Analyze show ref(const,const) when we are using index scan

create table t0 (a int);

INSERT INTO t0 VALUES (0),(0),(0),(0),(2),(0),(0),(1),(1),(0);

CREATE TABLE t1 (

a int(11) DEFAULT NULL,

b int(11) DEFAULT NULL,

d int(11) DEFAULT NULL,

KEY a_d (a,d),

KEY a_b (a,b)

) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;

insert into t1 select A.a , B.a, C.a from t0 A, t0 B, t0 C, t0 D;

MariaDB [test]> analyze select a,b,d from t1 where a=1 and d=2 order by b limit 20000;

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

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

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

|    1 | SIMPLE      | t1    | index | a_d,a_b       | a_b | 10      | const,const | 10266 | 10000.00 |     1.95 |       2.00 | Using where |

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