[GitHub] cloud-fan commented on a change in pull request #23556: [SPARK-26626][SQL] Maximum size for repeatedly substituted aliases in SQL expressions

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cloud-fan commented on a change in pull request #23556: [SPARK-26626][SQL] Maximum size for repeatedly substituted aliases in SQL expressions
URL: https://github.com/apache/spark/pull/23556#discussion_r248928260
 
 

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 File path: sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/Optimizer.scala
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 @@ -686,6 +687,28 @@ object CollapseProject extends Rule[LogicalPlan] {
     }.exists(!_.deterministic))
   }
 
+  private def hasOversizedRepeatedAliases(
+      upper: Seq[NamedExpression], lower: Seq[NamedExpression]): Boolean = {
+    val aliases = collectAliases(lower)
+
+    // Count how many times each alias is used in the upper Project.
+    // If an alias is only used once, we can safely substitute it without increasing the overall
+    // tree size
+    val referenceCounts = AttributeMap(
+      upper
+        .flatMap(_.collect { case a: Attribute => a })
+        .groupBy(identity)
+        .mapValues(_.size).toSeq
+    )
+
+    // Check for any aliases that are used more than once, and are larger than the configured
+    // maximum size
+    aliases.exists({ case (attribute, expression) =>
+      referenceCounts.getOrElse(attribute, 0) > 1 &&
+        expression.treeSize > SQLConf.get.maxRepeatedAliasSize
 
 Review comment:
   I'm not sure about using `treeSize` as the cost of an expression. UDF can be very expensive even if its `treeSize` is 1.
   
   How about we simplify it with a blacklist? e.g. UDF is expensive and we shouldn't collapse projects if udf is repeated.

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