[GitHub] [spark] ulysses-you commented on a diff in pull request #40446: [SPARK-42815][SQL] Subexpression elimination support shortcut expression

["ulysses-you (via GitHub)" <gi...@apache.org>]

ulysses-you commented on code in PR #40446:
URL: https://github.com/apache/spark/pull/40446#discussion_r1138102904


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sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/EquivalentExpressions.scala:
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@@ -128,10 +131,23 @@ class EquivalentExpressions {
   // There are some special expressions that we should not recurse into all of its children.
   //   1. CodegenFallback: it's children will not be used to generate code (call eval() instead)
   //   2. ConditionalExpression: use its children that will always be evaluated.
-  private def childrenToRecurse(expr: Expression): Seq[Expression] = expr match {
-    case _: CodegenFallback => Nil
-    case c: ConditionalExpression => c.alwaysEvaluatedInputs
-    case other => other.children
+  private def childrenToRecurse(expr: Expression): Seq[Expression] = {
+    val alwaysEvaluated = expr match {
+      case _: CodegenFallback => Nil
+      case c: ConditionalExpression => c.alwaysEvaluatedInputs
+      case other => other.children
+    }
+    if (shortcut) {
+      // The subexpression may not need to eval even if it appears more than once.
+      // e.g., `if(or(a, and(b, b)))`, the expression `b` would be skipped if `a` is true.
+      alwaysEvaluated.map {
+        case and: And => and.left
+        case or: Or => or.left
+        case other => other
+      }

Review Comment:
   It is why we add a new config with disabled by default. We can not decide which subexpression would be evaluated before running. When enable this config, it assumes that the left child is a shotcut, then the righ child can be skipped whatever it contains common subexpression.



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