title: Apply np.where against square bracket filtering for numpy filtering
body: <p>I could perform filtering of numpy arrays via</p>
  
  <pre><code>a[np.where(a[:,0]==some_expression)]
  </code></pre>
  
  <p>or </p>
  
  <pre><code>a[a[:,0]==some_expression]
  </code></pre>
  
  <p>What are the (dis)advantages of each of these versions - especially with regard to performance?</p>