MySQL-Comparison of B-Tree and Hash Indexes

Most MySQL indexes (PRIMARY KEY, UNIQUE, INDEX, and FULLTEXT) are stored in B-trees. Exceptions: Indexes on spatial data types use R-trees; MEMORY tables also support hash indexes; InnoDBuses inverted lists for FULLTEXT indexes.

没多少内容，直接看原文吧

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

、只要使用了Limit mysql 会尽力使用索引、

MySQL-Comparison of B-Tree and Hash Indexes

原文：http://my.oschina.net/httpssl/blog/528211