My full legal name is "Sai"; I'm mononymous. (
Proof.) To the best of my knowledge, I also have the shortest (personal, legal) name in the world.
My boarding pass (like the results of all poorly coded databases) lists this in a wide variety of ways — eg "SAI SAI", "MR SAI", "SIR SAI", "NFN SAI", "SAI NLN", etc etc.
So far, I have only once had trouble with the TSA (about this) — a STSO demanded that I furnish a second form of ID other than my passport, clearly stating that I would not be permitted to enter the checkpoint unless I did so (which is also clearly illegal, but I haven't bothered pursuing that).
Otherwise they (and customs) do a doubletake, which by now I can time pretty exactly and say "yes, that's correct", sometimes express surprise, and let me go on my way.
It's been in a major PITA with some things (like credit report agencies, Amazon Payments) and totally not a problem at all with others (like driver's license, passport, some of my bank accounts but not others).