前言
MySQL8.0 开始支持不可见索引。 优化器根本不使用不可见索引,但会以其他的方式正常维护。
默认情况下 索引是可见的。 通过不可见索引,可以方便数据库管理人员 检查 索引对查询性能的影响,而不会进行破坏性的更改 。
应用场景: 软删除,灰度发布
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-- 创建测试表 mysql> create table t1(i int ,j int ); Query OK, 0 rows affected (0.03 sec) -- 创建索引 mysql> mysql> create index idx_i on t1(i); Query OK, 0 rows affected (0.02 sec) Records: 0 Duplicates: 0 Warnings: 0 mysql> -- 创建 一个隐藏索引 mysql> create index idx_j on t1(j) invisible ; Query OK, 0 rows affected (0.02 sec) Records: 0 Duplicates: 0 Warnings: 0 -- 查看索引 mysql> show index from t1\G *************************** 1. row *************************** Table : t1 Non_unique: 1 Key_name: idx_i Seq_in_index: 1 Column_name: i Collation: A Cardinality: 0 Sub_part: NULL Packed: NULL Null : YES Index_type: BTREE Comment: Index_comment: Visible: YES Expression: NULL *************************** 2. row *************************** Table : t1 Non_unique: 1 Key_name: idx_j Seq_in_index: 1 Column_name: j Collation: A Cardinality: 0 Sub_part: NULL Packed: NULL Null : YES Index_type: BTREE Comment: Index_comment: Visible: NO Expression: NULL 2 rows in set (0.00 sec) |
上面 对 i
,j
分别创建了索引, 其中j
为不可见索引 ,即默认优化器不会使用到该索引的。
索引使用情况测试
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mysql> explain select * from t1 where i= 10\G *************************** 1. row *************************** id: 1 select_type: SIMPLE table : t1 partitions: NULL type: ref possible_keys: idx_i key : idx_i key_len: 5 ref: const rows : 1 filtered: 100.00 Extra: NULL 1 row in set , 1 warning (0.00 sec) mysql> explain select * from t1 where j= 10\G *************************** 1. row *************************** id: 1 select_type: SIMPLE table : t1 partitions: NULL type: ALL possible_keys: NULL key : NULL key_len: NULL ref: NULL rows : 1 filtered: 100.00 Extra: Using where 1 row in set , 1 warning (0.00 sec) |
如何临时让优化器可以看到这个索引呢?
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mysql> select @@optimizer_switch\G *************************** 1. row *************************** @@optimizer_switch: index_merge= on ,index_merge_union= on ,index_merge_sort_union= on ,index_merge_intersection= on ,engine_condition_pushdown= on ,index_condition_pushdown= on ,mrr= on ,mrr_cost_based= on ,block_nested_loop= on ,batched_key_access= off ,materialization= on ,semijoin= on ,loosescan= on ,firstmatch= on ,duplicateweedout= on ,subquery_materialization_cost_based= on ,use_index_extensions= on ,condition_fanout_filter= on ,derived_merge= on ,use_invisible_indexes= off ,skip_scan= on 1 row in set (0.00 sec) -- 设置开关 mysql> set session optimizer_switch= 'use_invisible_indexes=on' ; Query OK, 0 rows affected (0.00 sec) mysql> select @@optimizer_switch\G *************************** 1. row *************************** @@optimizer_switch: index_merge= on ,index_merge_union= on ,index_merge_sort_union= on ,index_merge_intersection= on ,engine_condition_pushdown= on ,index_condition_pushdown= on ,mrr= on ,mrr_cost_based= on ,block_nested_loop= on ,batched_key_access= off ,materialization= on ,semijoin= on ,loosescan= on ,firstmatch= on ,duplicateweedout= on ,subquery_materialization_cost_based= on ,use_index_extensions= on ,condition_fanout_filter= on ,derived_merge= on ,use_invisible_indexes= on ,skip_scan= on 1 row in set (0.00 sec) -- 此时可以看到 优化器已经可以看到这个索引啦 mysql> explain select * from t1 where j= 10\G *************************** 1. row *************************** id: 1 select_type: SIMPLE table : t1 partitions: NULL type: ref possible_keys: idx_j key : idx_j key_len: 5 ref: const rows : 1 filtered: 100.00 Extra: NULL 1 row in set , 1 warning (0.00 sec) |
修改索引的可见性
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mysql> ALTER TABLE t1 ALTER INDEX idx_j INVISIBLE; Query OK, 0 rows affected (0.01 sec) Records: 0 Duplicates: 0 Warnings: 0 mysql> ALTER TABLE t1 ALTER INDEX idx_j VISIBLE; Query OK, 0 rows affected (0.02 sec) Records: 0 Duplicates: 0 Warnings: 0 |
注意: 主键索引 是不能设置 不可见的 。
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create table t2(`id` int not null auto_increment, primary key (`id`)); mysql> show index from t2\G *************************** 1. row *************************** Table : t2 Non_unique: 0 Key_name: PRIMARY Seq_in_index: 1 Column_name: id Collation: A Cardinality: 0 Sub_part: NULL Packed: NULL Null : Index_type: BTREE Comment: Index_comment: Visible: YES Expression: NULL 1 row in set (0.01 sec) mysql> alter table t2 alter index PRIMARY key (`id`) invisible; ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'PRIMARY invisible' at line 1 |
测试 添加 主键 为不可见索引:
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mysql> create table t3(id int not null ); Query OK, 0 rows affected (0.03 sec) mysql> alter table t3 add primary key pk_t3(id) invisible; ERROR 3522 (HY000): A primary key index cannot be invisible |
primary key
是必须可见的, 不能设置为不可见索引。
在生产环境中 也可以利用隐藏索引进行 SQL 语句的性能测试,或者对索引进行逻辑删除,索引的灰度发布测试,以及软删除索引等。
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原文链接:https://blog.csdn.net/u010339879/article/details/127043431