If a table fits almost entirely in main memory, the fastest way to perform queries on it is to use hash indexes. InnoDB has a mechanism that monitors index searches made to the indexes defined for a table. If InnoDB notices that queries could benefit from building a hash index, it does so automatically.
The hash index is always built based on an existing B-tree index on the table. InnoDB can build a hash index on a prefix of any length of the key defined for the B-tree, depending on the pattern of searches that InnoDB observes for the B-tree index. A hash index can be partial: It is not required that the whole B-tree index is cached in the buffer pool. InnoDB builds hash indexes on demand for those pages of the index that are often accessed.
In a sense, InnoDB tailors itself through the adaptive hash index mechanism to ample main memory, coming closer to the architecture of main-memory databases.
The configuration parameter innodb_adaptive_hash_index can be set to disable or enable the adaptive hash index. See Section 8.3.4, “Dynamically Changing innodb_adaptive_hash_index” for details.
2、hash index
哈希（hash）是一種非常快的查找方法，一般情況下查找的時間復雜度為O（1），常用於連接（join）操作，如SQL Server和Oracle中的哈希連接（hash join）。但是SQL Server和Oracle等常見的數據庫並不支持哈希索引（hash index）。MySQL的Heap存儲引擎默認的索引類型為哈希，而InnoDB存儲引擎提出了另一種實現方法，自適應哈希索引（adaptive hash index）
3、自適應哈希
InnoDB存儲引擎會監控對表上索引的查找，如果觀察到建立哈希索引可以帶來速度的提升，則建立哈希索引，所以稱之為自適應（adaptive） 的。自適應哈希索引通過緩沖池的B+樹構造而來，因此建立的速度很快。而且不需要將整個表都建哈希索引，InnoDB存儲引擎會自動根據訪問的頻率和模式 來為某些頁建立哈希索引。
根據InnoDB的官方文檔顯示，啟用自適應哈希索引後，讀取和寫入速度可以提高2倍；對於輔助索引的連接操作，性能可以提高5倍。在我看來，自適應哈希索引是非常好的優化模式，其設計思想是數據庫自優化（self-tuning），即無需DBA對數據庫進行調整。
Adaptive Hash Index是針對B+樹Search Path的優化，因此所有會涉及到Search Path的操作，均可使用此Hash索引進行優化，這些可優化的操作包括：Unique Scan/Range Scan(Locate First Key Page)/Insert/Delete/Purge等等，幾乎涵蓋InnoDB所有的操作類型
Adaptive，意味著不是所有的葉頁面都會以Hash索引維護，葉頁面進入Hash 索引的條件是：同種類型的操作(Scan/Insert…)，命中同一葉頁面的次數，超過此頁面記錄數量的1/16，則可將當前葉頁面加入Hash索引， 用以優化後續可能的相同Search Path。
mysql> show engine innodb status \G ------------------------------------- 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 553229, node heap has 17 buffer(s) 0.00 hash searches/s, 0.00 non-hash searches/s mysql> show variables like '%adaptive_hash%'; +----------------------------+-------+ | Variable_name | Value | +----------------------------+-------+ | innodb_adaptive_hash_index | ON | +----------------------------+-------+
不過我們可以通過參數innodb_adaptive_hash_index來禁用或啟動此特性，默認為開啟
參考文章
http://hedengcheng.com/?p=458
https://dev.mysql.com/doc/refman/5.0/en/innodb-adaptive-hash.html
https://dev.mysql.com/doc/innodb-plugin/1.0/en/innodb-performance-adaptive_hash_index.html
