title: Postgresql: what is different between DEFAULT NULL vs NULL on alter table
body: <p>Is any different between this two queries? If there is what will be faster?</p>
  
  <pre><code>ALTER TABLE mytable ADD COLUMN newcolumn VARCHAR(64) DEFAULT NULL;
  ALTER TABLE mytable ADD COLUMN newcolumn VARCHAR(64) NULL;
  </code></pre>
  
  <p>How does it work under the hood? If we have a table with a millions of rows will any one of queries rewrite every row to set NULL value to a new column?</p>