cloud-fan commented on a change in pull request #30178:
URL: https://github.com/apache/spark/pull/30178#discussion_r525826043
##########
File path:
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/Optimizer.scala
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@@ -806,6 +807,18 @@ object CollapseRepartition extends Rule[LogicalPlan] {
}
}
+/**
+ * Replaces first(col) to nth_value(col, 1) for better performance.
+ */
+object OptimizeWindowFunctions extends Rule[LogicalPlan] {
+ def apply(plan: LogicalPlan): LogicalPlan = plan resolveExpressions {
+ case we @ WindowExpression(AggregateExpression(first: First, _, _, _, _),
spec)
+ if spec.orderSpec.nonEmpty &&
+ spec.frameSpecification.asInstanceOf[SpecifiedWindowFrame].frameType
== RowFrame =>
Review comment:
shall we also check if the lower bound is `UnboundedPreceding`?
otherwise we can't use the offset optimization for nth_value and `first` is
probably faster than `nth_value(1)`
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