I know ORDER BY RAND() works by generating a random number for each row and then sorts by that. Generating that number is extra work. Sorting by this number or by creation date would be same. I would say ORDER BY RAND() will have an overhead. I haven't measured the performance, so I don't know the overhead. I tried it on my site (just a few hundred questions) and reloaded the page and didn't notice any visible delay in loading the page.