[GitHub] [spark] JoshRosen opened a new pull request #24693: [SPARK-27829][SQL] In Dataset.joinWith inner joins, don't nest data before shuffling

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JoshRosen opened a new pull request #24693: [SPARK-27829][SQL] In Dataset.joinWith inner joins, don't nest data before shuffling
URL: https://github.com/apache/spark/pull/24693
 
 
   ## What changes were proposed in this pull request?
   
   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; see #13425).
   
   ## How was this patch tested?
   
   Existing tests, which I strengthened to also make assertions about the join result's nullability (since this guards against a bug I almost introduced during prototyping).

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