闲来无事，研究了一下mysql索引，场景如下：
有一张MyISAM 类型的zt_action表，数据大约230W行，建两个索引，
CREATE INDEX `read` ON zt_action(`read`)
CREATE INDEX `Type_Id_read` ON zt_action(`objectType,objectID,read`)
CREATE INDEX `actor_action_date` ON zt_action(`actor,action,date`)
EXPLAIN SELECT objectType FROM zt_action WHERE objectType='user' AND objectID=1 AND `read`='0' ('Using where; Using index') 使用索引Type_Id_read，type为ref
EXPLAIN SELECT * FROM zt_action WHERE `read`='3' ('Using index condition') 使用索引read，然后为了select * ,所以进行回表操作
EXPLAIN SELECT * FROM zt_action WHERE `read`='0' AND objectType='user' ('Using index condition') 使用索引Type_Id_read，然后进行回表操作
EXPLAIN SELECT `read` FROM zt_action ('Using index') 未使用索引，但是遍历了索引表read
EXPLAIN SELECT `read` FROM zt_action WHERE `read`='0' ('Using where; Using index') 使用索引read
EXPLAIN SELECT `read` FROM zt_action WHERE `read`='0' AND objectType='user' ('Using index condition; Using where') ('Using where; Using index')Type_id_read 使用索引read, 这个不清楚了
EXPLAIN SELECT `read` FROM zt_action WHERE objectID=1 ('Using where; Using index') 未使用索引，但是遍历索引表Type_Id_read
EXPLAIN SELECT `date` FROM zt_action WHERE objectID=1 ('Using where') 未使用索引，全表扫描
EXPLAIN SELECT `read` FROM zt_action WHERE objectType='user' ('Using where; Using index') 使用了索引Type_Id_read
EXPLAIN SELECT `date` FROM zt_action WHERE `action` LIKE '%ct%'; ('Using where; Using index') 未使用索引，但是遍历了索引表actor_action_date