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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