时序数据库关键功能
query time-related data efficiently 
easy to downsampling 
handle expiring data automatically 

保存策略  过期自动删除
降低精度 把原先每秒的数据，存为每小时的数据 ，节省空间

连续查询（Continuous Queries）


SHOW CONTINUOUS QUERIES
创建新的Continuous Queries

CREATE CONTINUOUS QUERY cq_30m ON testDB BEGIN SELECT mean(temperature) INTO weather30m FROM weather GROUP BY time(30m) END

cq_30m  连续查询的名字
mean(temperature)  平均温度
weather     当前表名
weather30m  新数据的表名
30m  时间间隔为30分钟

SHOW MEASUREMENTS
name: measurements
------------------
name
weather
weather30m

删除Continuous Queries
DROP CONTINUOUS QUERY cq_name ON database_name