Plenty of databases at the time had pretty lame string indexing because it simply wasn't something people relied on if they wanted performance, and memory sayet

Plenty of databases at the time had pretty lame string indexing because it simply wasn't something people relied on if they wanted performance, and memory sayeth mysql's were both not particularly efficient and had some nasty technical limitations. On the mysql she was using, breaking things out so it only needed to index ints was almost certainly a much better idea. On anything I'd be deploying to today, I'd start by throwing a compound index at the thing with the expectation that'd probably get me to Good Enough.