Map join代码实现原理是一种分布式数据处理技术，可以有效地减少大量数据的传输。它通过将少量的数据分发到多个节点上，然后在这些节点上执行join操作，从而使得总体上的join性能得到显著的改善。原理如下:
1. 首先，map join将要进行join的表中的一部分数据(即小表)加载到内存中;
2. 然后，将另一个表(即大表)中所有数据加载到内存中;
3. 接下来，使用MapReduce API对这些已加载的数据进行map处理;
4. 最后，在map阶段根据需要对已加载的大表和小表进行join。...
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