title: Does adding a null column to a postgres table cause a lock?
body: <p>I think I read somewhere that running an <code>ALTER TABLE foo ADD COLUMN baz text</code> on a postgres database will not cause a read or write lock. Setting a default value causes locking, but allowing a null default prevents a lock.</p>
  
  <p>I can't find this in the documentation, though. Can anyone point to a place that says, definitively, if this is true or not?</p>