MySQL 8 新特性之Invisible Indexes

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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。

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