If you go to a Youtube video by Justin Bieber or Rebecca Black, comments are constantly being posted and it seems like the autorefresh for the comments kicks in every half second. I don't plan on having millions of users viewing my page, but still, wouldn't having every user make a [mysql] request to the server every half second really take a toll on the server (especially since I don't plan to ever have the resources that Youtube has)? Facebook also seems to have instantaneous auto-refreshes, be it in comment boxes or their chat interface.

So what would be the best method to implement a near-instantaneous autorefresh system with minimal resource usage?