CREATE TABLE test1 (
   id INTEGER NOT NULL DEFAULT 0,
   flag enum('y','n') NOT NULL DEFAULT 'n',
   KEY(id), KEY(flag)
) ENGINE=InnoDB;

MariaDB [db1]> show variables like 'optimizer_switch'\G
*************************** 1. row ***************************
Variable_name: optimizer_switch
        Value: index_merge=on,index_merge_union=on,index_merge_sort_union=on,
index_merge_intersection=on,index_merge_sort_intersection=on,
engine_condition_pushdown=off,index_condition_pushdown=on,derived_merge=on,derived_with_keys=on,firstmatch=on,
loosescan=on,materialization=on,in_to_exists=on,semijoin=on,partial_match_rowid_merge=on,partial_match_table_scan=on,
subquery_cache=on,mrr=off,mrr_cost_based=off,mrr_sort_keys=off,outer_join_with_cache=on,semijoin_with_cache=on,
join_cache_incremental=on,join_cache_hashed=on,join_cache_bka=on,optimize_join_buffer_size=off,table_elimination=on,
extended_keys=on,exists_to_in=on,orderby_uses_equalities=off
1 row in set (0.01 sec)

MariaDB [db1]> EXPLAIN SELECT * FROM test1 WHERE id = 1 AND flag = 'n'; 
+------+-------------+-------+------+---------------+------+---------+-------+------+-------------+
| id   | select_type | table | type | possible_keys | key  | key_len | ref   | rows | Extra       |
+------+-------------+-------+------+---------------+------+---------+-------+------+-------------+
|    1 | SIMPLE      | test1 | ref  | id,flag       | id   | 4       | const |    1 | Using where |
+------+-------------+-------+------+---------------+------+---------+-------+------+-------------+
1 row in set (0.00 sec)

MariaDB [db1]> EXPLAIN FORMAT=JSON SELECT * FROM test1 WHERE id = 1 AND flag = 'n'\G
*************************** 1. row ***************************
EXPLAIN: {
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "test1",
      "access_type": "ref",
      "possible_keys": ["id", "flag"],
      "key": "id",
      "key_length": "4",
      "used_key_parts": ["id"],
      "ref": ["const"],
      "rows": 1,
      "filtered": 100,
      "attached_condition": "(test1.flag = 'n')"
    }
  }
}
1 row in set (0.00 sec)