MySQL中的自适应哈希索引

innodb_adaptive_hash_index"; +----------------------------+-------+ | Variable_name | Value | +----------------------------+-------+ | innodb_adaptive_hash_index | ON | +----------------------------+-------+

存储引擎会自动对个索引页上的查询进行监控，如果能够通过使用自适应哈希索引来提高查询效率，其便会自动创建自适应哈希索引，不需要开发人员或运维人员进行任何设置操作。

自适应哈希索引是对innodb的缓冲池的B+树页进行创建，不是对整张表创建，因此速度很快。

 

可以通过查看innodb的status来查看自适应哈希索引的使用情况。

mysql> show engine innodb  status \G
*************************** 1. row ***************************
  Type: InnoDB
  Name: 
Status: 
=====================================
2019-03-07 23:37:23 0x7f1f2d34c700 INNODB MONITOR OUTPUT
=====================================
Per second averages calculated from the last 6 seconds
------------------------------------------------------
INSERT BUFFER AND ADAPTIVE HASH INDEX
-------------------------------------
Ibuf: size 1, free list len 0, seg size 2, 0 merges
merged operations:
 insert 0, delete mark 0, delete 0
discarded operations:
 insert 0, delete mark 0, delete 0
Hash table size 34679, node heap has 0 buffer(s)
Hash table size 34679, node heap has 0 buffer(s)
Hash table size 34679, node heap has 0 buffer(s)
Hash table size 34679, node heap has 0 buffer(s)
Hash table size 34679, node heap has 0 buffer(s)
Hash table size 34679, node heap has 0 buffer(s)
Hash table size 34679, node heap has 0 buffer(s)
Hash table size 34679, node heap has 0 buffer(s)
0.00 hash searches/s, 0.00 non-hash searches/s
-------------------------------
END OF INNODB MONITOR OUTPUT
============================

可以看到自适应哈希索引大小，每秒的使用情况。

注意从哈希表的特性来看，自适应哈希索引只能用于等值查询，范围或者大小是不允许的。

等着查询： select * from xx where name = "xxx";

MySQL中的自适应哈希索引

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