create table `instance` ( `id` int(11) unsigned not null auto_increment, `timestamp` int(10) unsigned default null, `cpu` decimal(8,3) default null, primary key (`id`)) engine=innodb;
统计每半小时平均值，实现可以如下：
select `timestamp`-`timestamp`% (5*60) , avg(cup)from `instance`group by `timestamp`-`timestamp`% (5*60)
另外还有：
select queuename, hour(dt) as h, floor(minute(dt) / 30) as v , avg(memoryused), sum(memoryused), count(memoryused) from queue_resource where dt>='2014-04-14 00:00:00' group by h, v, queuename;