Or they could be speaking from experience, because back in the day when webforums where the place to make online communities, as they grew beyond a certain size

Or they could be speaking from experience, because back in the day when webforums where the place to make online communities, as they grew beyond a certain size this sort of thing became THE bottleneck, especially for large threads. I empirically know this was an issue back then. I also agree it's something a DB engine should handle, and I do hope that databases do a better job today. So instead of calling out "premature optimization", maybe instead come with a practical example of a DB engine that properly handles this?