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
标签：
MySQL-Comparison of B-Tree and Hash 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.