title: Why does `libpq` use polling rather than notification for data fetch?
body: <p>I am reading <code>libpq</code> reference. It has both of sync and async methods. Bu I discovered something strange.</p>
  
  <p>When I see <code>PQsendQuery</code> function, it seems to send a query and return immediately. And I expected a callback function to get notified, but there was no such thing and the manual says to <em>poll</em> for data availability. </p>
  
  <p>I don't understand why async method is written in polling way. Anyway, as <code>libp</code> is the official client implementation, I believe there should be a good reason for this design. What is that? Or am I missing correct callback stuffs mentioned somewhere else?</p>