title: Add timestamp column with default NOW() for new rows only
body: <p>I have a table that has thousands of rows. Since the table wasn't constructed with created_at column initially, there is no way of getting their creation timestamp. It is crucial though to start getting the timestamps for future rows. </p>
  
  <p>Is there a way I can add a timestamp column with default value NOW() so that it won't populate the values to previous rows but only for the future ones?</p>
  
  <p>If I do the <code>ALTER</code> query, it populates all rows with timestamp:</p>
  
  <pre><code>ALTER TABLE mytable ADD COLUMN created_at TIMESTAMP DEFAULT NOW()
  </code></pre>