Github user marmbrus commented on a diff in the pull request:
https://github.com/apache/spark/pull/2345#discussion_r17452614
--- Diff:
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/Optimizer.scala
---
@@ -47,6 +48,53 @@ object Optimizer extends RuleExecutor[LogicalPlan] {
}
/**
+ * Pushes operations to either side of a Union.
+ */
+object UnionPushdown extends Rule[LogicalPlan] {
+
+ /**
+ * Maps Attributes from the left side to the corresponding Attribute
on the right side.
+ */
+ def buildRewrites(union: Union): AttributeMap[Attribute] = {
+ assert(union.left.output.size == union.right.output.size)
+
+ AttributeMap(union.left.output.zip(union.right.output))
+ }
+
+ /**
+ * Rewrites an expression so that it can be pushed to the right side
of a Union operator.
+ * This method relies on the fact that the output attributes of a
union are always equal
+ * to the left child's output.
+ */
+ def pushToRight[A <: Expression](e: A, union: Union, rewrites:
AttributeMap[Attribute]): A = {
--- End diff --
Nit: `union` is not used.
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