Mastering Hadoop 3

Reduce side join

The simplest form of join available in the MapReduce framework and nearly any type of SQL join such as inner, left outer, full outer, and so on can be done using reduce side join. The only difficulty is that nearly all the data will be shuffled across the network to go to the reducer. Two or more datasets will be joined together using a common key. Multiple large datasets can be joined by a foreign key. Remember that you should go with map side join if one of the datasets can fit into the memory. Reduce side join should be used when both datasets cannot fit into memory.

MapReduce has the capability of reading data from multiple inputs and different formats in the same MapReduce program and it also allows different mappers ...