Replicated Join- Same as Distributed cache, where smaller table is loaded to RAM and performs Map side Join.
Skewed Join - Used when data is unbalanced. The cases wherein one redcer gets more number of records and other reducer gets less records.
Eg: Records for USA might be more when compared to India. Here the records are unbalanced and recucer for processing USA records takes more time.
Skewed Join in Pig handles this scenario by presampling and identifies keys which have skewed data. The skewed data will be automatically split across multiple reducers
Merge Join - If the data is pre-sorted, then this kind of Join can be used which can skip sort phase. Sort operation is an expensive process and this can be skipped.
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