MySQL 8.0.0 INVISIBLE KEYとやらはどうやって指定すればいいのか

INVISIBLEなるキーワードがインデックスに指定できるようになったので、取り敢えずADD KEYしてみたけれど…。

mysql80> INSERT INTO t1 VALUES (1, 'one'), (2, 'two'), (3, 'three');
Query OK, 3 rows affected (0.01 sec)
Records: 3  Duplicates: 0  Warnings: 0

mysql80> ALTER TABLE t1 ADD KEY (val) INVISIBLE;
Query OK, 0 rows affected (0.03 sec)
Records: 0  Duplicates: 0  Warnings: 0

*************************** 1. row ***************************
       Table: t1
Create Table: CREATE TABLE `t1` (
  `num` bigint(20) unsigned NOT NULL AUTO_INCREMENT,
  `val` varchar(32) DEFAULT NULL,
  UNIQUE KEY `num` (`num`),
  KEY `val` (`val`) /*!50800 INVISIBLE */
) ENGINE=InnoDB AUTO_INCREMENT=4 DEFAULT CHARSET=utf8mb4
1 row in set (0.01 sec)

mysql80> explain SELECT * FROM t1 WHERE val = 'two';
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra       |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | t1    | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    3 |    33.33 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

mysql80> explain SELECT * FROM t1 FORCE INDEX(val) WHERE val = 'two';
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra       |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | t1    | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    3 |    33.33 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

おお、ホントだ FORCE INDEXでも使われない！！
けど、これってどうやって使わせるの…？ たぶん SELECT /*+ .. */ スタイルの構文で食わせそうな気がするんだけど、まだ情報が見当たらない…(´･ω･｀)

テーブルの1/3をフェッチするからコスト的に選ばれてないアレじゃないよね？ と思ってフツーのINDEXも作ったけど、こっちはちゃんと選ばれる。ということはちゃんとインビジボーなのだね。

mysql80> ALTER TABLE t1 ADD KEY (val);
Query OK, 0 rows affected, 1 warning (0.04 sec)
Records: 0  Duplicates: 0  Warnings: 1

mysql80> SHOW CREATE TABLE t1\G
*************************** 1. row ***************************
       Table: t1
Create Table: CREATE TABLE `t1` (
  `num` bigint(20) unsigned NOT NULL AUTO_INCREMENT,
  `val` varchar(32) DEFAULT NULL,
  UNIQUE KEY `num` (`num`),
  KEY `val` (`val`) /*!50800 INVISIBLE */,
  KEY `val_2` (`val`)
) ENGINE=InnoDB AUTO_INCREMENT=4 DEFAULT CHARSET=utf8mb4
1 row in set (0.00 sec)

mysql80> explain SELECT * FROM t1 WHERE val = 'two';
+----+-------------+-------+------------+------+---------------+-------+---------+-------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key   | key_len | ref   | rows | filtered | Extra       |
+----+-------------+-------+------------+------+---------------+-------+---------+-------+------+----------+-------------+
|  1 | SIMPLE      | t1    | NULL       | ref  | val_2         | val_2 | 131     | const |    1 |   100.00 | Using index |
+----+-------------+-------+------------+------+---------------+-------+---------+-------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

で、どうやって選ばせればいいの…。ドキュメントはよ…。