Github user gatorsmile commented on a diff in the pull request:
https://github.com/apache/spark/pull/16933#discussion_r101341551
--- Diff:
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/Optimizer.scala
---
@@ -563,23 +563,27 @@ object CollapseProject extends Rule[LogicalPlan] {
/**
* Combines adjacent [[Repartition]] and [[RepartitionByExpression]]
operator combinations
* by keeping only the one.
- * 1. For adjacent [[Repartition]]s, collapse into the last
[[Repartition]].
+ * 1. For adjacent [[Repartition]]s, collapse into the last
[[Repartition]] if their shuffle types
+ * are the same or the parent's shuffle is true.
* 2. For adjacent [[RepartitionByExpression]]s, collapse into the last
[[RepartitionByExpression]].
- * 3. For a combination of [[Repartition]] and
[[RepartitionByExpression]], collapse as a single
- * [[RepartitionByExpression]] with the expression and last number of
partition.
+ * 3. When a shuffle-enabled [[Repartition]] is above a
[[RepartitionByExpression]], collapse as a
+ * single [[RepartitionByExpression]] with the expression and the last
number of partition.
+ * 4. When a [[RepartitionByExpression]] is above a [[Repartition]],
collapse as a single
--- End diff --
`RepartitionByExpression ` always uses `ShuffleExchange`. Thus, it is like
Repartition with enabled shuffle.
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