as you can see, the index is simply a sorted list of student numbers. to process enrollment sequentially on student number, we simply process the index sequentially, obtaining enrollment data by reading the records indicated by the pointers. figure H-8(c) shows another index for enrollment - one that maintains class number order.
to use an index, the data to be ordered (here,enrollment) must reside on an indexed sequential or direct file, although the indexes can reside on any type of file. in practice, almost all DBMS products keep both the data and the indexes on direct files.
if you compare the linked list with the index, you will notice the essential difference between them. in a linked list, the pointers are stored along with the data. each record contains a link field containing a pointer to the address of the next related record. but in an index, the pointers are stored in indexes, separate from the data. thus, the data records themselves contain no pointers. both techniques are used by commercial DBMS products.