For an index to work you need to re-write the query.
AFAIK you don’t need to re-write a query to gain benefit from an added index. I’ve witnessed this on new Firefly installations. I normally add an index idx_artist on songs(artist,album). If I browse my database (18,000 songs) on my Soundbridge (Browse Artist, then select an artist to see their albums) before adding the above index , it is very slow. After I add the index and browse again, the Albums for an Artist are returned much more quickly.