[MySQL] mysql explain的bug

select a.host,avg(b.value) from
(select a.host,b.itemid,b.key_ from hosts a,items b where
a.hostid=b.hostid and a.host like '%hadoop-datanode%' and b.key_='impala.get[mem]')a
join
(select itemid,clock,value from history) b on a.itemid=b.itemid
and b.clock between unix_timestamp('2014-02-28 00:00:00') and
unix_timestamp('2014-03-06 00:00:00') group by a.host;

select
a.host,avg(c.value) from hosts a,items b,history c where
a.hostid=b.hostid and a.host like '%hadoop-datanode%' and
b.key_='impala.get[mem]'
and
b.itemid=c.itemid and c.clock between
unix_timestamp('2014-02-28 00:00:00') and unix_timestamp('2014-03-06 00:00:00') group by a.host;