I am curious to know if an index in a database which is valid and used take up space in the disk or not.
Your question is very vague, if you asked the question
does an Index take up "extra" space? Then the answer would be ......
No not in the case of a
Clustered Index, a clustered index is the actual data stored in a sorted manner.
But on the other hand
Non-Clustered Index is a separate structure to the actual table/data itself, containing pointers to the actual data, in this case Yes it would take up extra space .