Yes, I was using Mysql exclusively at the time. I don't recall which version.
I also tested this once years later when doing a Python app with sqlite. Similar result, but admittedly that was not a very big table to begin with.
I am meticulous with my database schemas, and periodically review my indexes and covering indexes. I'm no DBA, but I believe that the database is the only real value a codebase has, other than maybe a novel method here and there. So I put care into designing it properly and testing my assumptions.
Do you recall what the database server was?