Hive面试题：multi group by的好处，举例说明

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multi group by 可以将查询中的多个group by操作组装到一个MapReduce任务中，起到优化作用

例子：

select Provice,city,county,count(rainfall) from area where data="2018-09-02" group by provice,city,count

select Provice,count(rainfall) from area where data="2018-09-02" group by provice

 

#使用multi group by

from area

 insert overwrite table temp1

  select Provice,city,county,count(rainfall) from area where data="2018-09-02" group by provice,city,count

 insert overwrite table temp2

  select Provice,count(rainfall) from area where data="2018-09-02" group by provice

 

使用multi group by 之前必须配置参数：

<property>

    <name>hive.multigroupby.singlemr</name>

    <value>true</value>

</property>