尝试优化更新内容

db/message_storage.py

@@ -214,7 +214,8 @@ class MessageStorageDB(BaseDBOperator):
             sql = """
             UPDATE messages
             SET image_path = %s
-            WHERE message_id = %s
+            WHERE message_id = %s 
+            AND image_path IS NULL
             """
             params = (image_path, message_id)
             result = self.execute_update(sql, params)