title: Adding column to PostgreSQL with default value but without impacting current rows
body: <p>I want to add a column in a Postgres table. I want that the value for existing rows will be null but for new rows, it will take the default value that I'll specify in the query.</p>
  <p>I'm running this query-</p>
  <pre><code>ALTER TABLE events 
  ADD COLUMN event_timestamp timestamp default current_timestamp
  </code></pre>
  <p>This is running fine but it is populating the current timestamp for existing rows as well. How to change the query so that it will not add a default value for existing rows?</p>