Optimizer trace: make best_access_path.chosen_access_method show the which index is used

create table ten(a int primary key);

insert into ten values (0),(1),(2),(3),(4),(5),(6),(7),(8),(9);

create table one_k(a int primary key);

insert into one_k select A.a + B.a* 10 + C.a * 100 from ten A, ten B, ten C;

create table t10 (a int, b int, c int, key(a), key(b));

insert into t10 select A.a, A.a, A.a from one_k A, ten B;

set optimizer_trace=1;

explain select * from ten,t10 where t10.a=ten.a and t10.b=ten.a;

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

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

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

|    1 | SIMPLE      | ten   | index | PRIMARY       | PRIMARY | 4       | NULL       | 10   | Using index |

|    1 | SIMPLE      | t10   | ref   | a,b           | a       | 5       | test.ten.a | 1    | Using where |

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

select * from information_schema.optimizer_trace\G

    "plan_prefix": ["ten"],

    "table": "t10",

    "best_access_path": {

      "considered_access_paths": [

        {

          "access_type": "ref",

          "index": "a",

          "used_range_estimates": false,

          "cause": "not available",

          "rows": 1,

          "cost": 20.00134269,

          "chosen": true

        },

        {

          "access_type": "ref",

          "index": "b",

          "used_range_estimates": false,

          "cause": "not available",

          "rows": 1,

          "cost": 20.00134269,

          "chosen": false,

          "cause": "cost"

        },

        {

          "type": "scan",

          "chosen": false,

          "cause": "cost"

        }

      ],

    "chosen_access_method": {

      "type": "ref",

      "records": 1,

      "cost": 20.00134269,

      "uses_join_buffering": false

    }

  },