Using this pattern completely eliminates the need to shuffle and sort all the data to the reduce phase. However, it requires the data to be already organized or prepared in a very specific way.

Composite joins are particularly useful if you want to join very large data sets together. However, the data sets must first be sorted by foreign key, partitioned by foreign key, and read in a very particular manner in order to use this type of join. With that said, if your data can be read in such a way or you can prepare your data, a composite join has a huge leg-up over the other types.