MySQL 8 新特性之Invisible Indexes

背景

索引是把双刃剑，在提升查询速度的同时会减慢dml的操作。毕竟，索引的维护需要一定的成本。所以，对于索引，要加上该加的，删除无用的。前者是加法，后者是减法。但在实际工作中，大家似乎更热衷于前者，而很少进行后者。究其原因，在于后者，难。难的不是操作本身，而是如何确认一个索引是无用的。

如何确认无用索引

在不可见索引出现之前，大家可以通过sys.schema_unused_indexes来确定无用索引。在mysql 5.6中，即使没有sys库，也可通过该视图的基表来进行查询。

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: utf8mb4
collation_connection: utf8mb4_0900_ai_ci
1 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: t1
create 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_ci
1 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