title: Why is Spark's show() function very slow?
body: <p>I have</p>
  
  <pre><code>df.select("*").filter(df.itemid==itemid).show()
  </code></pre>
  
  <p>and that never terminated, however if I do</p>
  
  <pre><code>print df.select("*").filter(df.itemid==itemid)
  </code></pre>
  
  <p>It prints in less than a second. Why is this?</p>