invisible index特点
默认创建的索引都是visible，如果需要invisible索引创建的时候需要指定invisible参数。
默认无法使用invisible index索引，需要开启参数optimizer_switch='use_invisible_indexes=on'才能使用；
使用force index也无法使用，会报错；
索引的visible和invisible属性可以相互转换，即使表很大，这个过程也是很快的；
primary key不能改成invisible;
invisible index作用
在生产环境中，往往一个表是非常大的，我们想要测试一条SQL不使用某个索引的执行效率，如果直接删除这个索引，可能代价比较大，但是把索引改成不可见模式，再去测试，时间是很快的。
mysql> create index idx_emp_no on t_group(emp_no) invisible; Query OK, 0 rows affected (0.22 sec) Records: 0 Duplicates: 0 Warnings: 0 mysql> show index from t_group; +---------+------------+------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+ | Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment | Visible | Expression | +---------+------------+------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+ | t_group | 1 | idx_emp_no | 1 | emp_no | A | 10 | NULL | NULL | | BTREE | | | NO | NULL | +---------+------------+------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+ 1 row in set (0.08 sec) mysql> desc select * from t_group where emp_no=31112; +----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+ | 1 | SIMPLE | t_group | NULL | ALL | NULL | NULL | NULL | NULL | 10 | 10.00 | Using where | +----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+ 1 row in set, 1 warning (0.00 sec) mysql> desc select * from t_group force index(idx_emp_no) where emp_no=31112; ERROR 1176 (42000): Key 'idx_emp_no' doesn't exist in table 't_group' mysql> desc select /*+ set_var(optimizer_switch='use_invisible_indexes=on') */ * from t_group where emp_no=31112; +----+-------------+---------+------------+------+---------------+------------+---------+-------+------+----------+-------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+---------+------------+------+---------------+------------+---------+-------+------+----------+-------+ | 1 | SIMPLE | t_group | NULL | ref | idx_emp_no | idx_emp_no | 4 | const | 1 | 100.00 | NULL | +----+-------------+---------+------------+------+---------------+------------+---------+-------+------+----------+-------+ 1 row in set, 1 warning (0.00 sec)
参考链接