title: Intersection of files
body: <p>I two large files (27k lines and 450k lines). They look sort of like:</p>
  
  <pre><code>File1:
  1 2 A 5
  3 2 B 7
  6 3 C 8
  ...
  
  File2:
  4 2 C 5
  7 2 B 7
  6 8 B 8
  7 7 F 9
  ... 
  </code></pre>
  
  <p>I want the lines from both files in which the 3rd column is in both files (note lines with A and F were excluded):</p>
  
  <pre><code>OUTPUT:
  3 2 B 7
  6 3 C 8
  4 2 C 5
  7 2 B 7
  6 8 B 8
  </code></pre>
  
  <p>whats the best way? </p>