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 14.2.13.3 Adaptive Hash Indexes
 ...............................
 
 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.
 
 Note that 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.
 

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