1. 日期格式转化（）

select beg_time, end_time, extract(epoch from to_timestamp(end_time,'yyyy-mm-dd-HH24-MI-SS-US'))-extract(epoch from to_timestamp(beg_time,'yyyy-mm-dd-HH24-MI-SS-US')) from cdb_all_iu_data where beg_time > '2017-09-21'

注：beg_time, end_time以TEXT形式存储，求时间差时转化为时间戳再相减得到结果（s）

2. select * from (中间结果) t

select count(*) from (select chkid, count(*) from abc_table GROUP BY chkid) t