This indexing algorithm provides a fast lookup and is very light on disk usage. It is durable and transactional, but does not support range queries. It is similar to a HashMap, which makes it faster on punctual lookups and it consumes less resources than other index types. The Hash index algorithm supports four index types, which have been available since version 1.5.x:
UNIQUE_HASH_INDEXDoes not allow duplicate keys, it fails when it encounters duplicates.
NOTUNIQUE_HASH_INDEXDoes allow duplicate keys.
FULLTEXT_HASH_INDEXIndexes to any single word.
DICTIONARYDoes not allow duplicate keys, it overwrites when it encounters duplicates.
For more information on
FULLTEXT_HASH_INDEX, see FullText Index.
Hash indexes are able to perform index read operations in one I/O operation and write operations in a maximum of three I/O operations. The Hash Index algorithm is based on the Extendible Hashing algorithm. Despite not providing support for range queries, it is noticeably faster than SB-Tree Index Algorithms, (about twice as fast when querying through ten million records).
NOTE: There is an issue relating to the enhancement of Hash indexes to avoid slowdowns introduced by random I/O operations using LSM Tree approaches. For more information, see Issue #1757.