Github user cloud-fan commented on a diff in the pull request: https://github.com/apache/spark/pull/12719#discussion_r62454855 --- Diff: sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/Optimizer.scala --- @@ -617,6 +618,46 @@ object NullPropagation extends Rule[LogicalPlan] { } /** + * Propagate foldable expressions: + * Replace all attributes with aliases of the original foldable expressions except Union queries. + * Aliases and ordinal expressions are the main target to be transformed after propagation. 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 isFoldableStatement = plan.find { + case _: Union => true + case _: Command => true + case _ => false + }.isEmpty + + val foldableExprSet = ExpressionSet(plan.flatMap { + case Project(projectList, _) => projectList.collect { + case a: Alias if a.resolved && a.child.foldable => a + } + case _ => Nil + }) + + if (!isFoldableStatement || foldableExprSet.isEmpty) { --- End diff -- So we won't do this optimization if there exists a un-foldable plan(`Union` and `Command`) in the plan tree? I think we can make it more fine-grained, e.g. we can still apply this optimization for the children of `Union`. A possible approach is: find one foldable project, collect foldable expressions, transformUp the plan tree and replace attributes until we reach `Union` or `Command`
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