[GitHub] [spark] hvanhovell commented on a change in pull request #27558: [SPARK-30806][SQL]Evaluate once per group in UnboundedWindowFunctionFrame

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hvanhovell commented on a change in pull request #27558: [SPARK-30806][SQL]Evaluate once per group in UnboundedWindowFunctionFrame
URL: https://github.com/apache/spark/pull/27558#discussion_r378835132
 
 

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 File path: sql/core/src/main/scala/org/apache/spark/sql/execution/window/WindowFunctionFrame.scala
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 @@ -284,9 +284,7 @@ final class UnboundedWindowFunctionFrame(
 
   /** Write the frame columns for the current row to the given target row. */
   override def write(index: Int, current: InternalRow): Unit = {
-    // Unfortunately we cannot assume that evaluation is deterministic. So we need to re-evaluate
-    // for each row.
-    if (processor != null) {
+    if (processor != null && (index == 0)) {
 
 Review comment:
   I can't remember why I added this. Are you sure that computing the result of an aggregate (on its buffer) is always deterministic? If so then I am ok with this change.

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