Primary keys must be unique, and titles of recordings are often not unique. In the sample file, for instance, the title “Symphony No. 9” is shared by several recordings.

Whenever we try to find a canonical form for a key that contains information we run into the problem that there may be many ways to express that information. For instance, the following titles all contain exactly the same information:

Symphony Number Nine
Symphony Number 9
Symphony No. Nine
Symphony No. 9
Ninth Symphony

We need to insure that either the exact same form is used whenever referring to a particular title, or that our programs are smart enough to transform a request for a key value into some equivalent canonical form.

Four types of changes that can affect indexes are those that:
1. change the value of the primary key
2. change the value of a secondary key
3. increase the length of the data record
4. decrease the length of the data record
These types of changes are not necessarily mutually exclusive. For instance, it may be that a change in the value of a primary key will result in an increase in the length of the data record. Here is a table indicating the types of changes and their effects on indexes, assuming the indexes are similar to the ones developed in the chapter:



Type of Change	Fixed Length	Variable Length
Primary key value changed	Primary key index must be resorted to accommodate new position of primary key.	Same as for fixed.
Secondary key value changed	Secondary key index must be resorted to accommodate new position of secondary key.	Same as for fixed.
Record length increases	Can not happen.	Reference field in primary key index must be changed to new byte offset of record.
Record length decreases	Can not happen.	Record not moved: no change. Record is moved: reference field primary index must be changed to new byte offset of record.

    ABC   1234   Fidelio   Beethoven   Maazel

• For the Secondary Index File, change the link field of record number 0 (BEETHOVEN) to 11.
• The following changes are made to the Label ID List File:
  • Append record number 11 with Label ID ABC1234 .
  • Insert the number 3 in the link field of record number 11 (ABC1234).



Secondary Index File  0 BEETHOVEN 11 1 COREA 2 2 DVORAK 7 3 PROKOFIEV 10 4 RIMSKY-KORSAKOV 6 5 SPRINGSTEEN 4 6 SWEET HONEY IN THE R 9

Label ID List File  0 LON2312 -1 1 RCA2626 -1 2 WAR23699 -1 3 ANG3795 8 4 COL38358 -1 5 DG18807 1 6 MER75016 -1 7 COL31809 -1 8 DG139201 5 9 FF245 -1 10 ANG36193 0 11 ABC1234 3