I am reading `libpq` reference. It has both of sync and async methods. Bu I discovered something strange.

When I see `PQsendQuery` 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 *poll* for data availability. 

I don't understand why async method is written in polling way. Anyway, as `libp` 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?