But wasn't the 16 million estimate based on the increasing ID numbers, which includes comments and self-posts, neither of which don't need to be hashed or check

But wasn't the 16 million estimate based on the increasing ID numbers, which includes comments and self-posts, neither of which don't need to be hashed or checked? The number of submitted URL's that need to be checked against is presumably much smaller, and the number of those that have at least one comment is much smaller than that. Edit: Estimating quickly from the new submissions for the last couple hours, it looks like there are on the order of 100 submissions per hour. Of these, it seems like less than 10% have any comments. So if we assume that HN has been up for 12 years (100,000 hours) we get about 1 million commented URL's with comments, and that's probably an overestimate (dups, increasing usage, etc).