【mysql】Innodb三大特性之adaptive hash index

一、Adaptive Hash Indexes 定義

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.

二、hash index

哈希（hash）是一種很是快的查找方法，通常狀況下查找的時間複雜度爲O（1），經常使用於鏈接（join）操做，如SQL Server和Oracle中的哈希鏈接（hash join）。可是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爲哈希，而InnoDB存儲引擎提出了另外一種實現方法，自適應哈希索引（adaptive hash index）

三、自適應哈希

InnoDB存儲引擎會監控對錶上索引的查找，若是觀察到創建哈希索引能夠帶來速度的提高，則創建哈希索引，因此稱之爲自適應（adaptive） 的。自適應哈希索引經過緩衝池的B+樹構造而來，所以創建的速度很快。並且不須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 來爲某些頁創建哈希索引。

根據InnoDB的官方文檔顯示，啓用自適應哈希索引後，讀取和寫入速度能夠提升2倍；對於輔助索引的鏈接操做，性能能夠提升5倍。在我看來，自適應哈希索引是很是好的優化模式，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，所以全部會涉及到Search Path的操做，都可使用此Hash索引進行優化，這些可優化的操做包括：Unique Scan/Range Scan(Locate First Key Page)/Insert/Delete/Purge等等，幾乎涵蓋InnoDB全部的操做類型

Adaptive，意味着不是全部的葉頁面都會以Hash索引維護，葉頁面進入Hash 索引的條件是：同種類型的操做(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    |
+----------------------------+-------+

不過咱們能夠經過參數innodb_adaptive_hash_index來禁用或啓動此特性，默認爲開啓

 

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