Grant Fritchey searches for included columns:
A common issue in performance is the lookup, key or RID, but the question frequently asked is, which columns are looked up? You need to know in order to try to address the issue. The answer is easy enough to find. Let’s take a quick look.
Read on for the answer, which you can find in SQL Server Management Studio. Or by shredding a bunch of XML if that’s the kind of thing you’re into.