he benefits of having an index are well known, you can get the same results by reading a smaller amount of data so the improvement in performance can be from several minutes to seconds or even less.
That sounds awesome and it certainly is and there are people out there making a living of it, so it’s a huge deal for sure.
But it’s not always like that, and things can go wrong very easily and make all these shiny indexes just a pile of useless burden.
Let me show you some examples, where we can see our indexes in use, but also how they can be ignored by the query processor and become totally useless. I’m going to use the Microsoft sample database [WideWorldImporters] so you can follow along if you want.
Read on to learn more.