为什么做增量
数据量大，只需要增量最新被更改的数据。如何做增量
（1）insert into
比如行为数据，发生一条记录就插入一条，数据不会被update。
严格T+1，初始化时候限定created_at的时间。否则凌晨之后的数据会被重复插入。
（2）insert overwrite
初始化的时候不限定时间。
增量的时候where to_date(a.updated_at) >= '%s'
（选择updated_at 做为增量字段，接近凌晨创建的数据2016-12-20 23:59:58的更新时间是T日，此数据不会被增量进，接近严格T+1 。以updated_at做为初始化时候时间的限制字段，会将T日凌晨之后更新创建于几日前的数据剔除，此为bug。）
增量的方法步骤如下：
1）建表
use default;
create table if not exists loan_f_order_info
(
ord_no string comment "订单号",
uid string comment "用户ID",
crt_tim timestamp comment "创建时间",
upd_tim timestamp comment "更新时间"
)
COMMENT "test表";
2）初始化
insert overwrite table loan_f_order_info
select order_no as ord_no,
uid as uid,
created_at as crt_tim,
updated_at as upd_tim
from ods_loan.ods_loan_dsloan_loan_stage_order
3）增量
a）insert into table default.loan_f_order_info
select order_no as ord_no,
uid as uid,
created_at as crt_tim,
updated_at as upd_tim
from ods_loan.ods_loan_dsloan_loan_stage_order
where to_date(updated_at) >= date_sub(from_unixtime(unix_timestamp(),'yyyy-MM-dd'),1)
b）insert overwrite table default.loan_f_order_info
select ord_no,
uid,
crt_tim,
upd_tim
from (
select *,
row_number() over(partition by uid, ord_no order by upd_tim desc) as row_num
from default.loan_f_order_info
) za
where row_num = '1'
