title: What Do $$ and :: do in Postgresql
body: <p>I'm looking through a mess of (Postgresql) SQL code that I've inherited, and there are two pieces of notation that are being used that I don't follow. One is using <code>$$</code>. E.g. <code>Create function ... returns FOO as $$</code> and <code>select NULL::int field_name from table</code>.  </p>
  
  <p>Having trouble googling because symbols, and I'm not even sure what I should be searching for.</p>