where 1=1是否造成性能影响？

讨论见http://www.iteye.com/topic/710338

帖子中说如果使用了where 1=1，则索引查询会失效。通过实验，该结论是错误的：

mysql> explain select * from pu_user_message where userid=1\G

*************************** 1. row ***************************

           id: 1

  select_type: SIMPLE

        table: pu_user_message

         type: ref

possible_keys: userid_index

          key: userid_index

      key_len: 5

          ref: const

         rows: 1

        Extra: Using where

1 row in set (0.00 sec)

mysql> explain select * from pu_user_message where 1=1 and userid=1\G

*************************** 1. row ***************************

           id: 1

  select_type: SIMPLE

        table: pu_user_message

         type: ref

possible_keys: userid_index

          key: userid_index

      key_len: 5

          ref: const

         rows: 1

        Extra: Using where

1 row in set (0.00 sec)

两次执行完全一样，均使用了索引。