如下：
有一张表user_action_log(id,user,action_time,action_desc),数据量有点大。
现在要查询某个月用户的记录，展示如下:（用户，时间，第一次操作时间）
user,action_time,first_time，如何比较高效的查询呢？
我的做法如下，但是太慢了：
select a.*,(select min(b.action_time) from user_action_log b where a.user=b.user and b.action_time!=a.action_time) as first_time from user_action_log a where substr(action_time,0,6)='201507'
select a.*, min(b.action_time) as first_time
from user_action_log a, user_action_log b
where substr(action_time, 0, 6) = '201507'
and a.user = b.user
and b.action_time != a.action_time