如果要join两张表，其中一张很大，另外一张很小，我们应该broadcast小表，把小表广播到各个节点。
还有另外一种，使用Cartesian product笛卡尔积，也就是直接join 不on
那么必须使用map join
举例，如果要计算会员数量及所占比例，需要除以会员总数，而这个总数我们就需要进行广播
with
customer as
(select userid, customeruid from dwd_store_member_ticket_di where pt>=20210221 and customeruid is not null and customeruid!='0' group by userid,customeruid),
customer_cnt as
(select COUNT(userid) as cnt_total from customer)
SELECT
/*+ mapjoin(d) */
c.*,round(c.cnt/d.cnt_total,4) as ratio FROM
(select b.parentuserid,b.parent_account,b.parent_company,COUNT(a.userid) as cnt
FROM
customer a
INNER JOIN
(select userid, parentuserid, parent_account, parent_company from dim_usergroup_df where pt='20210221') b
on a.userid=b.userid
group by b.parentuserid,b.parent_account,b.parent_company
having COUNT(a.userid)>300
) c
left join
customer_cnt d
order by c.cnt desc
;
只要在select之后加上/*+ mapjoin(d) */， 不用逗号
left join 左边必须是大表，d是小表