title: What's the default window frame for window functions
body: <p>Running the following code:</p>
  <pre><code>val sales = Seq(
    (0, 0, 0, 5),
    (1, 0, 1, 3),
    (2, 0, 2, 1),
    (3, 1, 0, 2),
    (4, 2, 0, 8),
    (5, 2, 2, 8))
    .toDF(&quot;id&quot;, &quot;orderID&quot;, &quot;prodID&quot;, &quot;orderQty&quot;)
  
  val orderedByID = Window.orderBy('id')
  
  val totalQty = sum('orderQty').over(orderedByID).as('running_total')
  val salesTotalQty = sales.select(*, totalQty).orderBy('id')
  salesTotalQty.show()
  </code></pre>
  <p>The result is:</p>
  <pre><code>+---+-------+------+--------+-------------+
  | id|orderID|prodID|orderQty|running_total|
  +---+-------+------+--------+-------------+
  |  0|      0|     0|       5|            5|
  |  1|      0|     1|       3|            8|
  |  2|      0|     2|       1|            9|
  |  3|      1|     0|       2|           11|
  |  4|      2|     0|       8|           19|
  |  5|      2|     2|       8|           27|
  +---+-------+------+--------+-------------+
  </code></pre>
  <p>There is no window frame defined in the above code, it looks the default window frame is <code>rowsBetween(Window.unboundedPreceding, Window.currentRow)</code></p>
  <p>Not sure my understanding about default window frame is correct</p>