viirya commented on a change in pull request #31656:
URL: https://github.com/apache/spark/pull/31656#discussion_r583797946
##########
File path:
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/Optimizer.scala
##########
@@ -157,8 +157,8 @@ abstract class Optimizer(catalogManager: CatalogManager)
// since the other rules might make two separate Unions operators
adjacent.
Batch("Union", Once,
RemoveNoopOperators,
- RemoveNoopUnion,
- CombineUnions) ::
+ CombineUnions,
+ RemoveNoopUnion) ::
Review comment:
Added test is fine. But there will be an
[error](https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder/135486/testReport/org.apache.spark.sql/SQLQueryTestSuite/subquery_in_subquery_in_set_operations_sql/)
of "Once strategy's idempotence is broken for batch Union".
It is because `CombineUnions` might combine two or more `Union` and they
have redundant children under `Distinct`. So the rule order `RemoveNoopUnion`
-> `CombineUnions` is not idempotence for Once strategy.
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