Explain output difference in possible_keys column

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

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

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

|    1 | PRIMARY     | t1         | ALL   | NULL          | NULL | NULL    | NULL    | 4    | Using where                                               |

|    1 | PRIMARY     | <derived2> | ref   | key0          | key0 | 6       | m8.t1.b | 1    |                                                           |

|    2 | DERIVED     | t2         | range | a             | a    | 58      | NULL    | 7    | Using index for group-by; Using temporary; Using filesort |

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

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

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

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

|    1 | PRIMARY     | t1         | ALL   | NULL          | NULL | NULL    | NULL    | 4    | Using where                                               |

|    1 | PRIMARY     | <derived2> | ref   | key0          | key0 | 6       | m8.t1.b | 1    |                                                           |

|    2 | DERIVED     | t2         | range | NULL          | a    | 58      | NULL    | 1    | Using index for group-by; Using temporary; Using filesort |

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

set optimizer_record_context=ON;

CREATE TABLE t1 (a varchar(35), b varchar(4)) ENGINE=InnoDB CHARSET=latin1;

INSERT INTO t1 VALUES

('Albania','AXA'),('Australia','AUS'),('American Samoa','AMSA'),('Bahamas','BS');

CREATE TABLE t2 (a varchar(4), b varchar(50), PRIMARY KEY (b,a), KEY (a)) ENGINE=InnoDB CHARSET=latin1;

INSERT INTO t2 VALUES

('BERM','African Methodist Episcopal'),('AUS','Anglican'),('BERM','Anglican'),('BS','Anglican'),('BS','Baptist'),('BS','Methodist');

explain SELECT t1.a

FROM (SELECT a FROM t2 GROUP BY a ORDER BY COUNT(DISTINCT b) LIMIT 1) dt

JOIN t1 ON dt.a=t1.b;

SELECT context INTO DUMPFILE 'context1.txt' FROM INFORMATION_SCHEMA.OPTIMIZER_CONTEXT;

source context1.txt