title: Implications of using ADD COLUMN on large dataset
body: <p>Docs for Redshift say:</p>
  
  <pre><code>ALTER TABLE locks the table for reads and writes until the operation completes.
  </code></pre>
  
  <p>My question is:<br>
  Say I have a table with 500 million rows and I want to add a column. This sounds like a heavy operation that could lock the table for a long time - yes?  Or is it actually a quick operation since Redshift is a columnar db?  Or it depends if column is nullable / has default value?</p>