Github user rxin commented on a diff in the pull request:
https://github.com/apache/spark/pull/22205#discussion_r213100428
--- Diff:
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/Optimizer.scala
---
@@ -130,6 +130,10 @@ abstract class Optimizer(sessionCatalog:
SessionCatalog)
// since the other rules might make two separate Unions operators
adjacent.
Batch("Union", Once,
CombineUnions) ::
+ // run this once earlier. this might simplify the plan and reduce cost
of optimizer
--- End diff --
can you put your comment above into the comment in code
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