cloud-fan commented on a change in pull request #29800:
URL: https://github.com/apache/spark/pull/29800#discussion_r511919183



##########
File path: 
sql/core/src/main/scala/org/apache/spark/sql/execution/window/WindowFunctionFrame.scala
##########
@@ -151,10 +169,69 @@ final class OffsetWindowFunctionFrame(
     }
     inputIndex += 1
   }
+}
 
-  override def currentLowerBound(): Int = throw new 
UnsupportedOperationException()
+/**
+ * The unbounded offset window frame calculates frames containing NTH_VALUE 
statements.
+ * The unbounded offset window frame return the same value for all rows in the 
window partition.
+ */
+class UnboundedOffsetWindowFunctionFrame(
+    target: InternalRow,
+    ordinal: Int,
+    expressions: Array[OffsetWindowSpec],
+    inputSchema: Seq[Attribute],
+    newMutableProjection: (Seq[Expression], Seq[Attribute]) => 
MutableProjection,
+    offset: Int)
+  extends OffsetWindowFunctionFrameBase(
+    target, ordinal, expressions, inputSchema, newMutableProjection, offset) {
 
-  override def currentUpperBound(): Int = throw new 
UnsupportedOperationException()
+  override def prepare(rows: ExternalAppendOnlyUnsafeRowArray): Unit = {
+    super.prepare(rows)
+    if (inputIndex >= 0 && inputIndex < input.length) {
+      val r = WindowFunctionFrame.getNextOrNull(inputIterator)

Review comment:
       The logic here is a bit weird. Why do we iterate the first `offset - 1` 
records in `super.prepare` method, and then accesee the `offset`th record here? 
Can we combine them?




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