What is a non-clustered index scan

I know what table scan, clustered index scan and index seek is but my google skills let me down to find a precise explanation into non clustered index scans. Why and when a query uses a non clustered index scan?

Thank you.

A non-clustered index scan may be chosen in this scenario:

• the optimizer determines that it is cheaper to scan all rows rather than perform seeks/range scans

• the non-clustered index is "skinnier" than the clustered index

• the non-clustered still covers the columns needed by the query (or it covers enough of them and a lookup for the remainder is still cheaper than a clustered index scan)

It can also happen, obviously, if you have a heap (no clustered index) and 3. is still true.

This is not meant to be an exhaustive list - there are other cases where a non-clustered index scan may be chosen, but this is probably the most common.

StackExchange Database Administrators Q#80841, answer score: 8