Under the Covers: MDX IF Statement

t is a well-known best practice in the MDX community to avoid run-time checks by choosing SCOPE over IIF function, and for the same reason, the IF statement. But what is the actual performance impact when you have no choice but to use IIF function or IF statement? I have described in details the block mode algorithm for IIF function in one of my previous blog posts. The other day, Teo Lachev asked whether he needed to worry about performance if he used IF statement in his cube script. In particular, Teo wanted to know if there was any performance impact on other calculations which kick in when the condition of IF statement is false. In my post about IIF function, I mentioned that IF statement is internally rewritten as IIF function calls. Today I am going to add a bit more details on how the rewriting is done and what limitations can prevent the rewriting from happening. What is described here applies equally to the CONDITION clause in a CREATE CELL CALCULATION statement since there is no internal difference between the two MDX features.