Originally Posted by felgall
But there's nothing to stop you making an additional key on those fields if you are going to need to access by fieldA/C as well as by fieldA/B
But then the question arises as to whether you are better off with a single compound index or three separate indexes or the compound key plus one index or...
And with MySQL, there's no good answer to that except to try variations and benchmark them in your actual situation. And then you also have the fun of learning that sometimes MySQL will purposely choose not to use an available index because it thinks a full table scan will be more efficient. And sometimes MySQL is right and sometimes it is not. Luckily, there are ways to gently guide MySQL to the best answer, but it takes experimentation to know just what is the best answer.