mysql> explain select * from userinfo where username='test1'\G

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

           id: 1

  select_type: SIMPLE

        table: userinfo

         type: ref

possible_keys: username

          key: username

      key_len: 768

          ref: const

         rows: 1

        Extra: Using where

1 row in set (0.00 sec)

mysql> explain select * from userinfo ignore index(username) where username='tes

t1'\G

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

           id: 1

  select_type: SIMPLE

        table: userinfo

         type: ALL

possible_keys: NULL

          key: NULL

      key_len: NULL

          ref: NULL

         rows: 4

        Extra: Using where

1 row in set (0.00 sec)