Can we join two tables using MapReduce? What are the requirements in this case -should one of the tables fit in to the memory or should both of the tables fit in to the memory? Or, is memory not a constraint at all?
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Answer:
None of the two tables have to fit in memory, although if they would, it would be lots and lots faster. The problem in this case is that there's going to be a lot of disk "spill": MapReduce jobs writing data to disk. As you might know, disk IO is very slow compared to in-memory computation.
Robin Verlangen at Quora Visit the source
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