(What's a 9-year-old girl doing in the temple? And what's with the yellow dress?)
The thought of having to perform baptisms for every person who has lived since the ministry of Christ gets increasingly complicated when you consider things like the sheer number of people who have lived in the time spanning the Resurrection to today - only the smallest fraction of a percentage of which were baptized "by authority" - and the fact that no recording of names was ever made for the vast majority of those people. Even if some record had been made of a birth, a death, a marriage, a purchase, or anything else, chances are it doesn't exist anywhere.
The way it was told me, the Mormon solution to the billions of forgotten and unresearchable names of the dead calls for the assistance of angels who will, I guess, whisper entire, unending genealogies to temple workers during the Millennium. Angels always get their facts straight so there won't be anything to worry about, right? Well, maybe. I didn't find it very convincing myself, but I doubted my logic and found faith in faith.