I understand that covering index will speed up read operation if all the columns in select list are covered in the index.
But what if I select and filter on columns in other tables as well? Would covering index still help in this case?
Here is my thought: Since the join would force the clustered key to be matched, so the clustered key look up couldn't be avoided. And by finding the clustered key we would have all the information we need thus covering index wouldn't help to speed up the read.