mysql InnoDB引擎支持hash索引吗

mysql InnoDB引擎支持hash索引吗

    查一下mysql官方文档：https://dev.mysql.com/doc/refman/5.7/en/create-index.html，



从上面的图中可以得知，mysql 是支持hash索引的，但支持和不支持又和具体的存储引擎有关系。从图中，
看到InnoDB是支持Btree索引，这是我们众所周知的。
我们具体查一下InnoDB文档那一部分：https://dev.mysql.com/doc/refman/5.7/en/innodb-introduction.html。



表中表明，InnoDB不支持hash索引，但又给出了一个特殊的解释，

InnoDB utilizes hash indexes internally for its Adaptive Hash Index
feature。

Performance benefits are not limited to giant tables with long-running queries. When the same rows are accessed over and over from a table, a feature called the Adaptive
Hash Index takes over to make these lookups even faster, as if they came out of a hash table.

adaptive hash index

An optimization for 

InnoDB

 tables
that can speed up lookups using 

=

 and 

IN

 operators,
by constructing a hash index in
memory. MySQL monitors index searches for 

InnoDB

 tables, and if queries could benefit from a hash index, it builds one automatically for index pages that
are frequently accessed. In a sense, the adaptive hash index configures MySQL at runtime to take advantage of ample main memory, coming closer to the architecture of main-memory databases. This feature is controlled by the 

innodb_adaptive_hash_index

 configuration
option. Because this feature benefits some workloads and not others, and the memory used for the hash index is reserved in the buffer
pool, typically you should benchmark with this feature both enabled and disabled.

The hash index is always built based on an existing 

InnoDB

 secondary
index, which is organized as a B-tree structure.
MySQL 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 against the index. A hash index can be partial; the whole B-tree index does not need to be cached in the buffer pool.

In MySQL 5.6 and higher, another way to take advantage of fast single-value lookups with 

InnoDB

 tables is to use the 

InnoDB

 memcached plugin.
See Section 14.20,
“InnoDB memcached Plugin” for details.

See Also B-tree, buffer
pool, hash index, memcached, page, secondary
index.

看到了吗。mysqlInnoDB存储引擎 是支持hash索引的，不过，我们必须启用，hash索引的创建由InnoDB存储引擎引擎自动优化创建，我们干预不了。

其实【MySQL技术内幕InnoDB存储引擎.姜承尧.扫描版】也提到了这点，【对不起作者，一向喜欢看电子书，实体书实在买不起，因为电子书携带也比较方便，搬家也不用乱丢书了。】



      

所以，当有人问你mysql InnoDB存储引擎是否支持hash索引的时候，你知道怎么回答了吧。