MepReduce面试题：MapReduce join

1.map join
缺点：只适合大小表join
优点：不会出现数据倾斜
实现：将小表数据加入缓存分发到各个计算节点,按连接关键字建立索引
job.addCacheFile(new URI(“xxxxxxx”));
job.setNumReduceTasks(0);
2.reduce join
缺点：会出现数据倾斜