Major Description
In order to support outer joins with null top-level objects, SPARK-15441 modified Dataset.joinWith to project both inputs into single-column structs prior to the join.
For inner joins, however, this step is unnecessary and actually harms performance: performing the nesting before the join increases the shuffled data size. As an optimization for inner joins only, we can move this nesting to occur after the join (effectively switching back to the pre- SPARK-15441 behavior).
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