drop table t1,t2; 的结果是（）【单选】？

mysql> explain select * from t1 where testkey=2;

+----+-------------+-------+------------+------+---------------+----------+---------+-------+------+----------+-------------+

| id | select_type | table | partitions | type | possible_keys | key      | key_len | ref   | rows | filtered | Extra       |

+----+-------------+-------+------------+------+---------------+----------+---------+-------+------+----------+-------------+

|  1 | SIMPLE      | t1    | NULL       | ref  | idx_test      | idx_test | 5       | const |    1 |   100.00 | Using index |

+----+-------------+-------+------------+------+---------------+----------+---------+-------+------+----------+-------------+

   关于输出结果中extra 字段：using index 描述正确的是（）【单选】？

CREATE TABLE rental (

rental_id int unsigned NOT NULL AUTO INCREMENT,

rental_date datetime NOT NULL,

inventory_id int unsigned NoT NULL,

customer_id int unsigned NOT NULL,

return_date datetime DEFAULT NULL,

staff_id int unsigned NoT NULL,

Iast_update timestamp NOT NULL DEFAULT CURRENT TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,

PRIMARY KEY (rental_id)

ENGINE=InnoDB;

   为该表加上什么索引可以使得如下SQL 语句执行效率最高（）【单选】？

SELECT rental_id,customer_id

FROM rental

where rental_date between now() - interval 1 month and now()

AND inventory_id=42

AND staff_id=1024;

mysql> show create table t1\G;

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

Create Table: CREATE TABLE `t1` (

  `id` int NOT NULL,

  `testkey` int DEFAULT NULL,

  `name` varchar(20) COLLATE utf8mb4_bin DEFAULT 'XNAME',

  PRIMARY KEY (`id`),

  KEY `idx_test` (`testkey`)

) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin

1 row in set (0.01 sec)

Mysql>select * from t1;

+----+---------+-------+

| id | testkey | name  |

+----+---------+-------+

|  2 |       2 | XNAME |

|  3 |       3 | XNAME |

|  4 |       4 | XNAME |

|  5 |       8 | XNAME |

|  9 |      10 | XNAME |

+----+---------+-------+

该SQL 语句select * from t1 where testkey>100 for update 加锁的情况是（）【单选】？

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

               Slave_IO_State: Waiting for master to send event

                  Master_Host: 172.16.20.187

                  Master_User: replnew

                  Master_Port: 3306

                Connect_Retry: 60

              Master_Log_File: binlog.000002

          Read_Master_Log_Pos: 191

               Relay_Log_File: relay.000002

                Relay_Log_Pos: 351

        Relay_Master_Log_File: binlog.000002

             Slave_IO_Running: Yes

            Slave_SQL_Running: Yes

              Replicate_Do_DB:

  Replicate_Ignore_Server_Ids: 

             Master_Server_Id: 351294564

                  Master_UUID: ef3b121f-0234-11eb-a6fb-000c29865847

             Master_Info_File: mysql.slave_master_info

                    SQL_Delay: 360

   关于如上输出结果中的SQL_Delay: 360 说法正确的是（）【单选】？

mysql> explain select * from t1 where testkey=2;

+----+-------------+-------+------------+------+---------------+----------+---------+-------+------+----------+-------------+

| id | select_type | table | partitions | type | possible_keys | key   | key_len | ref   | rows | filtered | Extra       |

+----+-------------+-------+------------+------+---------------+----------+---------+-------+------+----------+-------------+

|  1 | SIMPLE   | t1    | NULL    | ref  | idx_test  | idx_test | 5       | const |    1 |   100.00 | Using index |

+----+-------------+-------+------------+------+---------------+----------+---------+-------+------+----------+-------------+

   关于输出结果中possible_keys字段：idx_test 描述正确的是（）【单选】？

      Slave_SQL_Running_State: Slave has read all relay log; waiting for more updates

           Master_Retry_Count: 86400

                  Master_Bind: 

      Last_IO_Error_Timestamp: 

     Last_SQL_Error_Timestamp: 

               Master_SSL_Crl: 

           Master_SSL_Crlpath: 

           Retrieved_Gtid_Set: ef3b121f-0234-11eb-a6fb-000c29865847:34-38

            Executed_Gtid_Set: ef3b121f-0234-11eb-a6fb-000c29865847:1-38

                Auto_Position: 1

   如上输出中关于Retrieved_Gtid_Set、Executed_Gtid_Set两项结果输出说法正确的是（）【单选】？