title: Column with lists values to rows
body: <p>I have a data frame with 131 millions rows.</p>
  
  <p>The <code>data.frame</code> has 2 columns. <code>Column 1</code> is a number. <code>Column 2</code> is a list of values</p>
  
  <p>Something like this</p>
  
  <pre><code>Col1 | Col2  
  1    | a, b, c  
  2    | d, e, f  
  3    | a, e, f  
  </code></pre>
  
  <p>And I need to put it like this:</p>
  
  <pre><code>Col1 | Col2  
  1    | a  
  1    | b  
  1    | c  
  2    | d  
  2    | e  
  2    | f  
  3    | a  
  3    | e  
  3    | f  
  </code></pre>
  
  <p>It must be quick because of the 131 millions rows.</p>