Github user windpiger commented on a diff in the pull request:
https://github.com/apache/spark/pull/16255#discussion_r95095416
--- Diff:
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/Optimizer.scala
---
@@ -200,6 +200,8 @@ object RemoveAliasOnlyProject extends Rule[LogicalPlan]
{
case plan: Project if plan eq proj => plan.child
case plan => plan transformExpressions {
case a: Attribute if attrMap.contains(a) => attrMap(a)
+ case b: Alias if attrMap.exists(_._1.exprId == b.exprId)
+ && b.child.isInstanceOf[NamedExpression] => b.child
--- End diff --
As you said, if we find an alias-only project, e.g. alias a#1 to a#2, it's
safe to `remove this project` and `replace all a#2 with a#1` in this plan. So
another Alias which is also alias a#1 to a#2, but not the same object with the
first one, it will not be processed.
here, the logic shows that we process the situation.
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