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15.14.3 Adaptive Hash Indexes
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If a table fits almost entirely in main memory, the fastest way to
perform queries on it is to use hash indexes. `InnoDB' has an automatic
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' will build 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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