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diff --git a/sql/core/src/main/scala/org/apache/spark/sql/execution/window/RowBuffer.scala b/sql/core/src/main/scala/org/apache/spark/sql/execution/window/RowBuffer.scala
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+++ b/sql/core/src/main/scala/org/apache/spark/sql/execution/window/RowBuffer.scala
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+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.spark.sql.execution.window
+
+import scala.collection.mutable.ArrayBuffer
+
+import org.apache.spark.sql.catalyst.InternalRow
+import org.apache.spark.sql.catalyst.expressions.UnsafeRow
+import org.apache.spark.util.collection.unsafe.sort.{UnsafeExternalSorter, UnsafeSorterIterator}
+
+
+/**
+ * The interface of row buffer for a partition. In absence of a buffer pool (with locking), the
+ * row buffer is used to materialize a partition of rows since we need to repeatedly scan these
+ * rows in window function processing.
+ */
+private[window] abstract class RowBuffer {
+
+  /** Number of rows. */
+  def size: Int
+
+  /** Return next row in the buffer, null if no more left. */
+  def next(): InternalRow
+
+  /** Skip the next `n` rows. */
+  def skip(n: Int): Unit
+
+  /** Return a new RowBuffer that has the same rows. */
+  def copy(): RowBuffer
+}
+
+/**
+ * A row buffer based on ArrayBuffer (the number of rows is limited).
+ */
+private[window] class ArrayRowBuffer(buffer: ArrayBuffer[UnsafeRow]) extends RowBuffer {
+
+  private[this] var cursor: Int = -1
+
+  /** Number of rows. */
+  override def size: Int = buffer.length
+
+  /** Return next row in the buffer, null if no more left. */
+  override def next(): InternalRow = {
+    cursor += 1
+    if (cursor < buffer.length) {
+      buffer(cursor)
+    } else {
+      null
+    }
+  }
+
+  /** Skip the next `n` rows. */
+  override def skip(n: Int): Unit = {
+    cursor += n
+  }
+
+  /** Return a new RowBuffer that has the same rows. */
+  override def copy(): RowBuffer = {
+    new ArrayRowBuffer(buffer)
+  }
+}
+
+/**
+ * An external buffer of rows based on UnsafeExternalSorter.
+ */
+private[window] class ExternalRowBuffer(sorter: UnsafeExternalSorter, numFields: Int)
+  extends RowBuffer {
+
+  private[this] val iter: UnsafeSorterIterator = sorter.getIterator
+
+  private[this] val currentRow = new UnsafeRow(numFields)
+
+  /** Number of rows. */
+  override def size: Int = iter.getNumRecords()
+
+  /** Return next row in the buffer, null if no more left. */
+  override def next(): InternalRow = {
+    if (iter.hasNext) {
+      iter.loadNext()
+      currentRow.pointTo(iter.getBaseObject, iter.getBaseOffset, iter.getRecordLength)
+      currentRow
+    } else {
+      null
+    }
+  }
+
+  /** Skip the next `n` rows. */
+  override def skip(n: Int): Unit = {
+    var i = 0
+    while (i < n && iter.hasNext) {
+      iter.loadNext()
+      i += 1
+    }
+  }
+
+  /** Return a new RowBuffer that has the same rows. */
+  override def copy(): RowBuffer = {
+    new ExternalRowBuffer(sorter, numFields)
+  }
+}