ADSL拨号VPS包含了中国大陆（联通，移动，电信，）

中国香港，国外拨号VPS。

MySQL> show create table sys.schema_unused_indexes\G*************************** 1. row ***************************        View: schema_unused_indexes    Create View: CREATE ALGORITHM=MERGE DEFINER=`mysql.sys`@`localhost` SQL SECURITY INVOKER VIEW `sys`.`schema_unused_indexes` (`object_schema`,`object_name`,`index_name`) AS select `t`.`OBJECT_SCHEMA` AS `object_schema`,`t`.`OBJECT_NAME` AS `object_name`,`t`.`INDEX_NAME` AS `index_name` from (`performance_schema`.`table_io_waits_summary_by_index_usage` `t` join `information_schema`.`STATISTICS` `s` on(((`t`.`OBJECT_SCHEMA` = convert(`s`.`TABLE_SCHEMA` using utf8mb4)) and (`t`.`OBJECT_NAME` = convert(`s`.`TABLE_NAME` using utf8mb4)) and (convert(`t`.`INDEX_NAME` using utf8) = `s`.`INDEX_NAME`)))) where ((`t`.`INDEX_NAME` is not null) and (`t`.`COUNT_STAR` = 0) and (`t`.`OBJECT_SCHEMA` <> 'mysql') and (`t`.`INDEX_NAME` <> 'PRIMARY') and (`s`.`NON_UNIQUE` = 1) and (`s`.`SEQ_IN_INDEX` = 1)) order by `t`.`OBJECT_SCHEMA`,`t`.`OBJECT_NAME`character_set_client: utf8mb4collation_connection: utf8mb4_0900_ai_ci1 row in set, 1 warning (0.00 sec)

但这种方式也有不足，

1. 如果实例发生重启，performance_schema中的数据就会清零。

2. 如果基于上面的查询删除了索引，查询性能突然变差，怎么办？

不可见索引的出现，可有效弥补上述不足。将index设置为invisible，会导致优化器在选择执行计划时，自动忽略该索引，即便使用了FORCE INDEX。

当然，这个是由optimizer_switch变量中use_invisible_indexes选项决定的，默认为off。如果想看一个查询在索引调整前后执行计划的差别，可在会话级别调整use_invisible_indexes的值，如，

mysql> show create table slowtech.t1\G*************************** 1. row ***************************   Table: t1Create Table: CREATE TABLE `t1` ( `id` int(11) NOT NULL, `name` varchar(10) DEFAULT NULL, PRIMARY KEY (`id`), KEY `idx_name` (`name`) /*!80000 INVISIBLE */) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci1 row in set (0.00 sec)mysql> explain select * from slowtech.t1 where name='a';+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra   |+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+| 1 | SIMPLE   | t1  | NULL   | ALL | NULL     | NULL | NULL  | NULL |  6 |  16.67 | Using where |+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+1 row in set, 1 warning (0.00 sec)mysql> set session optimizer_switch="use_invisible_indexes=on";Query OK, 0 rows affected (0.00 sec)mysql> explain select * from slowtech.t1 where name='a';+----+-------------+-------+------------+------+---------------+----------+---------+-------+------+----------+-------------+| id | select_type | table | partitions | type | possible_keys | key   | key_len | ref | rows | filtered | Extra   |+----+-------------+-------+------------+------+---------------+----------+---------+-------+------+----------+-------------+| 1 | SIMPLE   | t1  | NULL   | ref | idx_name   | idx_name | 43   | const |  1 | 100.00 | Using index |+----+-------------+-------+------------+------+---------------+----------+---------+-------+------+----------+-------------+1 row in set, 1 warning (0.00 sec)

不可见索引的常见操作

create table t1(id int primary key,name varchar(10),index idx_name (name) invisible);alter table t1 alter index idx_name visible;alter table t1 alter index idx_name invisible;

如何查看哪些索引不可见

mysql> select table_schema,table_name,index_name,column_name,is_visible from information_schema.statistics where is_visible='no';+--------------+------------+------------+-------------+------------+| TABLE_SCHEMA | TABLE_NAME | INDEX_NAME | COLUMN_NAME | IS_VISIBLE |+--------------+------------+------------+-------------+------------+| slowtech  | t1    | idx_name | name    | NO    |+--------------+------------+------------+-------------+------------+1 row in set (0.00 sec)

注意

1. 主键索引不可被设置为invisible。

总结

以上所述是小编给大家介绍的MySQL 8 新特性之Invisible Indexes ，希望对大家有所帮助，如果大家有任何疑问欢迎给我留言，小编会及时回复大家的！

(责任编辑：admin)