A colleague of mine, as we are changing our DB indexing structure, has argued that indexes harm database writes since the data must be written to the database and to the index. This is a compelling argument. However, in my last job in working with Sybase SQL databases we had instances where indexes would greatly increase write efficiency apparently. I had specific experience that this was the case, but no one was able to explain to me how that was the case given what I could see where the data would have to be written in two separate places.
So, how do indexes effect the efficiency of database writes in mySQL?