);
+            cq.query(noRelations);
+            return ImmutableList.of(cq);
+        }
+        return ImmutableList.of(query);
+    }
+
     private static Relations optimizeRelations(Relations relations) {
         // Optimize and-relations in one query
         // e.g. (age>1 and age>2) -> (age>2)
diff --git a/hugegraph-core/src/main/java/org/apache/hugegraph/traversal/optimize/TraversalUtil.java b/hugegraph-core/src/main/java/org/apache/hugegraph/traversal/optimize/TraversalUtil.java
index 5c76edc67a..05e2f8c70e 100644
--- a/hugegraph-core/src/main/java/org/apache/hugegraph/traversal/optimize/TraversalUtil.java
+++ b/hugegraph-core/src/main/java/org/apache/hugegraph/traversal/optimize/TraversalUtil.java
@@ -107,6 +107,7 @@ public static HugeGraph getGraph(Step<?, ?> step) {
     }
 
     public static HugeGraph tryGetGraph(Step<?, ?> step) {
+        // TODO: remove these EmptyGraph judgments when upgrading tinkerpop to a fixed version
         Optional<Graph> graph = step.getTraversal()
                                     .getGraph()
                                     .filter(g -> {