Well, actually no, it's easy. And straightforward. And obvious. If the people who designed the website had been deliberately making it difficult, it probably would have been difficult, but it isn't. Here's how to do it: note the author of the QOTD (or enough of the name to be useful - in this case "Prakriti" is plenty, you don't need the rest of the name) and the topic of the question (in this case query cost). Then go to any of the discussion forums. Click on "Article Discussions by Author" in the heading (and wait for the page to load it will take a few 10s of second if you have a slow connection); then search for the query author's name; then in the set of discussion topics listed click on the one which ois the topic of the QOTD - bingo, you are there.

I know that you can do this because I saw the two wrongs and a right and concluded something pretty weird was going on (I could make a guess why - the QOTD in the email was clearly one of "those" QOTDs), wanted to know what that was before clicking on an answer, so thought about it for about 20 seonds and then went there. I hadn't ever done it, or even thought about it, before. So I know it is not difficult (thos big block headers make it pretty obvious how to do it).