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From dongjoon-hyun <...@git.apache.org>
Subject [GitHub] spark pull request: [SPARK-14939][SQL] Add FoldablePropagation opt...
Date Wed, 11 May 2016 08:48:32 GMT
Github user dongjoon-hyun commented on a diff in the pull request:

    https://github.com/apache/spark/pull/12719#discussion_r62811803
  
    --- 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 --
    
    Okay, maybe there exists some problem if the logic is not clear to you. Let's consider
the following example.
    ```
    scala> sql("select 1, a newAlias from (select 1 a)").show
    +---+--------+
    |  1|newAlias|
    +---+--------+
    |  1|       1|
    +---+--------+
    ```
    The collected alias map is {'a':Literal(1)}.
    And, we will replace `a newAliases` by creating new `Alias(Literal(1), "newAliases")`.
    Does this make sense to you?


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