[GitHub] [spark] dongjoon-hyun commented on a change in pull request #32586: [SPARK-35439][SQL] Children subexpr should come first than parent subexpr

[GitBox <gi...@apache.org>]

dongjoon-hyun commented on a change in pull request #32586:
URL: https://github.com/apache/spark/pull/32586#discussion_r635462518



##########
File path: sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/EquivalentExpressions.scala
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@@ -167,7 +167,25 @@ class EquivalentExpressions {
    * Returns all the equivalent sets of expressions.
    */
   def getAllEquivalentExprs: Seq[Seq[Expression]] = {
-    equivalenceMap.values.map(_.toSeq).toSeq
+    equivalenceMap.values.map(_.toSeq).toSeq.sortBy(_.head)(new ExpressionOrdering)
+  }
+
+  /**
+   * Orders [Expression] by parent/child relations. The child expression is smaller
+   * than parent expression. If there is child-parent relationships among the subexpressions,
+   * we want the child expressions come first than parent expressions, so we can replace
+   * child expressions in parent expressions with subexpression evaluation.
+   */
+  class ExpressionOrdering extends Ordering[Expression] {
+    override def compare(x: Expression, y: Expression): Int = {
+      if (x.semanticEquals(y)) {
+        0
+      } else if (x.find(_.semanticEquals(y)).isDefined) {
+        1
+      } else {
+        -1

Review comment:
       Some irrelevant expressions, e1 and e2, have e1 < e2 and e2 < e1 relation at the same time?




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