Lee Asher performs refactoring:
Over time the term “refactoring” has expanded and is sometimes used to mean code quality improvement in general, but here we are using it with its original meaning: condensing and eliminating redundant segments of code. Like factoring a number in math, we break the code into smaller blocks, identify any repeated elements, then replace them with a single reference.
I appreciate that Lee is sticking to the original meaning of the term here. Interestingly, Lee doesn’t cover T-SQL functions at all. On net, that’s probably a good thing, especially scalar functions. It’s easy to find cases where converting a function to an inline call can speed up query performance by 3x or more.
The mechanisms Lee does use could have an impact on query performance, especially lateral join/APPLY. But for some of these, as long as you do not overuse the technique, performance will be pretty similar.