In my experience, the more you learn about Haskell, the more you can pick out spots in your code where you could have made something more concise, more elegant, more efficient, etc. The excellent thing about pure functions and Haskell's strong typing, though, is that refactoring is a breeze -- changing a function will never break another function as long as the inputs and outputs to that function remain the same.

The was and continue to be a lot of refactorings, but they were/are pretty much all related to the complexity and sheer number of features in SQL. Haskell really gets out of the way, and helps make the refactorings much more easy.