title: Spark "replacing null with 0" performance comparison
body: <p>Spark 1.6.1, Scala api.</p>
  
  <p>For a dataframe, I need to replace all null value of a certain column with 0.
  I have 2 ways to do this.
  1.</p>
  
  <pre><code>myDF.withColumn("pipConfidence", when($"mycol".isNull, 0).otherwise($"mycol"))
  </code></pre>
  
  <p>2.</p>
  
  <pre><code>myDF.na.fill(0, Seq("mycol"))
  </code></pre>
  
  <p>Are they essentially the same or one way is preferred?</p>
  
  <p>Thank you!</p>