Github user cloud-fan commented on a diff in the pull request:
https://github.com/apache/spark/pull/12719#discussion_r62810329
--- Diff:
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/Optimizer.scala
---
@@ -618,6 +619,48 @@ object NullPropagation extends Rule[LogicalPlan] {
}
/**
+ * Propagate foldable expressions:
+ * Replace attributes with aliases of the original foldable expressions if
possible.
+ * Other optimizations will take advantage of the propagated foldable
expressions.
+ *
+ * {{{
+ * SELECT 1.0 x, 'abc' y, Now() z ORDER BY x, y, 3
+ * ==> SELECT 1.0 x, 'abc' y, Now() z ORDER BY 1.0, 'abc', Now()
+ * }}}
+ */
+object FoldablePropagation extends Rule[LogicalPlan] {
+ def apply(plan: LogicalPlan): LogicalPlan = {
+ val foldableExprSet = ExpressionSet(plan.flatMap {
+ case Project(projectList, _) => projectList.collect {
+ case a: Alias if a.resolved && a.child.foldable => a
+ }
+ case _ => Nil
+ })
+
+ if (foldableExprSet.isEmpty) {
+ plan
+ } else {
+ val foldableMap =
AttributeMap(foldableExprSet.toSeq.map(_.asInstanceOf[Alias])
+ .map(a => (a.toAttribute, a.child)))
--- End diff --
I haven't fully understood why we need to create new alias instead of
reusing the existing one. And even creating new alias may be not safe, as the
exprId changed. That's why I try to figure out all of the cases that will have
problem if we don't add `Alias`.
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