Under what conditions might such an index be used?

We discussed the choice of indexes to optimize the execution of the nested query (12.1) and pointed out that a clustered index on Teaching with search key CrsCode would not be considered. It might, however, be used in optimizing the execution of the equivalent, non-nested query

```
SELECT C.CrsName, P.Name
FROM Professor P, Teaching T, Course C
WHERE T.Semester=’S2003’ AND P.Department=’CS’
AND C.DeptId = ’MAT’
AND P.Id = T.ProfId AND T.CrsCode=C.CrsCode
```

If, in addition, there was a clustered index on Course with search key DeptId the computation of the second join in (12.2) could be done very quickly. If the join resulted in a small number of rows, the remaining computation could also be done quickly.