1. 时间索引不容许使用like
对时间索引适应like，会时间索引变成字符串操作，成为遍历动作，失去索引价值。
错误写法：
EXPLAIN SELECT AVG(data_value) AS avg_value FROM `host_monitor` WHERE host_ip= '192.168.106.11' AND data_type=3 AND add_time LIKE '2017-05-19 01%';
正确写法：
EXPLAIN SELECT AVG(data_value) AS avg_value FROM `host_monitor` WHERE host_ip= '192.168.106.11' AND data_type=4 AND add_time >= '2017-05-19 01:00:00' AND add_time<='2017-05-19 01:59:59';
2. 索引不容许使用Max，Min
直接对索引采用采用MAX，min关键字，将执行遍历操作，索引毫无价值。
错误写法:
EXPLAIN SELECT MAX(add_time) FROM host_monitor WHERE host_ip = '192.168.106.11' AND data_type = 6
正确写法：
EXPLAIN SELECT add_time FROM host_monitor WHERE host_ip = '192.168.106.11' AND data_type = 6 ORDER BY add_time DESC LIMIT 1
