The index also can be used for LIKE comparisons if the argument to LIKE is a constant string that does not start with a wildcard character.

http://dev.mysql.com/doc/refman/5.7/en/index-btree-hash.html

MySQL 5.7 Reference Manual  /  ...  /  Comparison of B-Tree and Hash Indexes

9.3.8 Comparison of B-Tree and Hash Indexes

Understanding the B-tree and hash data structures can help predict how different queries perform on different storage engines that use these data structures in their indexes, particularly for the MEMORY storage engine that lets you choose B-tree or hash indexes.